A theoretical ab initio approach to the S-S bond breaking process in hydrogen disulfide and in its radical anion

The S-S bond cleavage of dihydrogen disulfide, HSSH, and of its radical anion was studied theoretically with molecular orbital (MO) ab initio calculations at different levels of theory. The bond dissociation energy obtained from the total molecular energies of the undissociated HSSH and of the HS radicals is close to the best experimental value only with the G2 Moller-Plesset second-order perturbation (MP2) and complete basis set (CBS) methods and underestimated with the multiconfigurational self-consistent field (MCSCF) procedure regardless of the basis set and active space employed. The energy profile of the radical anion as a function of the S-S bond distance displays a minimum at 2.8 Angstrom, and the activation energy for the electron transfer was calculated from the crossing of the energy profiles of the neutral molecule and of the radical anion both in the vapor phase and in media of different polarity. Changes in the activation energy of the order of 1 kcal/mol were found when transitions between vibrational levels of the S-S bond in the neutral molecule and in the radical anion were taken into account. A method for estimating the energy at the crossing point is proposed, based on Morse-like potential functions constructed from the second derivative of the calculated energies of the equilibrium structure of the neutral molecule and of the radical anion.