text:
Removal of Sulfur Dioxide from Flue Gas Using the Sludge Sodium Humate

This study shows the ability of sodium humate from alkaline treatment sludge on removing sulfur dioxide (SO2) in the simulated flue gas. Experiments were conducted to examine the effect of various operating parameters, like the inlet SO2 concentration or temperature or O2, on the SO2 absorption efficiency and desulfurization time in a lab-scale bubbling reactor. The sludge sodium humate in the supernatant after alkaline sludge treatment shows great performance in SO2 absorption, and such efficiency can be maintained above 98% with 100 mL of this absorption solution at 298 K (flue gas rate of 0.12 m3/h). The highest SO2 absorption by 1.63 g SHA-Na is 0.946 mmol in the process, which is translated to 0.037 g SO2 g−1 SHA-Na. The experimental results indicate that the inlet SO2 concentration slightly influences the SO2 absorption efficiency and significantly influences the desulfurization time. The pH of the absorption solution should be above 3.5 in this process in order to make an effective desulfurization. The products of this process were characterized by Fourier transform infrared spectroscopy and X-ray diffraction. It can be seen that the desulfurization products mainly contain sludge humic acid sediment, which can be used as fertilizer components.

Introduction
Flue gas emissions, which mainly come from power plants by burning fossil fuels, have been causing serious air pollution for decades [1][2][3]. The reason for serious air pollution caused by flue gas emissions is that flue gas contains large amounts of sulfur dioxide (SO 2 ) and other pollutants [4]. Many researchers have been actively exploring technologies in effective flue gas desulfurization field [5][6][7][8]. Among these technologies, one of the most effective methods is the wet flue gas desulfurization which is mainly based on limestone [9]. However, it has many disadvantages, such as higher operating costs and greater water requirement and the potential to cause secondary pollution. Thus, cost-effective technologies in removing SO 2 have become the focus of investigations.
Common commercial sodium humate, which is derived from peat, brown coal, and weathered coal, is a cheap absorbent. Green and Manahan started to use sodium humate to absorb SO 2 from flue gas in the 1980s [10,11]. Sun et al. were using humic acid as an additive to modify adsorbents for flue gas desulfurization [12]. Our team used sodium humate solution to remove SO 2 and NO x in flue gas [13,14]. Sludge sodium humate (SHA-Na) can be extracted from sludge through alkaline treatment method [15]. This paper proposed a new process for the removal of SO 2 from flue gas by the absorption solution from sludge treatment and the production of fertilizer.
The new process shown in Figure 1 includes the following stages. (a) Excess sludge is disintegrated by sodium hydroxide in stirred reactor at 313 K. (b) The disintegrated sludge is centrifuged and the supernatant is concentrated through membrane filter to spray into a desulfurization tower. (c) SO 2 can be absorbed by SHA-Na in the desulfurization tower. The desulfurization liquid, which mainly contains sludge humic acid (SHA) and H 2 SO 3 , flows into the reactor. (d) In the reactor, SO 3 2− is oxidized to SO 4 2− through diffused aeration. (e) Afterward, the reaction liquid from the reactor flows into a sedimentation tank and stands for 12 h. Due to the poor solubility of SHA, SHA may be separated as sediment from acidic solution. The separated SHA can be used as a kind of material for compound fertilizer after drying in spray dryer. From the above process, it is believed that the removal of SO 2 by the supernatant from alkaline sludge treatment is a resourceful type of environmental protection technology for FGD. It  carries the following advantages: (a) realizes sludge reduction and (b) utilizes the desulfurization product as a useful fertilizer. Therefore, it is hopeful to be applied in the future.
In this paper, we attempt to use a kind of absorption solution from alkaline treatment sludge to remove SO 2 in flue gas. The effects of the inlet SO 2 concentration, temperature, and presence or absence of O 2 , on the SO 2 absorption efficiency, together with desulfurization time in a lab-scale bubbling reactor, are studied. Desulfurization products are characterized by Fourier transform infrared spectroscopy and X-ray diffraction.

Experimental Section
2.1. Materials. Sludge samples were collected from the thickening tank of a wastewater treatment plant. The sludge samples carry water of approximately 98%, a suspended solid (SS) content of 20.0 g/L, a volatile suspended solid (VSS) content of 16.0 g/L, and a soluble chemical oxygen demand (SCOD) of 270 mg/L. Sodium hydroxide (NaOH, AR) was collected from chemical reagent. HA-Na was supplied with purity of 99%. Sulfur dioxide gas with a purity of 99.95% was obtained from the market.

The Absorption Solution Extraction.
A slightly modified method [16] was used to extract SHA-Na. The extraction of SHA-Na was conducted in 2.0 L batch mixed reactor that was placed in a water bath to maintain the temperature for this reaction at 313 K while the NaOH is at 0.2 mol/L. Following alkaline sludge treatment, the samples were centrifuged at 5000 r/m for 10 min. After alkaline sludge treatment and centrifugal separation, the sample was filtered through a membrane with a mesh size of 0.45 m.

Desulfurization Test.
A schematic diagram of the experimental setup used in this study is shown in our previous paper [17]. The SO 2 gas supplied from cylinder was diluted with O 2 and N 2 in the gas-mixing chamber. Then, this gaseous mixture flew through the bubbling reactor at ambient pressure. Rota meters and valves were used to monitor the gas flow. The concentrations of the simulated flue gas components (SO 2 and O 2 ) were controlled by adjusting their flow rate and were monitored by a flue gas analyzer. In the progress of this experiment, the total rate of the gas stream was controlled at 0.12 m 3 /h during the experiment.

Characterization Methods.
In the experiment, the inlet and outlet concentrations of SO 2 were measured by a flue gas analyzer. The pH value measuring was conducted with a pH meter using a combination pH electrode PHB-5. Before each pH value was read, the pH buffer solution was used to check the measurements of the electrode. Water content, SS, and volatile dissolved solids (VDS) were measured according to the standard methods [18]. SHA-Na was measured according to the spectrophotometer method recommended by Ye [19]. The protein content was measured using the Coomassie brilliant blue method [20]. FTIR spectra were recorded in the 4000-500 cm −1 range using an EQUINOX 55 FTIR spectrometer, continuously purged with dry air. Spectra were obtained from pressed KBr pellets. Pellets (1 cm diameter) were prepared by mixing 1 or 2 mg samples with 200 mg KBr. Desulfurization products were characterized by XRD with a D/max-2200/PC type-ray diffraction instrument to study the composition of them. The heavy metals in products were analyzed by inductively coupled plasma atomic emission spectrometry.

Characteristics of the Supernatant after Alkaline Treatment
Sludge. The sludge samples were centrifuged after alkaline treatment, and the supernatant was analyzed. Characteristics of the supernatant after alkaline treatment are shown in Table 1. The supernatant has a high pH value which is similar to the common commercial sodium humate. The VDS indicated that there was a high content of organic substances including sodium humate, protein, polysaccharides, nucleic acid, and lipids [21]. It can be seen that sodium humate and The Scientific World Journal 3 protein were the primary constituents in this supernatant. Sodium humate accounted for approximately 25% of all of the dissolved organic matter by weight. After alkaline sludge treatment, the humic acid concentration in the supernatant was 1.63 g/L. However, the concentration of sodium humate in the absorption solution should be 10∼40 g/L in order to get a better absorption solution of SO 2 in our previous papers. Therefore, when the supernatant was concentrated 10 times by ultrafiltration, the content of sodium humate in the retentate reached the required concentration in the process.

Absorption Process of SO 2 in the Absorption Solution.
The change of the SO 2 concentration in outlet flue gas with time is shown in Figure 2. It can be observed that the SO 2 absorption curve in absorption solution is divided into three sections: a descending section, a nearly horizontal section, and an ascending section. Also, the pH of the absorption solution in this process was clearly illustrated in Figure 2. The change of pH in the absorption solution is related to the SO 2 concentration in outlet flue gas. In the descending section of SO 2 absorption curve, the pH value of the absorption solution drops rapidly from 10.8 to 7.4, because of the faster consumption of OH − in the absorption solution. The pH value of the absorption solution decreases slowly from 7.4 to 3.4 in the nearly horizontal section of SO 2 absorption curve, as the SHA-Na in the absorption solution is a sort of pH buffer solution [22], this behavior was interpreted as a buffer action of -COOH [23], which may lower the rate of pH value decrease.
In the ascending section of SO 2 absorption curve the pH value of SHA-Na maintains a constant number. It can be explained as follows: the absorption solution loses the desulfurization capability as the SHA-Na in the absorption solution has converted to SHA sediment. From the above analyses, it can be seen that when the pH value of the absorption solution drops to 3.4, the absorption solution will loose the desulfurization capability. Hence, the pH value of the absorption solution should be above 3.4 in this process in order to make it an effective desulfurization.

