Role of weakly bound complexes in temperature-dependence and relative rates of M_x O_y^- + H₂O (M = Mo, W) reactions

Results of a systematic comparison of the Mo_xO_y^- + H₂O and W_xO_y^- + H₂O reaction rate coefficients are reported and compared to previous experimental and computational studies on these reactions. W_xO_y^- clusters undergo more direct oxidation by water to yield W_xO_y+1^- + H₂, while for Mo_xO_y^- clusters, production of Mo_xO_yH₂^- (trapped intermediates in the oxidation reaction) is comparatively more prevalent. However, Mo_xO_y^- clusters generally have higher rate coefficients than analogous W_xO_y^- clusters if Mo_xO_y+1H₂^- formation is included. Results of calculations on the M₂O_y^- + H₂O (M = Mo, W; y = 4, 5) reaction entrance channel are reported. They include charge-dipole complexes formed from long-range interactions, and the requisite conversion to a Lewis acid-base complex that leads to M_xO_y+1H₂^- formation. The results predict that the Lewis acid-base complex is more strongly bound for Mo_xO_y^- clusters than for W_xO_y^- clusters. The calculated free energies along this portion of the reaction path are also consistent with the modest anti-Arrhenius temperature dependence measured for most Mo_xO_y^- + H₂O reactions, and the W_xO_y^- + H₂O reaction rate coefficients generally being constant over the temperature range sampled in this study. For clusters that exhibit evidence of both water addition and oxidation reactions, increasing the temperature increases the branching ratio toward oxidation for both species. A more direct reaction path to H₂ production may therefore become accessible at modest temperatures for certain cluster stoichiometries and structures.