Attila Kovács, Rudy J.M. Konings

Computed vibrational frequencies of actinide oxides AnO^0/+/2+ and AnO₂^0/+/2+ (An = Th, Pa, U, Np, Pu, Am, Cm)

Journal of Physical Chemistry A, 115 (2011) 6646-6656

The vibrational frequencies of the actinide oxides AnO and AnO₂ (An = Th, Pa, U, Np, Pu, Am, Cm) and of their mono- and dications have been calculated using advanced quantum chemical techniques. The stretching fundamental frequencies of the monoxides have been determined by fitting the potential function to single-point energies obtained by relativistic CASPT2 calculations along the stretching coordinate and on this basis solving numerically the ro-vibrational Schrödinger equation. To obtain reliable fundamental frequencies of the dioxides we developed an empirical approach. In this approach the harmonic vibrational frequencies of the AnO₂^0/+/2+ species were calculated using eight different exchange-correlation DFT functionals. On the basis of the good correlation found between the vibrational frequencies and computed bond distances, the final frequency values were derived for the CASPT2 reference bond distances from linear regression equations fitted to the DFT data of each species. As test, the approach provided excellent agreement with accurate experimental data of ThO, ThO⁺, UO and UO⁺. The joint analysis of literature experimental and our computed data facilitated the prediction of reliable gas-phase molecular properties for some oxides. They include the stretching frequencies of PuO, ThO₂, UO₂ and UO₂⁺ and the bond distance of PuO (1.818 Å, being likely within 0.002 Å of the real value). Also the derived equilibrium bond distances of ThO₂, UO₂, and UO₂⁺ (1.896, 1.790 and 1.758 Å, respectively) should approximate closely the (yet unknown) experimental values. On the basis of the present results we suggest that the ground electronic state of PuO₂ in Ar and Kr matrices is probably different from that in the gaseous phase, similarly to UO₂ observed previously.