Analysis of the Desulfurization Mechanism.
In removing SO 2 process with this absorption solution, the main reactions are that SO 2 from simulated flue gas react with SHA-Na, which is an acid-base reaction, and the acid-base theory predicts that SHA-Na should react with SO 2 by following neutralization reaction [10,11]: Simultaneously, when SO 2 is dissolved in the absorption solution, based on its alkalinity, the following reactions occur [24,25]: Generally, the above reactions can be described by the following equilibrium equations (6) and (7): Then, SO 3 2− is oxidized to SO 4 2− by catalysis with transition metal ions, which is the step for (6) and (7) to move to the right. The dissolved SO 2 also produced H + by ionization in this process. The acidic groups of SHA-Na, such as carboxyl (COO − ) and hydroxyl (OH − ), can react rapidly with H + , and SHA-Na is transferred to sludge humic acid. According to (1) and (5), this reaction also moves the reaction equilibrium to the right, which results in the fact that more SO 2 is dissolved into the absorption solution. When entire SHA-Na is consumed, the desulfurization reaction is terminated in the process. The Scientific World Journal where stands for the amount of SO 2 absorption, mmol and stands for the removal efficiency of SO 2 , %. The formula of the SO 2 absorption efficiency can be obtained from the literature [17]. stands for the gas flow, m 3 /h; 0 stands for the inlet concentration of SO 2 , ppm; stands for the molar mass SO 2 , g/mol; stands for the reaction time, s.
It is clear as shown in Table 2 that experimental results illustrate that SHA-Na performance much better than that of water and NaOH in both the SO 2 absorption efficiency and the duration time of high efficiency ((DTHE), the time of the SO 2 absorption efficiency is above 70%). Water absorbs the SO 2 more slowly because it is only a physical absorption process controlled by molecular diffusion [26]. However, when we use SHA-Na in this absorption solution to remove SO 2 , the hydroxyl radicals in SHA-Na can react rapidly with the SO 2 . Thus, it leads to the decrease of the SO 2 concentration in the gas-liquid interface. The SO 2 diffusion can be promoted due to this decrease of the SO 2 concentration in the gasliquid interface. Furthermore, SHA-Na can react with H + as what common commercial sodium humate does, and it is transformed into humic acid sediment. Therefore, it can be concluded that the desulfurization capability of SHA-Na mainly depends upon the consumption of the hydroxyl radicals as well as common commercial sodium humate. Compared with NaOH solution, it is also shown in Table 2 that the desulfurization capability of SHA-Na is stronger than NaOH solution at the same volume and pH value in Table 2. It can be explained that the hydroxyl radicals of NaOH solution are less than those of SHA-Na at the same volume and pH value. The contrast experiments of removing SO 2 with different sludge as the absorption solution at the same volume are also conducted, respectively. The experimental results illustrate that SHA-Na performance is also much better than that of activated sludge and excess sludge in both the SO 2 absorption efficiency and DTHE. It can be explained that the SO 2 absorption efficiency and desulfurization time depend on the content of fulvic acid that can excellently absorb SO 2 in the activated sludge or excess sludge [25]. The content of fulvic acid in the activated sludge is lower than the content of fuluic acid in the excess sludge, so the SO 2 absorption efficiency of the activated sludge is lower than that of the excess sludge and the desulfurization time of the activated sludge is shorter than that of the excess sludge. Therefore, SHA-Na can be practically used as a kind of absorbent for the removal of SO 2 . by sludge allows the concentration of SO 2 in the absorption solution to decrease. However, the absorption effect of SO 2 becomes worse because of higher concentration of SO 2 at the gas-liquid interface when the inlet SO 2 concentration increases. Hence the desulfurization time in the nearly horizontal section decreased. And the absorption amount of SO 2 at the inset in Figure 3, which decreases with the increase of the inlet SO 2 concentration, shows a reasonable agreement with it.

Effect of the Inlet SO
3.6. Effect of the Temperature. The effect of the temperature on the SO 2 absorption efficiency was also conducted. The experimental results are shown in Figure 4. It indicates that the absorption temperature has slight effect on the SO 2 absorption efficiency and the desulfurization time in the nearly horizontal section becomes shorter with the rise of the temperature. The SO 2 absorption efficiency increases slightly as the absorption temperature ascends from 298 K to 338 K. However, the desulfurization time in the nearly horizontal section decreases from 3500 s to 1300 s as the absorption temperature ascends from 298 K to 338 K. It can be explained as follows. On one hand, according to the dissolution equilibrium of SO 2 , the equilibrium partial pressure of SO 2 is increasing with the rise of the temperature, which results in the escape of some dissolved SO 2 from solution. In addition, the limit of "diffusional regime" is probably reached and this limit depends on temperature proposed by Lancia and Musmarra [28]. The solubility of SO 2 decreases with the rise of the absorption temperature in water. On the other hand, the contact time of flue gas and liquid is reduced because the relative velocity of gas molecules speeds up at high temperature. Hence, the absorption amount of SO 2 decreases with the increase of the temperature at the inset in Figure 4. Although the removal of SO 2 is promoted at low temperature, removing SO 2 with SHA-Na solution in the ambient condition at room temperature should be suitable for this process to reduce the desulfurization cost.

Effect of Oxygen.
Flue gas from power plant usually consists of about 5 vol% O 2 , trace gases, 15 to 20 vol% CO 2 , and balance N 2 [29]. Hence, oxygen (a concentration of 5 vol%) was introduced into a simple simulated flue gas (N 2 + SO 2 ), that is, the simulated flue gas for this study, to understand the potential influence of the presence of O 2 on the SHA-Na performances and interaction of SO 2 and SHA-Na in the absorption solution. Test was conducted without the presence of O 2 , where relative to the presence of O 2 , which is shown in Figure 5. The SO 2 absorption efficiency has a slight decrease under an aerobic condition, while the absorption amount of SO 2 and DTHE has little increase. It is known that the dissolved O 2 in the absorption solution may accelerate the oxidation of sulfite into sulfate. Sulphite oxidation rate can be greatly improved by very low metal concentration [30,31]. The mass-transfer resistance of the liquid phase in absorption solution is lessened, which results in the promotion of the SO 2 mass transfer. Thus, the SO 2 absorption efficiency with the presence of oxygen is higher than that with the absence of O 2 . However, it maybe that the consumption of O 2 makes a corresponding growth of the inlet SO 2 concentration, which results in the lower absorption amount of SO 2 and DTHE. Based on the flue gas, the oxygen concentration was maintained at 5% in the simulated flue gas of N 2 , O 2 and for subsequent experiments.

Analysis of Desulfurization Product.
After the desulfurization process, the absorption liquid shown in Figure 6 was separated into liquid and solid by filtration. The crystals were gained by drying the supernatant layer from the absorption liquid.

6
The Scientific World Journal Table 3: The contents of humic acid and heavy metals in the product and the comparison with the standard.

Indexes
In the standard In the product Humic acid (g/L) FTIR spectroscopy is a powerful and nondestructive tool for the investigation of decomposition in the desulfurization product. Figure 7 shows the FTIR spectra comparison diagram of desulfurization products which are from SHA-Na desulfurization process and HA-Na desulfurization process, respectively. The main absorbance bands of them are in common a broad band at the wavenumbers of 3424 cm −1 (H bonds, OH groups), 1384 cm −1 (COO − , CH 3 ), a small peak at 1253 cm −1 (aromatic C, C-O stretch), a slight shoulder around 1097 cm −1 (aliphatic CH 2 , OH, or C-O stretch of various groups), and a peak at 1050 cm −1 (C-O stretch of polysaccharide) [32,33]. Compared with spectra of humic acid, it is obvious that distinct differences in the spectra that resulted from the SHA-Na flue gas desulfurization process appear as peaks in the aliphatic region at 2921 and 2860 cm −1 , [34] in which sludge humic acid contains a lot of fat material. Three new bands (1652, 1539, and 1460 cm −1 ) appear in the 1700-1450 cm −1 spectra of all the products of desulfurization. HAs are distinguished by the presence of a strong absorption band near 1650 cm −1 , moderately strong absorption at 1540 cm −1 , strong absorption near 1050 cm −1 , and relatively pronounced absorption near 2900 cm −1 [35]. A unique feature of this spectrum also shows the presence of bands indicated proteins and carbohydrates. Due to its poor solubility, sludge humic acid may be separated as sediment from acidic aqueous solution and converted into humic acid compound fertilizer. The chief product of drying the supernatant layer was characterized by XRD. As shown in Figure 8, the diffraction peaks at the XRD patterns of the catalysts obviously belong to Na 2 SO 4 [36,37].
Heavy metals are a limiting factor for sludge fertilizer. The heavy metal content of humic acids extracted from sludge depends on the dissolution rate of sludge heavy metals and the rejection rate of heavy metals by the membrane. During NaOH treatment, the dissolved heavy metals in sludge generally comprise less than 60% of the total heavy metals [38]. Under the strong alkaline conditions, only part of the amphoteric metal in proper forms can be dissolved. Therefore, the heavy metal content in the supernatant was low in Table 3. It can be seen that after the desulfurization process, the desulfurization product contains mainly sludge humic acid sediment, which may be used as fertilizer components.

