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Mechanisms of the Reactions of W and W+ with H2O:  Computational Studies

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journal contribution
posted on 06.04.2006, 00:00 by D. G. Musaev, S. Xu, S. Irle, M. C. Lin
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(H2O) complexes with M−H2O 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 + H2O → 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 H2-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 H2 + 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) + H2O → H2 + WO(3Σ+), and W+(6D) + H2O → H2 + WO+(4Σ+) 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 H2O.

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