An MNDO molecular orbital study of the reactions of protonated oxirane with simple nucleophiles

The gas-phase bimolecular hydroxyethylation of simple nucleophiles by protonated oxirane has been studied using MNDO semiempirical molecular orbital (MO) theory with complete geometry optimization of all stationary points. The calculated reaction paths involved passage across low-lying transition states between the reactant clusters and the gauche ⇌ antiproduct manifolds. The transition-state geometries were calculated to be characteristically "early", and corresponded to concerted processes with activation energies somewhat lower than the 14.5 kcal mol-1 for the unimolecular ring opening. The lowest barrier was calculated for 3-alkylation of imidazole which parallels the known tendency of simple epoxides to predominantly modify the analogous 7-position of guanine in nucleic acids. Variations in the calculated transition-state geometries, which differ qualitatively from those expected in more typical SN2 reactions, are explained.