THEORETICAL STUDY OF BARRIERS TO INTERNAL ROTATION IN HYDROGEN PERSULFIDE, HSSH

Hydrogen persulfide, HSSH, has been studied by the ab initio self-consistent-field molecular-orbital method for the four dihedral angles of 0°, 90°, 135°, and 180°. The basis set consisted of Gaussian orbital expansions of best-atom Slater-type orbitals through 3p, tested in other calculations on H2S. The molecule has a calculated energy minimum for a dihedral angle in the range 90°-100°, in fair agreement with experiment (about 90.5°). The cis barrier is high (7.4 kcal), but the smaller trans barrier (1.9 kcal) seems low for consistency with experimental studies of the torsional vibrational levels. The S-S overlap population shows a pronounced maximum near a dihedral angle of 90°, just where the total energy shows a minimum. This reflects larger S-S bonding character near equilibrium and furnishes semiquantitative support for the discussion based on hyperconjugation used by the experimentalists to explain the structure. A partitioning of the total energy into attractive and repulsive components shows that both cis and trans barriers are dominated by the repulsive component (this is also true if the wavefunctions are scaled to satisfy the virial theorem).