Comparative study of water reactivity with Mo2Oy- And W2Oy- Clusters: A combined experimental and theoretical investigation

Abstract

A computational investigation of the Mo2Oy- + H2O (y = 4, 5) reactions as well as a photoelectron spectroscopic probe of the deuterated Mo2O6D2- product have been carried out to understand a puzzling question from a previous study: Why is the rate constant determined for the Mo2O5- + H2O/D2O reaction, the terminal reaction in the sequential oxidation of Mo2Oy- by water, higher than the W2O5- + H2O/D2O reaction? This disparity was intriguing because W3Oy- clusters were found to be more reactive toward water than their Mo3Oy- analogs. A comparison of molecular structures reveals that the lowest energy structure of Mo2O5- provides a less hindered water addition site than the W2O5- ground state structure. Several modes of water addition to the most stable molecular and electronic structures of Mo2O4- and Mo2O5- were explored computationally. The various modes are discussed and compared with previous computational studies on W2Oy- + H2O reactions. Calculated free energy reaction profiles show lower barriers for the initial Mo2Oy- + H2O addition, consistent with the higher observed rate constant. The terminal Mo2Oy- sequential oxidation product predicted computationally was verified by the anion photoelectron spectrum of Mo2O6D2-. Based on the computational results, this anion is a trapped dihydroxide intermediate in the Mo2O5- + H2O/D2O → Mo2O6- + H2/D2 reaction.