Conclusion
In the present work, alkaline pretreatment of the activated sludge represented a suitable and effective method for the preparation of SHA-Na. The characteristics of SO 2 absorption into the SHA-Na absorption solution have been investigated in a bubbling reactor. The experimental results show, that compared with water (with the same volume) and NaOH solution (with the same pH), SHA-Na shows greater performance in SO 2 absorption. The effects on the SO 2 absorption efficiency and desulfurization time have been studied, while the inlet SO 2 concentration and temperature were The Scientific World Journal changed. Characterization of the desulfurization products was performed by FTIR and XRD analysis. It was found that the desulfurization products can be used as the compound fertilizer. The concept of combat environmental problems by methods such as removing a waste by using another waste source is of economic interest. Therefore, it is possible for the desulfurization process of SHA-Na to be used in future.


extfieldsofstudy: Chemistry Medicine

s2fieldsofstudy:
Chemistry
Materials Science

text:
4-{(E)-[2-(Pyridin-3-ylcarbonyl)hydrazinylidene]methyl}phenyl acetate

The title compound, C15H13N3O3, exists in the E conformation with respect to the azomethane C=N double bond. The pyridyl and phenyl rings form dihedral angles of 35.67 (8) and 36.65 (7)°, respectively with the central C(=O)N2C unit. In the crystal, N—H⋯O and C—H⋯O hydrogen bonds connect the molecules into chains along the b axis. Another C—H⋯O interaction connects molecules along the c-axis direction, forming layers.

Comment
Hydrazone derivatives show excellent spectrum of biological activities (Sreeja et al., 2004). The chemical and pharmacological properties of aroylhydrazones have been extensively investigated recently owing to their potential application as antineoplastic, antiviral and antiinflammatory agents (Rakha et al., 1996;Takahama, 1996).
The compound ( Fig. 1) crystallizes in the monoclinic space group P2 1 /n. This molecule adopts an E configuration with respect to the C7=N3 bond and it exists in the amido form with a C6=O1 bond length of 1.2277 (18) Å which is very close to the reported C=O bond length of a related structure (Reshma et al., 2012). The O1 and N2 atoms are in a Z configuration with respect to C6-N2 having a torsion angle of 9.9 (3)°. The central C(=O)N 2 C unit has dihedral angles of 35.67 (8) and 36.65 (7)°, respectively with the pyridyl and the phenyl rings.
There is a classical intermolecular N-H···O hydrogen bond interaction between the H atom attached at the N2 and O1 atom of neighbouring molecule with D···A distance of 2.9107 (18) Å and two C-H···O hydrogen bond interactions (Fig. 2) between the H atoms attached at the C7 & C15 and O1 & O3 atoms of neighbouring molecules with D···A distances of 3.251 (2) and 3.469 (4) Å, respectively. The classical hydrogen bond together with C-H···O interaction connect the molecules along b axis while the other C-H···O interaction chain the molecules along c axis in the crystal system. Four types of C-H···π interactions are also found in the molecular system ( Fig. 3) with H···Cg distances of 3.1267, 3.2825, 3.3911 and 3.1984 Å forming a three-dimensional-supramolecuar architecture together with the intermolecular hydrogen bonding interactions. Although there are very weak short ring interactions found in the crystal system, they are not significant to support the network since centroid-centroid distances are above 4 Å. Fig. 4 shows a packing diagram of the title compound viewed along the b axis.

Experimental
The title compound was prepared by adapting a reported procedure (Emmanuel et al., 2011). A solution of pyridine-3carbohydrazide (0.137 g, 1 mmol) in ethanol (10 ml) was mixed with a methanolic solution (10 ml) of 4-formylphenyl acetate (0.164 g, 1 mmol). The mixture was refluxed for 6 h after adding few drops of glacial acetic acid and then cooled to room temperature. The formed crystals were collected, washed with few drops of methanol and dried over P 4 O 10 in vacuo. Colorless block shaped crystals, suitable for SXRD studies, were obtained after slow evaporation of the solution in air for a few days.

Refinement
The atom H2′ was located from a difference Fourier map and N-H3′ distance was restrained to 0.

Figure 1
ORTEP view of the title compound drawn with 50% probability displacement ellipsoids for the non-H atoms.

Figure 4
Packing diagram of the compound along the b axis.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.


text:
Stereoselectively fluorinated N-heterocycles: a brief survey

The stereoselective incorporation of fluorine atoms into N-heterocycles can lead to dramatic changes in the molecules’ physical and chemical properties. These changes can be rationally exploited for the benefit of diverse fields such as medicinal chemistry and organocatalysis. This brief review will examine some of the effects that fluorine substitution can have in N-heterocycles, including changes to the molecules’ stability, their conformational behaviour, their hydrogen bonding ability, and their basicity. Finally, some methods for the synthesis of stereoselectively fluorinated N-heterocycles will also be reviewed.

Introduction
A cursory inspection of the medicinal chemistry literature will reveal two obvious themes in the structures of current drug candidates: the ubiquity of nitrogen heterocycles, and the popularity of organofluorine moieties. Therefore, it seems natural that a combination of these two features will offer rich possibilities in the future of drug development. To date, the introduction of fluorine into medicinal entities [1,2] has mostly taken the form of aryl fluorination [3,4] or trifluoromethylation [5,6], and fascinating developments in synthetic methodology of this type are continuing to occur [7][8][9]. However, with the advent of stereoselective fluorination methods [10,11] it seems clear that the subset of stereoselectively fluorinated N-heterocycles [12] offers particularly rich possibilities. We therefore felt that it would be worthwhile to examine in a brief review some of the unique features of this emerging class of molecules.
We have not attempted to cover this topic comprehensively; rather, in the following pages we aim to provide selected examples of the ways that fluorine can influence N-heterocycles' stability and their conformational behaviour; we will see that fluorine can be used as a tool to probe the importance of hydrogen bonding in bioactive molecules; and we will observe how fluorine can affect the basicity of N-heterocycles. Finally, we will survey some of the various ways in which stereoselec-Scheme 1: Fluorination makes β-lactam derivatives more reactive towards lipase-catalysed methanolysis. tively fluorinated N-heterocycles can be synthesised. Throughout, it will become clear that medicinal chemistry is not the only field that stands to benefit from a deeper understanding of these fascinating molecules: for example, attractive prospects are also clear in the field of organocatalysis [13].

Fluorination can influence N-heterocycles' stability and reactivity
If a highly-polarised C-F bond is incorporated into a nitrogen heterocycle, it can be expected to have a dramatic influence on the molecules' physical and chemical properties [14]. The influence that fluorine can have on chemical reactivity is illustrated by considering the smallest N-heterocycles, the aziridines. Aziridines (1, Figure 1) are generally very stable, in marked contrast with their oxygenated counterparts, the epoxides. However, if one or two fluorine atoms are attached to the aziridine backbone, the resulting molecule is much more susceptible to hydrolysis. De Kimpe and co-workers have investigated the reactivity of mono-and difluoroaziridines 2 and 3 ( Figure 1) [15,16]. As well as the enhanced reactivity that 2 and 3 both show towards nucleophilic ring opening, there is an additional subtlety regarding the regioselectivity. While ab initio calculations predict that both 2 and 3 should favour nucleophilic ring opening at C3 [17], preliminary experiments showed that the mono-and difluorinated aziridines actually behave differently in the presence of nucleophiles, with monofluorinated aziridines 2 experiencing C2 attack and the difluorinated counterparts 3 favouring C3 attack.
Fluorination has also been shown to influence reactivity in fourmembered N-heterocycles (Scheme 1). Kanerva and co-workers [18] investigated a series of β-lactam derivatives (4a-c) in a lipase-catalysed methanolysis process. While the non-fluorinated derivative 4a was found to be unreactive under the reaction conditions specified, successive introduction of one or two fluorine atoms (4b and 4c) led to a marked increase in reactivity. The enantioselectivity of this approach is also worthy of note, and will be discussed further in a later section of this review. Now that our survey of N-heterocycles has reached ring sizes of four atoms or larger, another important consideration emerges: fluorine can affect the molecules' conformational behaviour [19]. To illustrate this point a series of examples are presented below, drawing from heterocycles with ring sizes of up to eight atoms.

Four-membered rings
O'Hagan and co-workers observed an interesting conformational effect in a computational study of fluorinated azetidine derivatives ( Figure 2) [20]. The neutral molecule 6 was calculated to prefer a ring pucker which placed the fluorine atom far away from the neutral nitrogen atom (N-C-C-F dihedral angle = 137.2°). However, the story changed markedly with the charged derivative 7: in this case, the ring pucker inverted and the fluorine atom more closely approached the charged nitrogen atom (N + -C-C-F dihedral angle = 100.0°). This contrast was explained by invoking a favourable interaction between the C-F  dipole and the charged N + atom, and the magnitude of this charge-dipole effect is revealed by comparison with the nonfluorinated control molecule 8 in which the ring pucker is less pronounced (N-C-C-H dihedral angle = 102.3°). It transpires that this C-F … N + interaction is a general effect which has also been observed in larger N-heterocycles, as discussed below.

