Low inversion energy barrier of cytisine NH group—an explanation for the FT-IR bands splitting

The DFT calculations using B3PW91 hybrid functional and 6-311++G** basis set were performed for the two most stable isomers of cytisine (1a and 1b). The conformation 1a differs from 1b by the orientation (equatorial and axial, respectively) of the hydrogen atom at the piperidine nitrogen atom. In terms of the Gibbs free energy, the conformation 1a is less stable than 1b by 0.25 kcal mol−1, only. The isomers are separated by 3.6 kcal mol−1—an activation energy barrier, which is higher in energy than the vibrational modes inverting one isomer into another. Presence of such a barrier explains the existence of the two isomers (equatorial and axial) observed previously in apolar solvents.