Stabilization of the zwitterionic forms of three-membered rings by cationization in the gas phase

High level ab initio calculations have been carried out to investigate the relative stabilities of the zwitterionic forms which can be derived from three-membered heterocycles, and how these stabilities are affected by Li+ association in the gas phase. The total energies of the neutrals were obtained in the framework of the G2 theory, while those of the Lit complexes were evaluated at the MP4/6-311G(d,p) level. The following three-membered heterocycles have been considered: aziridine (1), diaziridine (2), triaziridine (3), oxirane (4), dioxirane (5), oxaziridine (6), oxadiaziridine (7) and dioxaziridine (8). The stability of the zwitterionic forms investigated decreases as the electronegativity difference between the atoms which behave as hydrogen donor and hydrogen acceptor increases. A further decrease of the relative stabilities is found when the electronegativity of the third group of the ring increases. In all cases the calculated binding energies of the zwitterionic forms are systematically greater than those of the corresponding parent compounds. Hence these zwitterionic forms are stabilized by Li+ association in the gas phase. This stabilizing effect is particularly important for those systems which have H2N(+)-N- and H2N+-CH- bonds. (C) 1998 Elsevier Science B.V. All rights reserved.