Five-membered rings
The C-F … N + interaction can have a more dramatic impact on the conformations of pyrrolidines, since they are inherently more flexible than azetidines [21]. For example, O'Hagan and co-workers investigated the pyrrolidine-containing molecules 9 and 10 ( Figure 3) as ligands of G-quadruplex DNA [22]. The non-fluorinated ligand 9 had some conformational disorder because the pyrrolidine rings were able to interconvert between exo and endo puckers. In contrast, the pyrrolidine rings of fluorinated ligand 10 were more rigid, with the fluorine atoms preferring to occupy an axial position consistent with a favourable C-F … N + interaction (worth approximately 5.0 kcal/mol). This led to a number of changes to the DNA binding mode of 10, including a rotation of the entire pyrrolidine ring by 180° relative to that of 9, and several different H-bonding contacts with the DNA as a result.
In contrast with the strong charge-dipole effect evident in pyrrolidine 10 ( Figure 3), another more subtle interaction is observed in neutral fluorinated pyrrolidines. For example, Raines and co-workers found that (4R)-fluoroproline 12 adopts a Cγ-exo ring pucker (Figure 4), in contrast with natural proline 11 which has a more flexible pyrrolidine ring [23]. The increased rigidity of 12 was explained by a stabilising hyperconjugation phenomenon (Figure 4), in which an appropriatelyaligned σ CH orbital is able to donate electron density into the vacant σ* CF antibonding orbital. This stabilising interaction is only possible if the C-F and C-N bonds are aligned gauche to one another, and is analogous to the well-known fluorine gauche effect [24]. The importance of the rigid Cγ-exo ring pucker of 12 was demonstrated in spectacular fashion: Raines and co-workers showed that the thermal stability of collagen was increased when 12 was incorporated in place of collagen's naturally-present (4R)-hydroxyproline residues [25].
This hyperconjugation effect has also been exploited in the context of organocatalysis. Fluorination of proline itself, as well as related N-heterocycles, has been shown to increase enantioselectivity in certain organocatalytic processes [13]. For example, Alexakis and co-workers found that the non-fluorinated catalyst 13 (Scheme 2) catalysed an alkylation reaction (15→17) with only moderate enantioselectivity [26]. This was ascribed to the flexibility of the pyrrolidine moiety in the enamine intermediate 16. In contrast, the fluorinated catalyst 14 has a relatively strong (1.5 kcal/mol) preference for an endo pucker, stabilised by hyperconjugation, and this increased ridigity was credited with a dramatic improvement in the enantioselectivity.

Six-membered rings
The conformational analysis of six-membered rings is a cornerstone in physical chemistry. Substituted saturated six-membered compounds usually adopt a chair conformation with substituents preferring the equatorial positions. However, in 1993 Lankin and Snyder [27] observed that fluoropiperidine 18 preferentially adopted a conformation in which the fluorine substituent resides in the axial position ( Figure 5). This study was then extended to include piperidines 19 and 20, and in each case the axial conformers are preferred by a substantial 5.0 kcal/mol over the equatorial conformers (not shown) [28,29]. This pioneering work constituted the original discovery of the C-F … N + interaction which has already been discussed Scheme 2: Hyperconjugation rigidifies the ring pucker of a fluorinated organocatalyst 14, leading to higher enantioselectivity.
above in the context of azetidines and pyrrolidines. Interestingly, Lankin and Snyder were also able to rule out hydrogen bonding as the source of the axial preference, since the N,Ndimethyl analogue 20 exhibited a similar effect.

Seven-membered rings
Seven-membered rings exhibit much more complex conformational behaviour than six-membered rings. Hence, it is perhaps unsurprising that a twenty year gap separated the pioneering work of Lankin and Snyder ( Figure 5) from the first analysis of fluorinated seven-membered N-heterocycles. Liu and co-workers [30] have recently explored the conformational behaviour of the substituted azepanes 21-23 ( Figure 6), and observed that the rigidifying power of a fluorine substituent is strongly dependent on the other groups present. The non-fluorinated azepane 21 was found to exhibit extensive conformational disorder, and this was attributed to competing preferences for the OBn/N 3 substituents to adopt pseudoequatorial positions and for the azide group to align gauche to the ring nitrogen. The situation was not greatly changed upon introduction of a (6S)fluorine atom (compound 22): in this case, no single conformation of 22 was able to satisfy a C-F … N + gauche alignment as well as the two conformational preferences described for 21. In contrast however, introduction of a (6R)-fluorine atom (compound 23) greatly rigidified the ring system, to the extent that a single conformer of 23 dominated in solution. This work highlights the subtleties that can arise when fluorine atoms are incorporated into highly flexible molecules with preexisting substituents.

Eight-membered rings
The eight-membered ring is the largest stereoselectively fluorinated N-heterocycle that has been investigated to date [20]. O'Hagan and co-workers investigated the structure 24 (Figure 7), and calculated that the axial conformation of 24 should be strongly preferred over the equatorial conformation (9.2 kcal/mol) because of two stabilising C-F … N + interactions. An X-ray structure of 24 was also obtained (Figure 7), and it revealed a geometry consistent with the calculated minimumenergy structure, with no evidence of disorder.  So far in this review, we have primarily been considering fluorine as a replacement for hydrogen in N-heterocycles. However a new vista opens up if we consider fluorine as a replacement for the hydroxy group in bioactive molecules.

Fluorine can serve as a tool to probe the importance of hydrogen bonding
The replacement of a hydroxy group in a bioactive molecule with a fluorine atom can cause the loss of hydrogen bond donor ability, which may have profound effects on the ligand-receptor interaction. The study of fluorinated iminosugars serves as a good platform to discuss this issue.
Naturally occurring iminosugars, also referred as polyhydroxylated alkaloids or azasugars, are sugar mimics in which a nitrogen atom replaces the ring oxygen of the corresponding monosaccharide ( Figure 8) [31][32][33][34][35][36]. Iminosugars can competitively bind to glycosidase enzymes because of their structural resemblance to the terminal sugar moiety of natural substrates, or to the activated intermediate of hydrolysis (i.e. the oxocarbenium ion). As a consequence, iminosugars show great promise for the treatment of a variety of diseases including diabetes, viral infection, bacterial infection, and lysosomal storage disorders [37].
Fluorinated analogues of several of these privileged structures have been prepared, in order to probe the importance of hydrogen bonding in these systems [38][39][40][41][42][43]. For example, 1-deoxynojirimycin (28) is the C1-deoxy product of nojirimycin, the first iminosugar isolated from Nature. Iminosugar 28 is a potent inhibitor of yeast α-glycosidase (Figure 9), and the fluorinated analogues 32-34 suggest that the C2 and C4 hydroxy groups of 28 act as H-bond donors when binding to the enzyme, while the C6 hydroxy of 28 does not [44,45].
Miglitol (30, Figure 10) is an orally-available drug used for the treatment of type II diabetes. It was first marketed by Merck in 1996. The biological activity of the fluorinated analogues 35-37 ( Figure 10) suggest that the C6 hydroxy group of 30 acts as a hydrogen bond donor in its binding to yeast α-glycosidase, while the C2' and C2 hydroxy groups of 30 do not [46,47]. The fluorinated analogue 37 is particularly worthy of note, since this compound is five times more potent than the existing drug 30, and exhibits no toxicity in human cells.  However, a word of warning: in the fluorinated iminosugar examples discussed above (Figure 9 and Figure 10) the inhibition data must be interpreted with some caution, because another effect could be in operation. As well as changing the molecules' hydrogen bonding properties, fluorination can also affect the basicity of the amine group. This latter effect can be rationally exploited, for example to improve the bioavailability of a drug molecule; this concept is explored in the next section.

Fluorination alters the basicity of N-heterocycles
The 3-piperidinylindole derivative 38 (Table 1) binds to the human 5-HT 2A serotonin receptor, and was identified as a promising antipsychotic drug lead [48]. However, the bioavailability of 38 was poor, and this was attributed to the basicity of the secondary amine group which made the molecule positively charged at physiological pH and hence unable to traverse biological membranes. This problem was overcome by introducing a fluorine atom onto the piperidine ring (39): the basicity of the secondary amine was thereby reduced by nearly two orders of magnitude, and this led to a marked improvement in bioavailability. Incidentally, it is also worthy of note that the bioavailability (and 5-HT 2A binding affinity) could be further improved by the introduction of a second fluorine atom, this time onto the indole moiety (40); this further improvement in bioavailability was attributed to blockage of the metabolic degradation of 38 and 39 which commenced with hydroxylation of the indole moiety.
In the next example, we return to the world of iminosugars. Isofagomine (31, Figure 11) is an inhibitor of the β-glucosidase from sweet almond, and it is thought to exert its inhibitory activity by mimicking the oxocarbenium intermediate of glycoside cleavage [49]. Several analogues of 31 have been investigated (41-44, Figure 11) [50][51][52], and on first inspection it is difficult to rationalise the observed trends in biological activity. One possible explanation for the dramatically improved activity of e.g. 43 over 42 is to invoke the "polar hydrophobic" nature of the fluorine substituent [53,54]. But another important factor is the basicity of the amine group [55]. To best mimic the oxocarbenium ion, the iminosugars 31 and 41-44 ( Figure 11) must bear a positive charge, and since the pK aH values vary considerably amongst the different derivatives, it follows that each derivative has a different "optimal" pH for maximal inhibitory potency. This explains why the K i values in Figure 11 do not seem to follow a clear trend: the quoted K i values were all measured at the same pH, whereas it would be more revealing to consider the K i of each molecule at its "optimal" pH. This is  We have now seen that fluorination can affect N-heterocycles' stability, their conformational behaviour, their hydrogen bonding ability, and their basicity. It is hopefully clear to the reader that these effects have already led to several benefits in fields such as medicinal chemistry and organocatalysis. If these concepts are to be continued to be exploited in the future, then robust methods must be available for the synthesis of new fluorinated N-heterocycles. Hence, in the final section of this review we will examine some of the stereoselective synthetic methods that have been developed in recent years.
6. There are many ways to synthesise stereoselectively fluorinated N-heterocycles

