STABILIZATION OF THE ZWITTERIONIC FORMS OF 3-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 deriv ed from three-membered heterocycles, and how these stabilities are aff ected by Li+ association in the gas phase. The total energies of the n eutrals were obtained in the framework of the G2 theory, while those o f the Lit complexes were evaluated at the MP4/6-311G(d,p) level. The f ollowing three-membered heterocycles have been considered: aziridine ( 1), diaziridine (2), triaziridine (3), oxirane (4), dioxirane (5), oxa ziridine (6), oxadiaziridine (7) and dioxaziridine (8). The stability of the zwitterionic forms investigated decreases as the electronegativ ity difference between the atoms which behave as hydrogen donor and hy drogen acceptor increases. A further decrease of the relative stabilit ies is found when the electronegativity of the third group of the ring increases. In all cases the calculated binding energies of the zwitte rionic forms are systematically greater than those of the correspondin g parent compounds. Hence these zwitterionic forms are stabilized by L i+ association in the gas phase. This stabilizing effect is particular ly important for those systems which have H2N(+)-N- and H2N+-CH- bonds . (C) 1998 Elsevier Science B.V. All rights reserved.