Comparative study of water reactivity with Mo2Oy− and W2Oy− clusters: A combined experimental and theoretical investigation

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.