Deoxyfluorination
Because of the ease of synthesis of enantiomerically pure alcohols, and the ever-increasing availability of deoxyfluorination reagents [10], the deoxyfluorination of N-protected alcohols is the most obvious strategy for synthesising fluorinated N-hetero-cycles (Scheme 3). Deoxyfluorination methods have already been extensively reviewed [8], and of course they are not limited in scope to N-heterocyclic systems, so we will not attempt to comprehensively cover this topic here. Instead, we will focus on two recent developments in deoxyfluorination methods that are particularly relevant to N-heterocyclic targets. Late stage deoxyfluorination is an attractive method for synthesising multifunctional fluorinated N-heterocycles, but mild and selective reagents are required if this is to be successfully achieved. One such reagent, PhenoFluor (45, Figure 12), was originally developed by Ritter and co-workers for the direct fluorination of phenols [56]. Recent work showed that 45 can also be used to effect late-stage fluorination of hydroxy groups within complex molecular architectures. For example, 45 can react selectively with primary and allylic alcohols in the presence of secondary and tertiary alcohols, and the reaction will also tolerate the presence of carbonyl groups [57]. Some N-heterocyclic targets that have been synthesised in one step using 45 as the deoxyfluorination reagent are highlighted in Figure 12.
A unique complication sometimes arises when deoxyfluorination is attempted in N-heterocyclic systems: side reactions can occur, bought about by neighbouring group participation (Scheme 4) [58]. Such processes can lead to rearrangement, and this outcome has been rationally exploited to synthesise fluorinated five- [59], six- [60] and seven-membered [61] N-heterocycles that may have been otherwise difficult to access (e.g. 48→49, Scheme 4). Alternatively, neighbouring group participation sometimes results in an unexpected pattern of substitu- tion with retention (e.g. 50→51, Scheme 4); in the latter example, note that the ring nitrogen does not directly engage in the anchimeric process [62].

The fluorinated building block approach
An alternative to the strategy of deoxyfluorination (section 6.1) is to synthesise fluorinated N-heterocycles starting from fluorine-containing organic building blocks. Such an approach benefits from the wide variety, and frequently the low cost, of today's commercially available organofluorine molecules [63,64].
For example, the fluorinated aziridines 2 and 3 presented earlier ( Figure 1) were synthesised through a building block approach. De Kimpe and co-workers [15,16] developed a strategy to synthesise such targets via cyclization of β-fluoro-β-chloroamines (54, Scheme 5), which in turn are derived from the readilyavailable fluoroacetate derivatives 52.
Percy and co-workers' synthesis of a difluorinated analogue of calystegine B (63, Scheme 6) is a more elaborate example of the strategy of using a readily available fluorinated starting material for the synthesis of a complex target [65]. Percy's approach commenced with protected trifluoroethanol 55 (Scheme 6), and the multistep route to 63 featured a [2,3]-Wittig rearrangement, a diastereoselective epoxidation, and a microwave assisted transannular epoxide opening reaction. It is also noteworthy that the starting material 55 contains an extraneous fluorine atom which is deleted during the synthetic sequence; this approach takes advantage of the often low cost and ready availability of perfluorinated building blocks. It should be noted, however, that access to enantiopure targets is not straightforward via the building block approach. Such targets may be better obtained through diastereoselective or enantioselective fluorination methods, and examples of these types of approaches are examined in the following sections.

Diastereoselective fluorocyclisation
The use of fluorocyclisation processes for the production of heterocycles and carbocycles has attracted considerable attention in recent years. Such processes have the advantage of forming multiple bonds in one pot [66]. Electrophilic fluorocyclisation involving the intrinsic nucleophilicity of nitrogen can be a powerful tool to synthesise stereoselectively fluorinated N-heterocycles. This concept was exemplified by Shibata . This synthesis was remarkable for its rapid generation of molecular complexity, which is a defining feature of the fluorocyclisation approach. Even more spectacular was the extension of this methodology to create analogues of the natural product gypsetin [69,70] (68) via a double fluorocyclisation sequence (Scheme 7). The one drawback of this approach was its disappointing lack of diastereoselectivity, which presumably arose because nonselective fluoroquaternisation of the indole moiety preceded the cyclisation event.

Enantioselective fluorocyclisation
The lack of diastereoselectivity seen in Scheme 7 is attributable to the fluorination event preceding the cyclisation event, and this is a significant issue which inhibits the further development of diastereoselective processes. However, this issue does not preclude the development of enantioselective variants, provided the initial fluorination event can be controlled [11]. Gouverneur and co-workers recently reported the first enantioselective electrophilic fluorocyclisation (Scheme 8) [71]. Their substrates (e.g. 69) were indole derivatives bearing a pendant nitrogen nucleophile, and the source of chirality was a substoichiometric quantity of the cinchona alkaloid derivative (DHQ) 2 PHAL (70). This method was shown to work very well with several different pendant nucleophiles, but the N-acetamido nucleophile was found to be optimal, giving the corresponding product 71 in an impressive 92% ee. Elucidating the mechanism of chiral induction in this type of process is not straightforward, but preliminary experiments showed that associative complexation between the substrate 69 and the alkaloid catalyst 70 may account for the observed enantioselectivity. from N-(tert-butylsulfinyl)imine 75 through a nucleophilic addition/radical cyclisation sequence. The selectivity during the radical cyclisation (77→78) can be explained by the Beckwith-Houk transition-state model [74,75]. The 3,3-difluoropyrrolidine moiety (e.g. 78) is found in a variety of enzyme inhibitors such as thrombin inhibitors and cathepsin inhibitors, and so this synthetic methodology (Scheme 10) is likely to have valuable future applications in medicinal chemistry.

Chemoenzymatic synthesis
Enzyme catalysis was presented earlier (Scheme 1) as a strategy for synthesising fluorinated β-lactams (4) [18]. At that time, we were interested in the effect that the fluorine substituents had on the reactivity of the β-lactam derivatives. However this work now merits further attention, because it also illustrates a strategy for achieving stereoselectivity in C-F bond formation. The racemic β-lactam 4b was synthesised as a single diastereoisomer from the Schiff base 79 (Scheme 11), by a Reformatsky addition followed by spontaneous cyclisation; removal of the amine protecting group under oxidative conditions then furnished rac-4b, the substrate for enzymatic resolution. Using an immobilized lipase enzyme as the catalyst (and under slightly different conditions from those described in Scheme 1), Scheme 10: Synthesis of 3,3-difluoropyrrolidine 78 via a radical cyclisation.
one enantiomer of the racemic β-lactam 4b was completely transformed into the ester 5b, while the other enantiomer of β-lactam 4b remained intact. The net result was that a fluorinated stereocentre was rapidly constructed, with defined absolute configuration, within a nitrogen heterocycle.

Conclusion
When the concept of selective fluorination is applied in the context of N-heterocycles, the resulting molecules have a variety of unique properties. In this brief review, we have examined some of these features, including the effects on stability, conformation, hydrogen bonding ability and basicity, and we have also surveyed some of the synthetic methods that are currently available for the production of such molecules. Throughout, we have seen that the unique properties of stereoselectively fluorinated N-heterocycles have led to a variety of valuable applications of these molecules, particularly in the field of medicinal chemistry.
What does the future hold? It is interesting to note that the molecules described in this review all comprise ring sizes of three to eight atoms; in contrast, macrocyclic structures have been little explored to date. It will be fascinating to learn whether similar effects operate in much larger ring sizes, for example in fluorinated analogues of cyclic peptides [76][77][78]. More generally however, it seems safe to predict that the unique properties of stereoselectively fluorinated N-heterocycles will ensure that their importance and utility continue to grow in the future.


text:
1-Ethyl-2-phenyl-3-[2-(trimethylsilyl)ethynyl]-1H-indole

The title compound, C21H23NSi, was synthesized by Sonogashira-type reaction of 1-ethyl-3-iodo-2-phenyl-1H-indole with trimethylsilylacetylene. The indole ring system is nearly planar [maximum atomic deviation = 0.0244 (15) Å] and is oriented at a dihedral angle of 51.48 (4)° with respect to the phenyl ring. The supramolecular aggregation is completed by weak C—H⋯π interactions of the methylene and phenyl groups with the benzene and pyrrole rings of the indole ring system. The methyl groups of the trimethylsilyl unit are equally disordered over two sets of sites.

1-Ethyl-2-phenyl-3-[2-(trimethylsilyl)ethynyl]-1H-indole Iaroslav Baglai, Valérie Maraval, Carine Duhayon and Remi Chauvin Comment
Indole is one of the three heterocycles occurring in the 20 standard natural amino-acids (in (L)-trhyptophane). Its unique bicyclic aromatic structure makes it a key rigid structural basis of one of the largest classes of alkaloids, comprising more than 4000 natural products (Huang et al., 2004;Seferoğlu et al., 2007a,b;Ruiz et al., 2012). The synthesis and functionalization of indoles have been the object of research for over one and a half century (Shiri, 2012). Few examples of Sonogashira-type reactions on the indole core have been described (Hussain et al., 2011;Prateeptongkum et al., 2010).
In the molecule (Fig. 1), the indole ring is almost planar [maximum deviation for ring atoms = 0.0244 (15) Å], and the dihedral angle between the pyrrole and benzene ring of the indole system is equal to 1.83 (5)°. Methyl groups of the trimethylsilyl moieties are disordered into two positions with the corresponding occupancy of 0.5 for each part. The whole molecule is almost planar excluding methyl groups of trimethylsilyl and ethyl moieties as well as the benzene ring [maximum deviation for atoms = 0.0539 (16) Å]. The interplanar angle between indole and phenyl planes is equal to 51.48 (4)°. Inspite of the three aromatic rings of the molecule, the packing shown in Fig. 2 does not reveal any particular intermolecular π-stacking or columnar arrangement but weak C-H···π interactions (Table 1).

