Vibrational spectroscopy of anionic nitrate complexes of UO22+ and Eu3+ isolated in the gas phase

Abstract

Wavelength-selective infrared multiple photon photo-dissociation (IRMPD) was used to generate spectra of anionic nitrate complexes of UO₂²⁺ 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(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, 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)(NO₂)(O₂)]⁻ structure.