For a linear triatomic molecule such as CO 2, there are only 5 degrees of freedom (3 translations and 2 rotations), assuming vibrational modes are not excited. However, as mass increases and the frequency of vibrational modes decreases, vibrational degrees of freedom start to enter into the equation at far lower temperatures than is typically the case for diatomic molecules. For example, it requires a far larger temperature to excite the single vibrational mode for H 2, for which one quantum of vibration is a fairly large amount of energy, than for the bending or stretching vibrations of CO 2.