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

UO(2) (+)-solvent complexes having the general formula [UO(2)(ROH)](+) (R=H, CH(3), C(2)H(5), and n-C(3)H(7)) 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 laser scanning through the 10 mum region of the infrared spectrum. Asymmetric O=U=O stretching frequencies (nu(3)) are measured over a very small range [from approximately 953 cm(-1) for H(2)O to approximately 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 nu(3) values generally decrease with increasing nucleophilicity of the solvent, except for the methanol (MeOH)-containing complex, which has a measured nu(3) value equal to that of the n-PrOH-containing complex. The nu(3) 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. nu(3) values for the U(V) complexes are comparable to those for the anionic [UO(2)(NO(3))(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 nu(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 nu(3) frequencies for the H(2)O-MeOH-EtOH-n-PrOH series are not reproduced by the calculations, specifically for the MeOH complex, which has a lower than expected value.