Energy effects of substituents in three-membered nitrogen-containing rings

The energy effects of substituents in three-membered nitrogen-containing heterocycles were studied by quantum-chemical calculations. Within the examined series of substituents, the dependence of the angular strain on the nature of substituent is mainly described via variation of the force constants of vibrations of endocyclic angles. Strong sigma-electron donors and accepters destabilize the ring. The dependence of the strain energy on the total charge of the ring is quadratic, with the minimum corresponding to the maximal stabilization of the heterocycles and optimal delocalization of the sigma-electron density in the ring plane. Substituents with polar conjugation effects increase the strain energy. Amino group exerts a minor destabilizing effect owing to specific steric and electronic interactions in acyclic structures relative to which the strain energy was calculated using homodesmic reactions of group separation.