Prediction of the structures and heats of formation of MO, MO, and MO for M = V, Nb, Ta, Pa.

Structures for the mono-, di-, and tri-bridge isomers of MO as well as those for the MO and MO fragments for M = V, Nb, Ta, and Pa were optimized at the density functional theory (DFT) level. Single point CCSD(T) calculations extrapolated to the complete basis set (CBS) limit at the DFT geometries were used to predict the energetics. The lowest energy dimer isomer was the di-bridge for M = V and Nb and the tri-bridge for M = Ta and Pa. The di-bridge isomers were predicted to be composed of MO and MO fragments, whereas the mono- and tri-bridge are two MO fragments linked by an O. The heats of formation of MO dimers, as well as MO and MO neutral and ionic species were predicted using the Feller-Peterson-Dixon (FPD) approach. The heats of formation of the MF species were calculated to provide additional benchmarks. Dimerization energies to form the MO dimers are predicted to become more negative going down group 5 and range from -29 to -45 kcal mol. The ionization energies (IEs) for VO and TaO are essentially the same at 8.75 eV whereas the IEs for NbO and PaO are 8.10 and 6.25 eV, respectively. The predicted adiabatic electron affinities (AEAs) range from 3.75 eV to 4.45 eV for the MO species and vertical detachment energies from 4.21 to 4.59 eV for MO. The calculated MO bond dissociation energies increase from 143 kcal mol for M = V to ∼170 kcal mol for M = Nb and Ta to ∼200 kcal mol for M = Pa. The M-O bond dissociation energies are all similar ranging from 97 to 107 kcal mol. Natural bond analysis provided insights into the types of chemical bonds in terms of their ionic character. PaO is predicted to behave like an actinyl species dominated by the interactions of approximately linear PaO groups.