Energetics, structures, bonding, and kinetics of the HSCl-HClS system

The [H,S,C1] potential-energy surface has been investigated at the self-consistent field (SCF), complete active space self-consistent field (CASSCF), second-order Moller-Plesset, coupled-cluster single-double and perturbative triple excitation, [CCSD(T)]/6-31G(d,p), 6-31G(2df,2pd), and correlation-consistent polarized valence triple zeta (cc-pVTZ) levels of theory. CCSD(T)/ cc-pVTZ results predict a very stable HSC1 species, an isomer HC1S, 51.84 kcal/mol higher in energy, and a transition state 57.68 kcal/mol above HSC1. Independent of the level of theory, results with the smaller 6-31G(d,p) basis set turned out to be poor, especially for HC1S. Vibrational analysis indicates that both species can be easily differentiated if isolated. Bonding differences between these molecules are illustrated by contour plots of valence orbitals. Viewed classically, bonding in HC1S involves a dative bond. Transition-state rate constants. and equilibrium constants for the HSC1 --> HC1S isomerization have been estimated for various temperatures (200-1000 K). At 298.15 K, the forward rate is predicted to be 7.95 x 10(-29) s(-1), and the equilibrium constant to be 2.31 x 10(-38). Tunneling corrections vary from 1.57 at 298.15 K to 1.05 at 1000 K. Activation energies have been obtained by a two-points linear fit to the Arrhenius equation.