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We present that recent studies on reactions between Mo_xO_y^- cluster anions and H₂O/C₂H₄ mixtures revealed a complex web of addition, hydrogen evolution, and chemifragmentation reactions, with chemifragments unambiguously connected to cluster reactions with C₂H₄. To gain insight into the molecular-scale interactions along the chemifragmentation pathways, the anion photoelectron (PE) spectra of MoC₂H₂^-, MoC₄H₄^-, MoOC₂H₂^-, and MoO₂C₂H₂^- formed directly in Mo_xO_y^- + C₂H₄ (x > 1; y ≥ x) reactions, along with supporting CCSD(T) and density functional theory calculations, are presented and analyzed. The complexes have spectra that are all consistent with η^2- acetylene complexes, though for all but MoC₄H₄^-, themore » possibility that vinylidene complexes are also present cannot be definitively ruled out. Structures that are consistent with the PE spectrum of MoC₂H₂^- differ from the lowest energy structure, suggesting that the fragment formation is under kinetic control. The PE spectrum of MoO₂C₂H₂^- additionally exhibits evidence that photodissociation to MoO₂^- + C₂H₂ may be occurring. In conclusion, the results suggest that oxidative dehydrogenation of ethylene is initiated by Lewis acid/base interactions between the Mo centers in larger clusters and the π orbitals in ethylene.« less

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 includemore » 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. In conclusion, a more direct reaction path to H₂ production may therefore become accessible at modest temperatures for certain cluster stoichiometries and structures.« less