Infrared spectroscopy of dioxouranium(V) complexes with solvent molecules: Effect of reduction

UO2+-solvent complexes having the general formula [UO2-(ROH)](+) (R=H, CH3 C2H5, and n-C3H7) are formed using electrospray ionization and stored in a Fourier transform ion cyclotron resonance mass spectrometer, where they are isolated by mass-to-charge ratio, and then photofragmented using a free-electron loser scanning through the 10 mu m region of the infrared spectrum. Asymmetric O=U=O stretching frequencies (v) are measured over a very small range [from similar to 953cm(-1) for H2O to similar to 944 cm(-1) for n-propanol (n-PrOH)] for all four complexes, indicating that the nature of the alkyl group does not greatly affect the metal centre. The v(3) values generally decrease with increasing nucleophilicity of the solvent, except for the methanol (MeOH)-containing complex, which has a measured v(3) value equal to that of the n-PrOH-containing complex. The v, frequency values for these U(V) complexes are about 20 cm(-1) lower than those measured for isoelectronic U(VI) ion-pair species containing analogous alkoxides. v(3) values for the U(V) complexes are comparable to those for the anionic [UO2(NO3)(3)](-) complex, and 40-70 cm(-1) lower than previously reported values for ligated uranyl(VI) dication complexes. The lower frequency is attributed to weakening of the O=U=O bonds by repulsion related to reduction of the U metal centre, which increases electron density in the antibonding pi* orbitals of the uranyl moiety. Computational modelling of the v(3) frequencies using the B3LYP and PBE functionals is in good agreement with the IRMPD measurements, in that the calculated values fall in a very small range and are within a few cm(-1) of measurements. The values generated using the LDA functional are slightly higher and substantially overestimate the trends. Subtleties in the trend in v(3) frequencies for the H2O-MeOH-EtOH-n-PrOH series are not reproduced by the calculations, specifically for the MeOH complex, which has a lower than expected value.