Theory of one-dimension rotational isomerization: A study of thecis-trans isomerization of HS-NS compared to that of HO-NO

Abstract

A theoretical approach, in which the potential functions representing rotational isomerization processes are expressed in terms of linear combinations of local potentials, is presented. Partitioning the torsional potential allows identification of specific contributions that are at the origin of the shape of potential curves at different regions along the torsional variable. Key properties, such as barrier heights, may then be expressed parametrically in terms of properties associated to the stable conformations. Simple analytical expressions intended to explore, quantitatively and qualitatively, the main characteristics of the transition states connecting stable isomers are formulated. As a first step towards the study of complex systems, we use this procedure to analyseab initio results concerning thecis-trans isomerization reaction of two simple prototype molecules: HSNS and HONO. We determine the relative stabilities of the different isomers and molecular structures and evaluate the associated potential barriers. It is shown that the mathematical procedure used to get potential functions is quite convenient and may be applied to the study of more complex isomerization reactions. Numerical results concerning molecular structures, potential barriers, ionization potentials and dipole moments are discussed. Comparing the values for barrier heights suggests that S(O)-S(O) π bonding through the mechanism of hyperconjugation may be present, to some extent, especially in HSNS.