Tetrahydrides of third-row transition elements: Spin-orbit coupling effects on the stability of rhenium tetrahydride

Abstract

The potential energy surfaces of low-lying states in rhenium tetrahydride (ReH4) were explored by using the multiconfiguration self-consistent field (MCSCF) method together with the SBKJC effective core potentials and the associated basis sets augmented by a set of f functions on rhenium atom and by a set of p functions on hydrogen atoms, followed by spin-orbit coupling (SOC) calculations to incorporate nonscalar relativistic effects. The most stable structure of ReH4 was found to have a D2d symmetry and its ground state is 4A2. It is found that this is lower in energy than the dissociation limit, ReH2 + H2, after dynamic correlation effects are taken into account by using second-order multireference Møller-Plesset perturbation (MRMP2) calculations. This reasonably agrees with previous results reported by Andrews [J. Phys. Chem. 107, 4081 (2003)]. The present investigation further revealed that the dissociation reaction of ReH4 cannot occur without electronic transition from the lowest quartet state to the lowest sextet state. This spin-forbidden transition can easily occur because of large SOC effects among low-lying states in such heavy metal-containing compounds. The minimum-energy crossing (MEX) point between the lowest quartet and sextet states is proved to be energetically and geometrically close to the transition state for the dissociation reaction on the potential energy surface of the lowest spin-mixed state. The MEX point (C 2 symmetry) was estimated to be 9184 cm-1 (26.3 kcal/mol) higher than the 4A2 state in D2d symmetry at the MRMP2 level of theory. After inclusion of SOC effects, an energy maximum on the lowest spin-mixed state appears near the MEX point and is recognized as the transition state for the dissociation reaction to ReH2 + H2. The energy barrier for the dissociation, evaluated to be MEX in the adiabatic picture, was calculated to be 5643 cm-1 (16.1 kcal/mol) on the lowest spin-mixed state when SOC effects were estimated at the MCSCF level of theory. © 2010 American Institute of Physics.