1-ethyl-3-iodo-2-phenyl-1H-indole (3).
To a solution of 1-ethyl-2-phenyl-1H-indole 2 (0.5 g, 2.26 mmol) in CHCl 3 (30 ml) at 0 °C was added N-iodosuccinimide (0.535 g, 2.37 mmol) as a small portions over 5 min. The mixture was then stirred at the same temperature for 3 h. After evaporating the solvent, the residue was extracted in dichloromethane and washed with H 2 O. The organic layer was separated and dried over MgSO 4 . The solvent was removed under reduced pressure, and the product was purified by silica gel chromatography using a mixture of acetone and pentane (2:98) as an eluent (yield 92%, 0.72 g).
R f (99/5 = C 5 H 12 /Ether) = 0.47. The title compound was dissolved in Et 2 O and CH 2 Cl 2 (1:1 mixture). After slow evaporation over two days, crystals of 1a suitable for X-ray diffraction analysis were obtained.

Refinement
The H atoms were located in a difference Fourier map and refined with riding constraints. Methyl groups are disordered over two positions in site occupancy ratio of 0.5:0.5.   Packing diagram of the title compound, viewed along the c axis.


EMPLOYING CONFORMATIONAL ANALYSIS IN THE MOLECULAR MODELING OF AGROCHEMICALS : INSIGHTS ON QSAR PARAMETERS OF 2 , 4-D

A common practice to compute ligand conformations of compounds with various degrees of freedom to be used in molecular modeling (QSAR and docking studies) is to perform a conformational distribution based on repeated random sampling, such as Monte-Carlo methods. Further calculations are often required. This short review describes some methods used for conformational analysis and the implications of using selected conformations in QSAR. A case study is developed for 2,4-dichlorophenoxyacetic acid (2,4-D), a widely used herbicide which binds to TIR1 ubiquitin ligase enzyme. The use of such an approach and semi-empirical calculations did not achieve all possible minima for 2,4-D. In addition, the conformations and respective energies obtained by the semi-empirical AM1 method do not match the calculated trends obtained by a high level DFT method. Similar findings were obtained for the carboxylate anion, which is the bioactive form. Finally, the crystal bioactive structure of 2,4-D was not found as a minimum when using Monte-Carlo/AM1 and is similarly populated with another conformer in implicit water solution according to optimization at the B3LYP/aug-cc-pVDZ level. Therefore, quantitative structure-activity relationship (QSAR) methods based on three dimensional chemical structures are not fundamental to provide predictive models for 2,4-D congeners as TIR1 ubiquitin ligase ligands, since they do not necessarily reflect the bioactive conformation of this molecule. This probably extends to other systems.

INTRODUCTION
The stereochemical control of molecules governs many micro and macroscopic properties of substances, like the biological activity of drug like compounds. Molecules with one or more rotational degrees of freedom (rotatable bonds) exhibit conformational isomerism, where the stable conformers correspond to energy minima and, therefore, to the most probable structures to be found in materials, pharmaceuticals and agrochemicals (FREITAS; . The three dimensional shape of molecules is ruled by intramolecular interactions, but it is also influenced by the medium (solvent, for instance). Certainly, hydrogen bonding is the main interaction stabilizing conformations, but others also take place in many cases, such as electrostatic effects and hyperconjugation. For example, the double helix in DNA is due to intermolecular hydrogen bonding between nucleotides of different strands (VOET et al., 2000), while the anomeric effect (a widely known effect operating in carbohydrate derivatives) (EDWARD, 1955) has been claimed to be originated either from electrostatic effects (the dipolar repulsion in the  anomer is supposed to be stronger than in the  anomer) (MO, 2011) or hyperconjugation (electron donation from the endocyclic oxygen lone pair toward the vicinal axial antibonding orbital) (KIRBY, 1983).
Because conformation plays a key role in biological activity, since most of action mechanisms are based on the correct fit between the active site of an enzyme and the substrate, many quantitative structure-activity relationship (QSAR) methods have been developed to account for the three dimensional structure of drug like molecules. In QSAR, descriptors representing molecular structures are usually calculated to be correlated with their biological properties; thus, a correlation model is built and the biological property of new molecules can be estimated using the calculated molecular descriptors in the equation obtained. Consequently, there is no need to synthesize and biologically test a new compound to obtain its biological property, since it can be predicted using the QSAR model. However, three dimensional QSAR methods and higher order analogs are usually difficult to be handled, especially because of the conformational screening and three dimensional alignment rules required by these methods to obtain molecular descriptors. In addition, there is not any evidence that a given conformation instead of another can affect the quality of QSAR models, despite the profound effects of conformation on the biological activity. Thus, simpler methods, like those based on connectivity indices and bidimensional shape, are supposed to perform appropriately, at least in many practical cases (BROWN; MARTIN, 1997;ESTRADA et al., 2001).
The most widely known three dimensional method is based on comparative molecular field analysis (CoMFA) (CRAMER III et al., 1988), in which aligned molecular structures of a data set are put inside a virtual grid cell, where probes reflecting steric, electrostatic and other effects pass through the grid intersections to generate thousands of descriptors. A similar approach including other indices, like hydrogen bond, was further developed and named Comparative Molecular Similarity Indices Analysis (CoMSIA) (KLEBE et al., 1994), but no significant improvement in comparison to CoMFA was achieved. In order to account for new dimensions, namely conformational and alignment freedom, receptor insight and solvation effects, 4D (HOPFINGER et al., 1997), 5D (VEDANI;DOBLER, 2002) and 6D QSAR (VEDANI et al., 2005) methods have been developed. In all cases, the three dimensional structure of a possible ligand is required and, consequently, several methods to generate and optimize molecular structures should be used to give rise an input geometry for the QSAR calculations. Thus, an overview about some classes of theoretical models used to obtain molecular conformations are briefly described as follows.

COMPUTATIONAL MOLECULAR MODELS
The conformational isomerism of organic compounds can be determined by means of experimental techniques, with emphasis on spectroscopic methods. For example, the conformational preferences of a given compound can be estimated by the relative band intensities of conformers possessing a functional group using infrared spectroscopy. Alternatively, since coupling constants in nuclear magnetic resonance is often an average value among conformers, their populations can be obtained if the corresponding intrinsic coupling constant is determined through low temperature experiments or using rigid derivatives. However, because computational estimation of geometries and energies has been progressively improved in the recent years (both in terms of computing time and accuracy), molecular modeling has replaced or at least corroborated experimental approaches in many research examples. The use of a variety of computational models to represent molecular properties is especially dependent upon the system size and/or type, the number of molecules to be analyzed and the availability of computational resource. The computational models are briefly described as follows.
The simplest computational model is based on molecular mechanics, which is quite used in the modeling of large molecular systems. Molecular mechanics (MM) methods use principles from classical physics to predict the structures and properties of molecules, while the different MM methods differ from one another by their force fields (FORESMAN;FRISCH, 1996). A force field has a set of equations defining how the potential energy of a molecule varies with the locations of its component atoms. The potential energy is dependent on the parameters included in the routine program, which are values used in the equations to relate atomic characteristics to energy Ciênc. agrotec., Lavras, v. 37, n. 6, p. 485-494, nov./dez., 2013 components, and structural data such as bond lengths and angles. Since QSAR models are usually built using large data sets, MM methods are frequently applied to obtain the molecular structures used in 3D QSAR. An optimal geometry is probably not generated using this approach, because its theoretical background is often not sufficiently rigorous to provide very accurate results, which is better achieved using electronic structure methods.
Electronic structure methods include semi-empirical and ab initio methods, whose similarity lies on trying to solve the Schrödinger equation (H  = E  ). On the other hand, the main difference between these methods is that semi-empirical methods use parameters derived from experimental data to simplify the computation (different semi-empirical methods are largely characterized by their differing parameter sets), while ab initio method do not use any parameter derived from experiment, but they are based solely on the laws of quantum mechanics (FORESMAN;FRISCH, 1996). Another class of electronic structure methods are the density functional methods (DFT), which are similar to ab initio methods in many ways, but they present some advantages, such as the inclusion of effects of electron correlation (the fact that electrons in a molecular system react to one another´s motion and attempt to keep out of one another´s way). Since the electron correlation approximation in DFT is quite simpler than in Hartree-Fock calculations, DFT methods are computationally less expensive and, therefore, have been preferred in many studies. For years, the Becke´s DFT method using the electron correlation approach of Lee, Yang and Parr (B3LYP) (BECKE, 1988;LEE et al., 1988) has been widely used in many studies about molecular structure. More recently, DFT analyses have been improved by including dispersion effects, becoming the so called DFT-D methods as accurate as the high level method CCSD(T) in some cases, even for highly dispersive systems (JOSA et al., 2013).
Thus, geometries and energies obtained by different methods not always converge to similar results; important discrepancies are observed between MM and electronic structure methods, but also between semiempirical and ab initio or DFT methods . Considering that chemical structures used in 3D QSAR derived from a simple optimization step can be obtained either by MM or electronic structure methods, the chosen molecular framework used in the alignment step may not be only different from the bioactive structure, but the order of stabilities in the energy minima will probably vary with the theoretical level. Consequently, the choice for a given conformer to be used in 3D QSAR from a simple optimization step is arbitrary, making traditional QSAR feasible to be revisited. In fact, the identification of bioactive conformations is an important part of understanding the relationship between the structure and the biological activity of a molecule. Consequently, derived experimental data on the bioactive conformation should serve as verification wherever possible ( CUNHA et al., 2005). It should be kept in mind that the MD resulting structures can also be applied to construct the conformational ensemble profile (CEP) of each ligand into the active site, thus overcoming the scenario of a poor molecular conformational sampling (CUNHA et al., 2012;SODERO et al., 2012). Furthermore, crystallographic data of the enzyme-inhibitor complex can generate useful information about the enzyme-inhibitor interaction, as well as on the bioactive conformation of ligand in order to validate the chemical structures in 3D QSAR (CUNHA et al., 2013).

