CALCULATION OF THE WEIGHTS OF RESONANCE STRUCTURES OF MOLECULES IN SOLUTION

Weights of polyelectron structures, including resonance structures, of the pi-electron system of formaldehyde, carbonyl oxide, nitrosimine, and 2-nitroethenamine in solution have been calculated by means of a g eneral polyelectron population analysis recently developed. Wave funct ions of a molecular system in solution have been obtained using the co ntinuum model of solvation developed by Rivail's group. Correlated (CI SD and MCSCF) wave functions with different basis sets (pseudopotentia l + minimal basis sets, STO-3G and 6-31G) have been employed. In the case of multi-xi basis sets, the natural atomic orbitals derived from Weinhold analysis have been used. Changes of weight with solvation are discussed at the SCF and CI levels and as a function of the basis set s. Polarity of the solvent does not necessarily increase the weight of all ionic structures, but only that of those structures being favored in the framework of a given topology. The stabilization of the closed -shell structure of the carbonyl oxide in solution is explained by the changes induced by the solvent in the relative weight of the diradica l/zwitterionic resonance structures. In contrast, nitrosimine does not present changes under solvation, and the singlet diradical canonical form remains the main one responsible for its open-shell configuration , For 2-nitroethenamine, it is shown that solvent polarity enhances th e weights of the iminic and push-pull canonical structures and reduces that of the ethylenic structure.