Infrared Spectroscopy of Discrete Uranyl Anion Complexes

Abstract

The Free-Electron Laser for Infrared Experiments (FELIX) was used to study the wavelength-resolved multiple photon photodissociation of discrete, gas-phase uranyl (UO22+) complexes containing a single anionic ligand (A), with or without ligated solvent molecules (S). The uranyl antisymmetric and symmetric stretching frequencies were measured for complexes with general formula [UO2A(S)n]+, where A was hydroxide, methoxide, or acetate; S was water, ammonia, acetone, or acetonitrile; and n = 0-3. The values for the antisymmetric stretching frequency for uranyl ligated with only an anion ([UO2A]+) were as low or lower than measurements for [UO2]2+ ligated with as many as five strong neutral donor ligands and are comparable to solution-phase values. This result was surprising because initial DFT calculations predicted values that were 30-40 cm(-1) higher, consistent with intuition but not with the data. Modification of the basis sets and use of alternative functionals improved computational accuracy for the methoxide and acetate complexes, but calculated values for the hydroxide were greater than the measurement regardless of the computational method used. Attachment of a neutral donor ligand S to [UO2A]+ produced [UO2AS]+, which produced only very modest changes to the uranyl antisymmetric stretch frequency, and did not universally shift the frequency to lower values. DFT calculations for [UO2AS]+ were in accord with trends in the data and showed that attachment of the solvent was accommodated by weakening of the U-anion bond as well as the uranyl. When uranyl frequencies were compared for [UO2AS]+ species having different solvent neutrals, values decreased with increasing neutral nucleophilicity.

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

“…These values are consistent with previous IRMPD measurements of solvated ion pairs [34]. The exception is the fluoride complex, which has a 3 frequency somewhat lower at 956 cm −1 , indicating stronger coordination in this case.…”

“…At 956 cm −1 , the value for the 3 stretching mode consistent with values measured for other ligated uranyl-anion complexes in the gas phase [34,35], but ∼50 cm −1 higher than that reported by Gal for uranyl fluoride complexes in aqueous solution [3]. The large difference between the gas-phase and solution values is somewhat surprising considering that the acetone ligands bound to the gas-phase complex are expected to be more electron donating than water.…”

“…The trend suggests that the [UO 2 F(ACO) 2 ] + intermediate formed by loss of one ACO during the experiment may be contributing to the IRMPD spectrum. This explanation is supported by the DFT calculations (Table 1) and by prior measurements that show that uranyl complexes with fewer equatorial ligands have 3 stretching modes at higher cm −1 [32,34].…”

“…After the second ACO loss, loss of the third ACO generates [UO 2 F] + at m/z 289, and this is competitive with the loss of HF, which forms [UO 2 (ACO-H)] − at m/z 327. The photofragments of the hydroxide complex show up 2 u lower than those of the fluoride complex, which enabled measurement of uranyl 3 frequency for the hydroxide [34].…”

“…However the FEL can deliver sufficient pulse energies so that complexes will absorb multiple photons, and are energized to their dissociation threshold [25]. This approach has been used to examine the IR spectra of a number of metal coordination complexes [13,[26][27][28][29][30][31], and has been applied to uranyl coordination complexes in several studies by our group, specifically [UO 2 (solvent) n ] 2+ [32], [UO 2 (solvent)] + [33], [UO 2 (anion)(solvent)] + [34], and [UO 2 (anion) 3 ] − complexes [35,36]. These measurements have provided a quantitative basis for evaluating the effects of the number of ligands, and ligand nucleophilicity on binding within the complexes, and have provided a useful basis for comparing the results of theoretical calculations.…”