HARDNESS PROFILE AND ACTIVATION HARDNESS FOR ROTATIONAL ISOMERIZATIONPROCESSES - APPLICATION TO NITROUS-ACID AND HYDROGEN PERSULFIDE

A theoretical study of the conformational dependence upon internal rot ation of molecular hardness and its correlation with the torsional pot ential energy is performed for two representative molecules that prese nt rotational isomerization. In order to characterize such correlation , the hardness and potential energy along a torsional angle alpha are expressed as functional forms (eta[omega] and V[omega], respectively) of a reduced variable omega(alpha) = 1/2(1 - coS alpha) that gives the statistical weight of the reference conformations along alpha. Correl ations among eta[omega] and V[omega] are then found by splitting eta[o mega] and V[omega] into symmetric and asymmetric parts. This leads to a formula defining the activation hardness in terms of the activation energy and the energy difference between two reference conformations. From this formula, we preform a qualitative analysis to characterize t he conditions under which the principle of maximum hardness (PMH) hold s. The procedure is used to analyze HF ab initio results of the intern al rotation of nitrous acid (HO-NO) and hydrogen persulfide (HSSH). Fo r both molecules we have found that the PMH is obeyed.