A COMPARATIVE MOLECULAR MECHANICS, SEMIEMPIRICAL AND AB-INITIO STUDY OF SATURATED 5-MEMBERED RINGS

The potential energy surfaces of five-membered homo- and heterocyclic saturated rings (cyclopentane, tetrahydrofuran, tetrahydrothiophene, p yrrolidine and N-methylpyrrolidine) have been studied using molecular mechanics, semiempirical (MNDO, AM1 and PM3) and ab initio methods (HF /6-31G*) to determine the ability of each in describing the geometry and stability of each of the mentioned compounds. The molecular mechan ics and ab initio calculations are consistent in the relative energies and geometries determined for the conformers of all ring systems, whe reas the semiempirical calculations yield flatter geometries and scatt ered energies. The PM3 method produces the best results among the semi empirical methods tested.