QSAR METHODS
The main advantage of 3D QSAR methods over traditional QSAR, according to their developers, is that chemical information can be obtained from the surfaces of molecular force fields, which indicate the groups leading to increase or decrease in biological activity. Since such surfaces depend on the ligand conformation, which is dependent on the theoretical level of calculation (which can lead or not the structure to the bioactive-like conformation), the value of 3D QSAR methods is not superior to those of traditional QSAR approaches. The systematic in QSAR was first introduced in the sixties by Hansch and Fujita (HANSCH;FUJITA, 1964), in which hydrophobicity was found to correlate with the biological activity of various compounds against mosquito larvae, bacteria, houseflies, rodents, guinea pigs and mice. Consequently, the biological activity could be indirectly determined by calculating logP (the logarithm of the octanol-water partition coefficient) and other constants. These parameters are independent of the three dimensional structure of a molecule.
Many molecular descriptors were subsequently investigated to provide a better correlation with bioactivity. For example, physicochemical descriptors representing topological and connectivity indices, constitutional parameters, charge, walk and path counts, etc. are widely used to create QSAR models (TODESCHINI; CONSONNI, 2000). Images representing bidimensional molecular structures have also been used to generate descriptors to use in QSAR; descriptors in the so called MIA-QSAR (Multivariate Image Analysis applied to QSAR) (FREITAS et al., 2005) are pixels. Because pixels can be managed numerically as binaries, the white color corresponds to 765, while black pixels are zero, according to the RGB system of colors. In MIA-QSAR, images correspond to chemical structures drawn with the aid of a software like ChemDraw and CheSketch. The structural changes or changes in the substituent positions in a congeneric series of compounds correspond to alterations in the pixel coordinates; these alterations explain the variance in the y block, which is the dependent variable block (e.g. biological activity). More recently, the MIA-QSAR method (a genuine 2D approach) was augmented by introducing new dimensions to better encode chemical information: different atomic radii and colors were found to represent size (like in steric effects) and the diverse periodic properties of elements (e.g. different colors encode the different atomic electronegativities) (NUNES; FREITAS, 2013).
In the next section, a case study is developed to show the dependence of structural conformation with various theoretical levels for geometry optimization, as well as the implications of this in QSAR.

CASE STUDY: 2,4-DICHLOROPHENOXYACETIC ACID (2,4-D)
2,4-Dichlorophenoxyacetic acid (2,4-D) is a synthetic auxin (a class of plant hormones) widely used in the control of broadleaf weeds. 2,4-D complexes with TIR1 ubiquitin ligase enzyme, then controlling plant growth and development (TAN et al., 2007). A variety of conformers can take place upon rotation of the four torsional angles of 2,4-D, but one corresponds to its crystal, bioactive conformation ( Figure 1).
Conformational search consists of finding lowenergy conformers from a pool of possibilities. A Monte-Carlo search follows a path that biases in favor of lowenergy conformers (but that does not completely exclude high-energy conformers). While there is no guarantee that the lowest-energy conformer (the global minimum) will actually be located, it can be shown that the set of con former s exam ined a pproaches a Boltzm ann distribution. Semi-empirical calculations are often coupled to Monte-Carlo to search for conformations of possible ligands, due to the lower computational cost compared to ab initio or DFT methods. This practice is particularly common to generate structures to be used in three dimensional (3D) QSAR, such as molecular field fit techniques (CRAMER III et al., 1988;KLEBE et al., 1994) and in docking studies.
Certainly, conformation plays a key role in describing the biological activity of a given molecule. However, QSAR models based on 3D descriptors are not fundamental to produce highly predictive models, since the most stable conformation of a given ligand may not correspond to the best choice, which is the bioactive conformation. Such QSAR methods are very informative, but one and two dimensional descriptors are supposed to be capable of estimating biological properties similarly well (TIAN et al., 2007;FREITAS et al., 2005, TODESCHINI;CONSONNI, 2000). In order to show the dependence of a conformational search method and the level of theory with conformer energies and structures, the conformations of 2,4-D (both undissociated and dissociated forms) were analyzed using semi-empirical and DFT methods and then compared with the crystal structure of this herbicide (in the carboxylate form).
Monte-Carlo search tries to find energy minima by random sampling; thus, the whole set of conformers is not necessarily found. Such a search was performed using the semi-empirical Austin Model 1 (AM1) method and 12 conformers were found for the undissociated 2,4-D, but two of them are equivalent (9 and 10, figure 2), resulting in 11 different conformers. Conformer 1 was calculated to be the most stable form in the gas phase at the AM1 level, while optimization of the conformers obtained by AM1 gave a different conformational profile when using density functional theory (B3LYP/aug-cc-pVDZ). In the gas phase, conformer 2 was found as the global minimum and ca. 1 kcal mol -1 more stable than 1 according to DFT; this preference remains in water solution (to approach a physiological medium), despite the changes in relative energies of the other conformers (Table 1). Moreover, conformers 6, 7, 9 and 11 found by AM1 were not located as energy minima at the B3LYP/aug-cc-pVDZ level, which converted those conformers to 6', 7', 9' and 11' (2 and 11', and 9' and 12 are structurally similar to each other; thus, 10 different conformers are shown in figure 3).
Figure 2 -Conformations obtained by the Monte-Carlo search at the AM1 level (relative energies are given in kcal mol -1 outside parenthesis). Conformers 9 and 10 are equivalent. Conformers 6, 7, 9 and 11 converge to different structures when optimized at the B3LYP/aug-cc-pVDZ level.
According to the stability profiles given in table 1 for the DFT results, 2 is the main conformer because of an intramolecular hydrogen bond O--H ... O; this is confirmed by the energy of stabilization of the corresponding hyperconjugative interaction n O   * OH of 4 kcal mol -1 , obtained by natural bond orbital (NBO) analysis. Not in a biological medium, the conformational equilibrium of undissociated 2,4-D is dictated by a balance of hyperconjugation and Lewis-type interactions. The full energy of a molecule can be decomposed into hyperconjugative (attractive) and Lewis-type (repulsive) interactions; NBO analysis provides individual interactions and the total energy of hyperconjugation by deleting twoelectron/two-orbital interactions and computing the resulting energy of the system. Therefore, given the relative energy of the conformers, the Lewis-type contribution can also be obtained. Importantly, those conformers with the C-O-C portion coplanar to the aromatic ring are capable of exhibiting strong n O   * C1C2 interactions, owing to the resonance of the lone pairs of the ether oxygen; for example, this interaction is ca. 28 kcal mol -1 stabilizing for conformer 2 and is similarly observed Figure 3 -Conformations optimized at the B3LYP/aug-cc-pVDZ level from that obtained by the semi-empirical AM1 calculations (relative energies are given in kcal mol -1 outside parenthesis). Conformers 2 and 11', as well as 9' and 12, are equivalent. Ciênc. agrotec., Lavras, v. 37, n. 6, p. 485-494, nov./dez., 2013 for conformers 4, 8, 10 and the crystal structure. On the other hand, such geometry imposes syn coplanarity to the fragment C-O-C=C and steric repulsion then takes place. Table 2 gives the contribution of hyperconjugative and Lewis-type interactions for the conformational stabilities of undissociated 2,4-D, both in the gas phase and implicit water. Table 1 -Relative energies (in kcal mol-1) in conformers of 2,4-dichlorophenoxyacetic acid (2,4-D). a a Equal letters (a, b and c) correspond to structures that converged to equivalent conformers. d Conformation obtained by AM1 which is different from that obtained by DFT The bioactive conformation of 2,4-D is in the carboxylate form, due to the lack of a proton when bound to the active site of TIR1 ubiquitin ligase enzyme (PDB code: 2P1N) (Figure 4). In order to compare the calculated conformations of 2,4-D with the bioactive crystal structure, a Monte Carlo conformational search at the semi-empirical AM1 level was performed for the dissociated 2,4-D, and two conformers with an energy difference of 3.6 kcal mol -1 were found ( Figure 5). These two conformers do not correspond to the bioactive-like geometry, while further optimization of the AM1 structures at the B3LYP/aug-cc-pVDZ level converged to other two structures ( Figure 5), with the most stable one (by 6.2 kcal mol -1 ) being very similar to the experimental, crystal 2-4-D. However, in water solution (using implicit solvent with the PCM model), the relative energy of the least stable conformer decreased to only 0.3 kcal mol -1 , indicating that it should be significantly populated in aqueous solution (mimicking a real situation). Thus, it would not be improbable to select the second stable conformer in a QSAR modeling, but it does n ot corr espon d to th e a ct ua l bioa ct ive conformation, which is induced by the enzyme active site, because of intermolecular hydrogen bond between the carboxylate oxygens and hydrogens in Arg403 and Ser438 amino acid residues. found by a density functional method. Some minima found by AM1 are not equivalent to that obtained by DFT. An additional conformational screening using molecular mechanics encountered 25 structures and the most stable one corresponds to the global minimum found by DFT; however, many structures correspond to saddle points. The crystal structure of the 2,4-D ligand (carboxylate) is not coincident with any conformation found by Monte-Carlo search and is not even an energy minimum at the AM1 level. At the B3LYP/aug-cc-pVDZ level, the global minimum is similar to the experimental crystal structure in the gas phase, but the other conformer must be similarly populated in water solution.
In mol ecul ar model in g, especi a ll y 3D QSAR, conformations (poses) that are obtained by simple optimization steps can not necessarily reflect the bioactive conformation; even though, the use of such three dimensional structures provide significant correlation with biological properties, at least in many practical cases. Obviously, molecular conformation plays a key role in describing biological properties of molecules, but the results of this work suggest that, in QSAR studies, satisfactory correlations can be obtained in dependen tl y of t he con form at ion used. Th is corroborates the fact that QSAR methods based on bidimensional representation of molecules, such as MIA-QSAR (FREITAS et al., 2005), are so predictive as the n-dimensional (n  3) techniques, while 3D methodologies are usually time and cost demanding, due to the necessity of conformational screening and 3D alignment of ligands.

