Conformational preference in isolated neutral cytisine

Two isomers (1a and 1b) of almost equal total energies were found for isolated cytisine by the semiempirical (AM1 and PM3) as well as ab initio methods (HF/6-31G*//HF/6-31G* and MP2/6-31G*//HF/6-31G*). They differ only by orientation of the hydrogen on the piperidine nitrogen. Geometrical parameters calculated at the AM1, PM3 and 6-31G* levels are close to those reported for crystal cytisine. Aromatic character of the pyridone moiety in the calculated molecules is slightly smaller than that in the experimental crystal cytisine.