Photoionization of Nb₃CO and Nb₃ (CO)₂: is CO molecularly or dissociatively adsorbed on niobium?

Abstract

The photoionization efficiency spectra of gas-phase Nb₃CO and Nb₃(CO)₂ have been acquired using a laserablation, photoionization mass spectrometer. The adiabatic ionization energies of the species are 5.82 ± 0.02 and 5.85 ± 0.02 eV, respectively, where error bars reflect precision, the values are accurate to within 0.1 eV. From these data and literature values of cation bond strengths, the bond energies of Nb₃-CO and Nb₃COCO are found to lie within the 2.8-3.4 and 1.7-1.8 eV ranges, respectively. The former is most consistent with dissociatively adsorbed CO, while the latter may correspond to molecularly or dissociatively adsorbed CO. These conclusions are supported by density-functional theory calculations. In addition to properties of Nb₃CO, the properties of the transition state to dissociative adsorption of CO have been calculated. It is found that the transition state lies lower in energy than the separated reagents. Spontaneous dissociation is expected to follow molecular adsorption of CO on Nb₃, accordingly. Over the 1540-2220 cm-1 range, no infrared multiphoton dissociation of Nb₃CO or Nb₃ (CO)₂ is observed, consistent with CO being dissociatively adsorbed for both species.