Torsional isomerization of biologically active bicyclic molecules

Rotational isomerization of bipyridines C5H4NC5H4N was studied by CNDO/2, PPP/CI, and CNDO/CI methods. It is shown that CNDO/2 overestimates the angle of rotation ϕ between the pyridine rings ca. two times. The angle ϕ was determined for 2,2′‐bipyridine by means of correlation of the theoretical (CNDO/2) and experimental dipole moment. It was also found from the correlation between the theoretical and experimental UV spectra. It is shown that there is an explicit dependence of the results upon the distance between heteroatoms (PPP/CI). It has been found that the CNDO/CI method correctly predicts the value of the rotational angles and their sequence in bipyridines.