Structure and vibrational IR spectra of a UCl4⋅2DMSO complex

Structural models are designed and spectral characteristics are computed based on DFT calculations for a complex of uranium tetrachloride with two molecules of dimethylsulfoxide (UCl4⋅2DMSO). The calculations were carried out using a B3LYP hybrid functional in the LANL2DZ effective core potential approximation for the uranium atom and a cc-pVDZ all-electron basis set for all other atoms. Two structural variants were found for the complex. In the first of them, which is more stable, DMSO molecules are coordinated to the central uranium atom through oxygen atoms whereas in the second one, whose energy is 225 kJ/mol higher, the coordination proceeds through sulfur atoms. The obtained spectral characteristics are analyzed and compared with experimental data. Spectral features that are characteristic of the complexation process are identified. The adequacy of the proposed models and the agreement between calculation and experiment are demonstrated.

Results and Discussion. Uranium tetrachloride (UCl 4 ). Equilibrium configurations of D 2d symmetry were obtained from our calculations [19] of isolated UCl 4 and UF 4 molecules (Fig. 1a). It was shown [19] that the structural and spectral-energetic characteristics of the uranium tetrahalides calculated by us for the D 2d model were in principle consistent with experimental results (Tables 1 and 2) that suggested a regular tetrahedral structure of T d symmetry [20]. The bond lengths for UCl 4  . (All bending vibrations were split in the IR spectrum. However, the intensities of the corresponding absorption bands were 1-2 orders of magnitude weaker than that of stretching vibration ν 3 .) Table 2 compares characteristics of the UCl 4 vibrational spectrum that were calculated in a harmonic approximation with experimental data for the gas phase at ~900 K [21]. The frequencies (in cm -1 ) and band intensities in the experimental spectra (in percent absorption) are also given. The intensities in the calculated spectra were normalized to the intensity of the strongest band in the corresponding spectrum. Figure  2a shows the calculated UCl 4 spectrum.
Dimethylsulfoxide (DMSO). The molecular structure of DMSO has been studied several times by experimental and theoretical methods [21,[24][25][26][27]. The geometric parameters of DMSO were determined highly accurately. The geometry of DMSO was optimized initially assuming that the equilibrium configuration possessed C s symmetry. However, it was discovered during calculation of the Hessian that the CH 3 antisymmetric torsion mode for such a structure had an imaginary frequency. Therefore, the final optimization was performed without symmetry constraints. The next calculation of the Hessian confirmed the stability of such a configuration (Fig. 1b). Table 1 compares calculated DMSO structural parameters and experimental data for the gas phase [21] (bond lengths are given in A°; angles, in degrees). These data showed that the calculation in the B3LYP/cc-pVDZ approximation reproduced adequately the experimental structural parameters for the isolated DMSO molecule. Deviations of the calculated S=O and CS bond lengths from the experimental values were 1.3%; of the CH bond lengths, 0.7%. The CSC angle was reproduced with an error 0.2%. The greatest error (-5.5%) was observed for the dihedral angle between a plane containing the S=O bond and the plane containing the two CS bonds. It is noteworthy that a similar error was also observed for DMF [28]. This is apparently typical of the approximation that was used.
An analysis of calculated vibrational spectra of DMF monomer and dimer [28] in addition to the complex UCl 4 ⋅2DMF [19] showed that the frequencies of CH stretching vibrations were elevated by 3%; of the carbonyl, by 4%, if the B3LYP/cc-pVDZ approximation was used. The frequencies of other stretching and bending vibrations of DMF were elevated less significantly. The frequencies of the stretching vibrations of the "core" molecule turned out to be relatively insensitive to scaling of the force constants. Therefore, the force constant matrices were partially scaled [19,28]. Calculation of the vibrational spectrum of the isolated DMSO molecule (Table 2) indicated that the frequencies of CH stretching vibrations were also elevated by an average of 4%. The frequency of the S=O stretching vibration (1086 cm -1 ) was lowered by 1.5% relative to its value for the gas phase (1102 cm -1 [1]) and elevated by  Table 2) were assigned based on our calculation and were consistent with published data [1,2]. Figure 2b shows a portion of the calculated spectrum.    [19], only one equilibrium structure (with trans ligands) of point symmetry C i was found for variant A. The spectrum of the model complex with coordination through the O atoms and cis ligands contained two imaginary frequencies for rocking vibrations of CSC groups in the DMSO molecules. This could be explained by steric hindrance arising upon close contact of the ligands. Hence, the formation of such a model complex seemed improbable. Therefore, it was not considered in analyzing and interpreting the experimental spectra [6,7]. The structure of variant B also had point symmetry C i although its energy according to the calculation was 225.4 kJ/mol greater. The shape of UCl 4 changed significantly upon forming the UCl 4 ⋅2DMSO complex (like for UCl 4 ⋅2DMF [19]). This was accompanied by a reduction of the local symmetry of the UCl 4 group to C 2h and the adoption of a flattened shape. The U bonds (four UCl and two U...O or two U...S) formed a regular octahedral structure for complex A and close to it for complex B. The UCl bonds in both variants were lengthened and became nonequivalent (Table 1). In contrast to the UCl 4 group, the DMSO ligands in the complex retained in general their structure (Fig. 1b-d). The greatest changes were typical of the S=O bonds. They lengthened by 0.078 and 0.020 A° in complexes A and B. The CS bond lengths underwent smaller changes. They contracted by 0.009 and 0.002 A°. The bond lengths of the methyl H atoms also changed insignificantly. Figure 2 shows portions of the two calculated (for variants A and B) IR spectra of the UCl 4 ⋅2DMSO complex. Table 2 compares the calculated frequencies and their corresponding band intensities with the experimental values. The strongest band in IR spectra of DMSO and the complex corresponded obviously to the S=O stretching vibration. The symmetric S=O stretching vibration was forbidden in IR spectra of both complex variants as a result of an alternate selection rule. The band for the S=O antisymmetric mode experienced a slight (8 cm -1 ) long-wavelength shift and had a simple structure in variant B because DMSO was coordinated through the S atom. The complexation in variant A caused a substantial (>160 cm -1 ) long-wavelength shift of the band for the S=O antisymmetric mode. Furthermore, this vibration was mixed (probably due to Fermi resonance) with one of the CH 3 rocking vibrations. This led to actual splitting of the band into two components (922 and 920 cm -1 ) of similar strengths (0.62:1.00). These results were confirmed by experimental spectra of the complex (940 and 918 cm -1 [6] and 951 and 941 cm -1 [7]).
Other noticeable differences between IR spectra of variant A and pure DMSO were due to complexation and were predicted by our calculation. The short-wavelength shift of bands for ρ CH 3 rocking vibrations (average ~60 cm -1 according to the calculation and ~40 cm -1 in the spectrum), the ν as CS antisymmetric stretching vibration (47 cm -1 according to the calculation and 88 cm -1 in the spectrum), and the γ S=O out-of-plane bending vibration (70 cm -1 according to the calculation and 46 cm -1 in the spectrum) should be pointed out. All these bands were strong in the IR spectrum and could act as a reliable signature of complexation. Short-wavelength shifts of the ρ CH 3 rocking modes and (to a lesser extent) the ν as CS stretching mode were also observed in the spectrum of variant B. It could be concluded from this that such spectral changes were typical of complexation in general regardless of the type of coordination (through the O or S atom). The band for the γ S=O vibration in the spectrum of variant B did not experience a short-wavelength shift. A broad weak band with a poorly resolved maximum near 630 cm -1 in the spectrum of the sample pressed in KBr [6] was most likely correlated with an electronic transition between the ground state and one of the excited sublevels of the U 4+ 3 H 4 ground state [23]. Splitting of the UCl antisymmetric stretching vibration into three components as a result of symmetry lowering of the UCl 4 group in the complex was accompanied by a long-wavelength frequency shift (292, 278, and 250 cm shapes that were due to the ligands and UCl 4 were located at lower frequencies. The frequencies and intensities of these bands were of little information value and were not included in Table 2.
Conclusion. Quantum-chemical modeling of the structure of the UCl 4 ⋅2DMSO complex predicted the existence of two stable configurations of C i symmetry in which the electron-donating organic ligands were coordinated through O and S atoms. The first variant turned out to be more stable. The structure of the DMSO molecule remained practically unchanged upon coordination whereas the shape of the UCl 4 group was noticeably distorted. Formation of the first structural variant was accompanied by significant long-wavelength shifts and splitting of bands for the S=O and UCl stretching vibrations in addition to short-wavelength shifts of bands for methyl rocking vibrations, the antisymmetric CS stretching vibration, and the out-of-plane S=O bending vibration. The spectrum of the second variant underwent less noticeable changes in comparison with spectra of the isolated DMSO molecule and UCl 4 . The shortwavelength shift of the S=O stretching vibration was not predicted by the calculation. Formation of the UCl 4 ⋅2DMSO complex with coordination through the S atoms seemed improbable. Thus, the structure of the first variant could serve as a successful model structure for the coordination sphere of U 4+ . The characteristics of the IR vibrational spectrum that were calculated on the basis of this variant reproduced adequately in general the experimental data.