The trans-pyramidalization distortion is taken as an example. The frontier molecular orbitals of the undistorted alkene possessing D 2h symmetry have symmetries a g (HOMO-1), b 2u (HOMO), b 1g (LUMO), and b 3u (LUMO+1). The symmetry of the trans-pyramidalization vibration is b 1g. A triple product of ground state, vibrational mode, and excited state that can be taken is b 2u (HOMO) x b 1g (trans-pyramidalizing vibrational mode) x b 3u (LUMO+1) = a 1g. Since a 1g is the totally symmetric representation, the b 2u and b 3u molecular orbitals participate in an allowed interaction through the trans-pyramidalizing vibrational mode. The molecule will distort in a trans-pyramidal fashion (into C 2h symmetry) in order to enable this interaction, which produces a more stabilized HOMO and more destabilized LUMO.