Vibrational Spectroscopy of Anionic Nitrate Complexes of UO22+ and EU3+ Isolated in the Gas Phase

Wavelength-selective infrared multiple photon photo-dissociation (IRMPD) was used to generate infrared spectra of anionic nitrate complexes of UO22+ and Eu3+ in the mid-infrared region. A pattern of absorptions were observed for both species, including splitting of the antisymmetric O-N-O stretch into high and low frequency components with the magnitude of the splitting consistent with attachment of nitrate to a strong Lewis acid center. The frequencies measured for [UO2(NO3)3]- were within a few cm-1 of those measured in the condensed phase, the best agreement yet achieved for a comparison of IRMPD with condensed phase absorption spectra. In addition, experimentally-determined values were in good general agreement with those predicted by DFT calculations, especially for the antisymmetric UO2 stretch. The spectrum from the [UO2(NO3)3]- was compared with that of [Eu(NO3)4]-, which showed that nitrate was bound more strongly to the Eu3+ metal center, consistent with its higher charge. The spectrum of a unique uranyl-oxo species having an elemental composition [UO9N2]- was also acquired, for which calculations suggested a [UO2(NO3)(NO2)(O2)]- structure