Coupled Cluster Study of the Interactions of AnO2, AnO2+, and AnO22+ (An = U, Np) with N2 and CO.

Abstract

Thermochemical and spectroscopic properties for actinyl complexes involving UO22+/1+/0 and NpO22+/1+/0 with N2 and CO, together with the UO2-O2, UO2+-O2, and UO2+-NO complexes, have been studied for the first time using an accurate composite coupled cluster approach. Two general bonding motifs were investigated, end-on (η1) and side-on (η2) relative to the metal center of the actinyls. For end-on CO complexes, both C-coordinated (An-C) and O-coordinated (An-O) structures were examined, with the former always being lower in energy. All of the η1 complexes were calculated to be stable, with dissociation energies ranging from 2 to 36 kcal/mol, except for that of UO2+-O2 (the η1 orientation for UO2+-NO was not amenable to single reference coupled cluster). In agreement with a previous study, the η2 structure for UO2+-O2 was calculated to be relatively strongly bound, by 22.3 kcal/mol in this work. The closely related NO complex, however, had a calculated dissociation energy of just 4.0 kcal/mol. The binding energy of O2 to neutral UO2 in a η2 orientation was calculated to be very strong, 75.4 kcal/mol, and strongly resembled a UO2+(O2-) complex at equilibrium. The N-N and C-O bonds were found to be somewhat activated for all the side-on (η2) neutral An(IV) complexes, with stretching frequencies of N2 or CO being red-shifted by as much as 480 cm-1 with a 0.06 Å bond length elongation. Dissociation energies for the η1 complexes are strongly correlated with the extent of electron transfer from ligand to actinyl. The nature of bonding in the actinyl complexes is examined using natural resonance theory (NRT). The correlation between bonding motif and small molecule activation is in agreement with experiments in condensed phases.