CO2 Activation by Nb+ and NbO+ in the Gas Phase. A Case of Two-State Reactivity Process

Gas-phase carbon dioxide activation by Nb+ and NbO+ was studied at the density functional level of theory using the hybrid exchange correlation functional B3LYP. Three reaction profiles corresponding to the quintet, triplet, and singlet multiplicities were investigated in order to ascertain the presence of some spin inversion during the CO2 reduction. Carbon dioxide activation mediated by metal cations was found to be an exothermic spin-forbidden process resulting from a crossing between quintet and triplet energetic profiles. The endothermic reaction of NbO+ with carbon dioxide was a barrierless process involving spin inversion. Geometries of minima along potential energy surfaces and reaction heats were in agreement with those from experimental studies carried out by using a guided ion beam tandem mass spectrometer. 1. Introduction The possibility of activating carbon dioxide, the main greenhouse gas, has received considerable attention in previous years.1-5 As it is not possible to reduce significantly CO2 emissions from anthropic sources, the interest in its chemical fixation6 and utilization as a starting material of chemically useful compounds is increased. However, only nature can effectively regenerate organic molecules from carbon dioxide by using it as a one-carbon building block.7 In previous years, chemical processes based on the use of metal species as catalysts for carbon dioxide reduction were investigated, both experimentally and theoretically.8-11 By using a guided ion beam mass spectrometer, Sievers and Armentrout elucidated the reaction mechanisms of bare zirconium12 and niobium13 cations and those of their respective monoxide and dioxide cations with CO2. Details of the zirconium-ion-assisted CO2 reduction potential energy surface were obtained by density functional theory.14 Experimental data highlight the importance of analyzing the electronic terms