ab Initio Study of Isomerization in the HONO System

摘要

Ab initio calculations, Moller-Plesset Perturbation Theory carried out to fourth order including triples, were used to study the cis-trans isomerization of HONO. The geometries were completely optimized and the trans-cis energy difference was calculated to be 3.5 KJ/mole. The activation energy for the process was found to be 53.5 KJ/mole. It was found necessary to employ a very large basis set, doubly split valence level with complete polarization, and a set of diffuse functions of both heavy and hydrogen atoms in order to obtain a reliable estimate of the energy difference between the isomers. The calculations also indicate that a reexamination of the structural assignments for both isomers might be in order. A harmonic analysis of the rotational barrier is carried out and its origin discussed. The barrier is principally traceable to the distortion in electronic structure which occurs under rotation and can be ascribed to large contributions from the ionic structure H-O(-) N(triple bond) O(+).