Structure and Vibrational Spectra of Uranyl Dinitrate Complexes with Water and DMSO

Structural models were designed and spectral characteristics were computed based on DFT calculations for uranyl dinitrate complexes with H2O and DMSO [UO2(NO3)2·2DMSO, UO2(NO3)2·2H2O·2DMSO, UO2(NO3)2·2H2O·4DMSO]. Vibrational IR and Raman spectra of UO2(NO3)2·2DMSO were interpreted using models for bidentate and monodentate coordination of nitrate ions to uranyl. Several spectral signatures that characterized DMSO complexation in the second coordination sphere were identified and had analytical significance.

the IR spectrum at ~3440, 3193, 2725, 2673, 2545, 1079, and 724 cm -1 and strong ones at 2953, 2924, 2870, 2853, 1465, 1377, 1305, and 1170 cm -1 belonged to the solvent (mineral oil). Therefore, they are not included in Table 1. Furthermore, broad bands and weak lines in the short-wavelength region (3370-3300 cm -1 ) were consistent with traces of H 2 O in the sample. This allowed the asymmetric shoulder of the strong band at 1513 cm -1 to be interpreted as a H 2 O bending vibration; the weak line at ~356 cm -1 , a H 2 O librational vibration.    Note. The presence of two (or four) identical fragments ( 3 NO − , H 2 O, or DMSO) in the complexes doubled the frequencies, the splitting of which was as a rule <3 cm -1 . Therefore, only one (the greater) wavenumber is given. C. is a combination frequency; BC, bidentate coordination; MC, monodentate coordination; ν, stretching; δ, bending; ρ, rocking; γ, out-of-plane; τ, torsion; s, symmetric; as, antisymmetric vibrations.
The starting model of the UO 2 (NO 3 ) 2 ·2DMSO structure was based on bidentate nitrate coordination in the equatorial plane of the uranyl, which was a more stable confi guration than the monodentate form [1]. The model equilibrium structure for bidentate coordinated UO 2 (NO 3 ) 2 ·2DMSO had C i symmetry. Its structure was analogous to those of UO 2 (NO 3 ) 2 complexes with two DMF, dibutylformamide, and dicyclohexylformamide molecules [5]. The organic ligands were located in transpositions relative to the central U atom with the oxygen atoms of the organic ligands situated in the uranyl equatorial plane. Spectra of UO 2 (NO 3 ) 2 ·2DMSO in the region <1000 cm -1 showed an anomalously large number of bands and lines that could not be adequately interpreted based only on bidentate NO 3 coordination. Therefore, we also examined the version with monodentate nitrate coordinated to uranyl. Because the computations indicated that the energy of monodentate coordinated UO 2 (NO 3 ) 2 ·2DMSO was ~51 kJ/mol greater than that of the bidentate complex, it was assumed that the intensities of the corresponding bands and lines of the fi rst complex would be signifi cantly less than those of the second. The equilibrium confi gurations were found ( Structural and spectral characteristics of the isolated uranyl ion and DMSO molecule in the B3LYP/cc-pVDZ approximation and their agreement with experimental data were discussed by us previously [6,7]. The standard designations ν 1 , totally symmetric stretching; ν 2 , doubly degenerate bending; and ν 3 , asymmetric stretching vibration were used to classify vibrations of the 2 2 UO + fragment (D ∞h symmetry). The structure and vibrational spectrum of 3 NO − were calculated for a model with point symmetry D 3h (free nitrate). The standard designations ν 1 , totally symmetric stretching, ν 2 ; out-of-plane bending; ν 3 , doubly degenerate stretching; and ν 4 , doubly degenerate in-plane bending vibration were used to classify vibrations of the 3 NO − fragment. Two models were also examined for the structure of isolated UO 2 (NO 3 ) 2 , i.e., bidentate and monodentate nitrates. In the fi rst instance, an equilibrium geometry with D 2h symmetry was obtained. The lack of imaginary frequencies in the calculated spectrum confi rmed that this confi guration was stable. Neither of the models with symmetry limitations (D 2h , C i , etc.) produced an equilibrium confi guration for monodentate coordination (calculated vibrational spectra of all models contained several imaginary frequencies). Optimization of the geometry of UO 2 (NO 3 ) 2 with monodentate nitrates without symmetry limitations produced an equilibrium confi guration with bidentate coordination. Thus, we supposed that free UO 2 (NO 3 ) 2 with monodentate coordination was unstable. This structure could be stabilized either by coordination of additional ligands in the fi rst coordination sphere of UO 2 (NO 3 ) 2 or in a crystalline sample [22].
The calculated vibrational frequencies (taking into account typical errors of the used approximation), their sequences, and activities in IR and Raman spectra of isolated nitrate and bidentate nitrate coordinated to UO 2 (NO 3 ) 2 corresponded to the published values [1,22,26] (Table 1). The local symmetry of the nitrate fragment was reduced to C 2v for both bidentate and monodentate coordination. As a result, the doubly degenerate modes were split (for typical values, see the literature [1]). The presence of two nitrates in the complex (regardless of their coordination) caused additional splitting of each vibrational mode into two components that were symmetric and asymmetric relative to the complex center. The sizes of these splittings could differ substantially for bi-and monodentate coordinated nitrate (Table 1). This allowed vibrational spectra of UO 2 (NO 3 ) 2 ·2DMSO to be interpreted as follows.
