Vibrational spectroscopy of anionic nitrate complexes of UO ₂²⁺ and Eu³⁺ isolated in the gas phase

Wavelength-selective infrared multiple photon photo-dissociation (IRMPD) was used to generate spectra of anionic nitrate complexes of UO₂ ²⁺ and Eu³⁺ in the mid-infrared region. Similar spectral patterns 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(NO ₃)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 Eu ³⁺ metal center, consistent with its higher charge. The spectrum of a unique uranyl-oxo species having an elemental composition [UO₉N ₂]^- was also acquired, that contained nitrate absorptions suggestive of a [UO₂(NO₃)₂(O)]^- structure; the spectrum lacked bands indicative of nitrite and superoxide that would be indicative of an alternative [UO₂(NO₃)(NO ₂)(O₂)]^- structure.