Vibrational spectra of discrete UO22+ halide complexes in the gas phase

The intrinsic binding of halide ions to the metal center in the uranyl molecule is a topic of ongoing research interest in both the actinide separations and theoretical communities. Investigations of structure in the condensed phases are frequently obfuscated by solvent interactions that can alter ligand binding and spectroscopic properties. The approach taken in this study is to move the uranyl halide complexes into the gas phase where they are free from solvent interactions, and then interrogate their vibrational spectroscopy using infrared multiple photon dissociation (IRMPD). The spectra of cationic coordination complexes having the composition [UO2(X)(ACO)(3)](+) (where X = F, Cl, Br and I; ACO = acetone) were acquired using electrospray for ion formation, and monitoring the ion signal from the photoelimination of ACO ligands. The studies showed that the asymmetric nu(3) UO2 frequency was insensitive to halide identity as X was varied from Cl to I, suggesting that in these pseudo-octahedral complexes, changing the nucleophilicity of the halide did not appreciably alter its binding in the complex. The nu(3) peak in the spectrum of the F-containing complex was 9 cm(-1) lower indicating stronger coordination in this complex. Similarly the ACO carbonyl stretches showed that the C=O frequency was relatively insensitive to the identity of the halide, although a modest shift to higher wavenumber was seen for the complexes with the more nucleophilic anions, consistent with the idea that they loosen solvent binding. Surprisingly, the vi stretch was activated when the softer anions Cl, Br and I were present in the complexes. IR studies of the anionic complexes [UO2X3](-) (where X =Cl-, Br- and I-) compared the nu(3) UO2 modes versus halide, and showed that the nu(3) values decreased with increasing anion nucleophilicity. This observation was consistent with DFT calculations that indicated that [UO2X2](-) -X-center dot and [UO2X2](center dot)-X- dissociation energies decreased on the order F > Cl > Br > I. The tri-fluoro complex could not be photodissociated in these experiments. (C) 2010 Elsevier B.V. All rights reserved.