COMPUTATIONAL METHODS
A conformer distribution was carried out for 2,4-D using the Monte-Carlo method at the semi-empirical AM1 level (DEWAR et al., 1985), available in the Spartan program (2000). Each minimum found was subsequently optimized at the B3LYP/aug-cc-pVDZ level (BECKE, 1988;LEE et al., 1988;KENDALL et al., 1992) of theory, both in the gas phase and water (using the polarizable continuum model, PCM, and using a cavity built up using the UFFradii with spheres around each solute atom) (TOMASI, et al., 2005). Frequency calculations were performed to guarantee that no imaginary frequencies were present. Natural bond orbital (NBO) calculations (GLENDENING et al., 2001) were performed at the same level of theory, also using the Gaussian09 program (FRISCH et al., 2009). The crystal structure of 2,4-D was obtained from the Protein Data Bank (PDB code: 2P1N) and submitted to optimization and NBO calculations at the B3LYP/aug-cc-pVDZ level.

CONCLUSIONS
Bioactivity of organic molecules is conformationdependent. However, theoretical conformational analysis is dependent on the method and level of theory. Intramolecular effects can drive the conformer stability and, therefore, the mode of interaction with an enzyme. On the other hand, the biological environment can also induce conformational changes, such as demonstrated in this work for 2,4-D. 3D QSAR considers the spatial structure of a molecule to generate variables to be used in the modeling, such as steric and electrostatic descriptors. Since the conformer chosen in 3D QSAR may not correspond to the effective, bioactive structure, the good correlation obtained by some 3D QSAR methods in som e cases could pr obabl y be not due to the conformation used; 2D shape, connectivity indices and fragment/atomic properties are some parameters supposed to be more descriptive, at least for the example of this work.


Acoustical studies on binary liquid mixtures of some 1,3,4-oxadiazole derivatives with acetone at 303.15 K

The wide spread use of 1,3,4-oxadiazoles as a scaffold in medicinal chemistry establishes this moiety as an important bioactive class of heterocycles. In the present study ultrasonic velocity (u), density (ρ) and viscosity (η) have been measured at frequency 2 MHz in the binary mixtures of 1,3,4-oxadiazole derivatives in acetone at 303.15 K using ultrasonic interferometer technique. The measured value of ultrasonic velocity, density and viscosity have been used to estimate the acoustical parameters namely adiabatic compressibility (β ad ), relaxation time (τ), acoustic impedance (Z i ), free length (L f ), free volume (V f ) and internal pressure (π i ), with a view to investigate the nature and strength of molecular interactions. The obtained result support the occurrence of molecular association through hydrogen bonding in the binary liquid mixtures.

INTRODUCTION
Ultrasonic offer the most exciting and fascinating field of scientific research among the researcher. since the ultrasonic and other related thermo acoustic parameters provide useful information regarding the structure of molecules, molecular order, molecular packing, inter and intra -molecular interaction 1-2 etc. Ultrasonic study of liquid and liquid mixture has gained much importance during the last two decades in assessing the nature of molecular interaction and investigating the physiochemical behavior of system [3][4] .
Molecular interaction in liquid mixtures has been extensively studied using ultrasonic technique by many workers 5 , because mixed solvents find practical applications in many chemical, biological and industrial processes. 1,3,4-oxadiazole derivatives are heterocyclic compounds containing one oxygen and two nitrogen atoms in a five-membered ring. 1,3,4oxadiazole derivatives have played a major role in the pharmaceutical chemistry. Therefore the applications of these compounds attract us to study their behaviour in acetone.
We report here the results of study on binary liquid mixture of 1,3,4-oxadiazole derivatives over the entire range of composition at T = 303.15 K. By using the experimentally measured values of ultrasonic velocity (u), density (r) and viscosity (η), various acoustical parameters like excess adiabatic compressibility, excess intermolecular free length, excess acoustic impedance, and excess ultrasonic velocity have been calculated the mixture. The calculated deviations and excess functions have been explained on the basis of the intermolecular interactions present in these mixtures. The binary liquid systems studied here is

EXPERIMENTAL SECTION
The Compounds were recrystalized before use. The solvent used in the present work of AR grade and were purified according to the standard procedure described in the literature 6 . Solutions of different molarity were prepared for each binary system. The ultrasonic velocity in the mixture was measured using a variable path fixed frequency ultrasonic interferometer working at 2 MHz frequency (Mittal enterprises, New Delhi). The accuracy of sound velocity was ±0.1 ms -1 .
The density was determined at the experimental temperature using 10 ml capacity specific gravity bottle immersed in a thermostatic bath (accuracy +0.01 °C). The volume of the bottle at the experimental temperatures, viz. 303.15 K was ascertained using doubly distilled water. The densities of water at these temperatures were obtained from literature. The viscosity of pure liquids and liquid mixtures at 303.15 K were determined using an Ostwald viscometer [30][31][32][33][34][35] .

THEORY & CALCULATIONS
The ultrasonic velocity, density and viscosity data described many parameters for understanding solvent interaction and structural effect. Various acoustical parameters like adiabatic compressibility (β ad ), intermolecular free length (L f ), specific acoustical impendence (Z i ), Rao's molar sound function (R), Vander Waals constant (b), etc., have been computed using the following standard equations 7-19 .  Tables 1 to 3 shows the measured ultrasonic velocity, density and related acoustical parameters of 1,3,4-oxadiazole derivatives with acetone at 303 K. These measured acoustical parameters of 1,3,4-oxadiazole derivatives with acetone at 303 K were also shown graphically in Figure 1 to 9. It is observed that ultrasonic velocity, density, viscosity and acoustic impedance shows nonlinear increasing variation with increase in molar concentration. This indicates the complex formation and intermolecular weak association may be due to hydrogen bond formation 20 . This behaviour is the result of structural changes occurring in the mixture.

RESULTS AND DISCUSSION
Adiabatic compressibility (β ad ) shows an inverse behaviour compared to the ultrasonic velocity. Adiabatic compressibility nonlinearly decreases with increase in concentration of 1,3,4-oxadiazole derivatives (shown in Table 2). The decrease in compressibility implies that there is an enhanced molecular association in the system with increase in solute concentration [21][22] .
The opposite trend of ultrasonic velocity and adiabatic compressibility indicate association among interacting 1,3,4-oxadiazole derivatives and acetone molecules. In the present systems, free length varies nonlinearly with increase in molar concentration suggest the significant interaction between solute and solvent due to which structural arrangement is also affected 23 .
Relaxation time decreases with increase in concentration. The relaxation time which is order of 10 -12 sec is due to structural relaxation 24-25 process in such a case it is suggested that molecules get rearranged due to co-operative process [26][27] .  The internal pressure increases & free volume decreases with increases in molar concentration indicate the association through hydrogen bonding. It shows the increasing magnitude of interaction between the 1,3,4-oxadiazole derivatives and acetone. The decrease in free volume shows that the strength of interaction decreases gradually with the increase in solute concentration. It represents that there is weak interaction 28 between the solute and solvent molecules.

International Letters of Chemistry, Physics and Astronomy Vol. 24
The interactions occurring in different solutions can also be confirmed by the solvation number (Sn), which is measure of structure forming or structure breaking tendency of solute in a solution. Fig. 8 shows the variation of solvation number (Sn) with molefractions for all the compounds in acetone. Sn values are found decrease with concentration of solute in acetone (shown in Table 3). Sn clearly represents the solvation of solute in solvent causes interactions giving rise to increase the solubility of solute 29 .
This weak association is further confirmed by low relative association value (RA), this is almost same for all compounds ( Table 3). The decrease of relative association with concentration substantiate that the solvent structure breaks up.