Mechanisms of the reactions of W and W ⁺ with H ₂O: Computational studies

The mechanism of the reactions of W and W ⁺ with the water molecule have been studied for several lower-lying electronic states of tungsten centers at the CCSD(T)/6-311G(d,p)+SDD and B3LYP/6-31G(d,p)+SDD levels of theory. It is shown that these reactions are essentially multistate processes, during which lower-lying electronic states of the systems cross several times. They start with the formation of initial prereaction M(H ₂O) complexes with M-H ₂O bonding energies of 9.6 and 48.2 kcal/mol for M = W and W ⁺, followed by insertion of the metal center into an O-H bond with 20.0 and 53.3 kcal/mol barriers for neutral and cationic systems, respectively. The overall process of M + H ₂O → t-HM(OH) is calculated to be highly exothermic, 48.4 and 48.8 kcal/mol for M -W and W ⁺. From the HM(OH) intermediate the reaction may proceed via several different channels, among which the stepwise HM(OH) → HMO + H → (H) ₂MO and concerted HM(OH) → (H) ₂MO pathways are more favorable and can compete (energetically) with each other. For the neutral system (M = W), the concerted process is the most favorable, whereas for the charged system (M = W ⁺), the stepwise pathway is slightly more favorable. From the energetically most favorable intermediate (H) ₂MO the reactions proceed via H ₂-molecule formation with a 53.1 kcal/mol activation barrier for the neutral system. For the cationic system, H-H formation and dissociation is an almost barrierless process. The overall reaction of W and W ⁺ with the water molecule leading to H ₂ + MO formation is found to be exothermic by 48.2 and 39.8 kcal/mol, respectively. In the gas phase with the collision-less conditions the reactions W( ⁷S) + H ₂O → H ₂ + WO( ³∑ ⁺), and W+( ⁶D) + H ₂O → H ₂ + WO ⁺( ⁴∑ ⁺) are expected to proceed via a 10.4 and 5.1 kcal/mol overall energy barrier corresponding to the first O-H dissociation at the TS1. On the basis of these PESs, we predict kinetic rate constants for the reactions of W and W ⁺ with H ₂O.