Methyl stretching and bending vibrations are usually found in the ranges 2970-2870 and 1450-1370 cm -1 [27]. The CH 3 asymmetric stretching vibration in DMSO exceeded the indicated upper limit of the corresponding range [7,21,28]. This allowed strong lines at 3019 and 2928 cm -1 in the Raman spectrum to be assigned to asymmetric and symmetric CH 3 stretching vibrations and a weak line at 2803 cm -1 to be interpreted as an overtone of a CH 3 bending vibration (1406 cm -1 ). The weak line at 1495 cm -1 lay signifi cantly higher than typical δ(CH 3 ) values and was most likely an overtone of bending mode ν 4 ( 3 NO − ) (bidentate coordination), the main frequency of which appeared as a strong band at ~748 cm -1 in the IR spectrum, and a weak line at ~753 cm -1 in the Raman spectrum. Bands and lines at 1421 (IR and Raman), 1415 (Raman), 1406 (IR), and 1310 cm -1 (Raman) were assigned to methyl bending vibrations based on calculations and the literature [7,21,28]. Bands and lines at 1030 (IR), 988 (Raman), 986 (IR), and 870 cm -1 (Raman and IR, shoulder) were assigned to rocking vibrations ρ(CH 3 ) [7,21,28].
Calculations predicted a signifi cant (164 cm -1 ) long-wavelength shift for the S=O stretching frequency in UCl 4 ·2DMSO [7]. This was confi rmed in the experimental spectrum of UCl 4 ·2DMSO [21] because ν S=O was shifted from 1039 cm -1 for pure DMSO to 940 cm -1 in the complex. Thus, it could be assumed that the quantum-chemical calculation using B3LYP/cc-pVDZ elevated ν S=O in the complex by ~20 cm -1 . It could be proposed that these frequencies in the spectrum were located at ~920 cm -1 because an analogous calculation for UO 2 (NO 3 ) 2 ·2DMSO with bidentate nitrates predicted for ν S=O values of 948 (symmetric mode) and 939 cm -1 (asymmetric). Thus, the strong band at 924 cm -1 and the shoulder at 911 cm -1 in the IR spectrum of UO 2 (NO 3 ) 2 ·2DMSO were assigned to S=O stretching vibrations for bi-and monodentate nitrates, respectively.
Frequencies of C-S stretching vibrations in pure DMSO are located at 695 (asymmetric mode) and 665 cm -1 (symmetric) [21,28]. The fi rst of these frequencies was shifted to 786 cm -1 for UCl 4 ·2DMSO [7,21]. One of the four possible C-S stretching modes (asymmetric relative to the S atom and symmetric relative to the U atom according to calculations) in the studied complex corresponded to a medium line at 687 cm -1 in the Raman spectrum. The corresponding band (asymmetric vibration relative to the S atom and U atom according to calculations) in the IR spectrum was very weak. Weak lines at 434 and 317 cm -1 in the Raman spectrum were assigned to out-of-plane bending γ and rocking ρ S=O vibrations (bands at 424 and 315 cm -1 in the spectrum of UCl 4 ·2DMSO corresponded to these vibrations [7,21]).
Strong bands at 950 (bidentate coordination) and 942 cm -1 (monodentate) in the IR spectrum were assigned to the asymmetric ν 3 uranyl stretching vibration; the strong line at 845cm -1 in the Raman spectrum, to totally symmetric ν 1 . Bands and lines for uranyl ν 2 bending mode were reported in the range 290-240 cm -1 depending on the type of coordination and number and type of ligands [1,22,29]. The ν 2 vibration was assigned to a lower-frequency region (210-190 cm -1 ) [24,25]. According to our calculations, which reproduced highly successfully the structure, and frequencies and intensities of uranyl stretching modes, the frequency of the ν 2 vibration was 259 cm -1 and; therefore, fell into the fi rst of the mentioned ranges. The calculated intensity of this mode was low. We supposed that this line was not observed in the Raman spectrum of the studied complex.
Frequencies of coordinated U…O stretching vibrations were reported at ~200 cm -1 for bidentate nitrate coordination. This allowed a weak line at 209 cm -1 in the Raman spectrum to be assigned to ν U…O (the mode at 199 cm -1 was the strongest of the four vibrations of this type predicted by the calculations). Two weak lines at 171 and 163 cm -1 corresponded according to the calculations to ligand (DMSO) bending vibrations relative to uranyl. The lower-frequency region contained according to the calculations numerous complicated mixed bending modes of the ligands and uranyl. The frequencies and shapes of these vibrations had little information value and are not given in Table 1. Two medium lines (117 and 108 cm -1 ) and a weak line at ~85 cm -1 in the Raman spectrum were assigned to crystal-lattice vibrations.
Conclusions. Quantum-chemical modeling of the structure of a UO 2 (NO 3 ) 2 complex with two DMSO molecules as electron-donating organic ligands predicted the existence of two stable confi gurations of C i symmetry with bidentate and monodentate nitrate coordination to the central U atom. Vibrational IR and Raman spectra of the complex could be interpreted suffi ciently completely only by assuming that both coordination types were present. Formation of the complexes was accompanied by splitting of bands and lines of nitrate vibrations and their shifts to short and long wavelengths that were predicted adequately by the calculations. The observed spectral shifts of the S=O and nitrate vibrational frequencies could be used for analytical purposes.