Solid-state vibrational spectra

The molecule S12, like Se, is of Dsd symmetry but in the soHd state it occupies sites of the much lower C211 symmetry [163]. Due to the low solubihty and the thermal decomposition on melting only solid state vibrational spectra have been recorded [2,79]. However, from carbon disulfide the compound Si2-CS2 crystallizes in which the S12 molecules occupy sites of the high Sg symmetry which is close to 03a [163]. The spectroscopic investigation of this adduct has resulted in a revision [79] of the earher vibrational assignment [2] and therefore also of the earlier force constants calculation [164]. In Fig. 24 the low-temperature Raman spectra of S12 and Si2-CS2 are shown. 

By far the most studied of Hg(II)-thiolate compounds are those with the stoichiometry Hg(SR)2- While no vibrational studies have been carried out in the vapor phase or in matrix isolation, a number of solution studies exist, and many solid-state vibrational spectra have been reported. In some cases, significant differences in solution and solid-state spectra are found. The vibrational data are consistent with linear coordination in the vast majority of compounds. Where important secondary interactions exist or when the geometry changes from linear to tetrahedral (as in the structurally characterized Hg(S-f-Bu)2, Fig. 7), the Raman and IR data reflect deviation from simple linear coordination, although in some cases, the proper assignment of Hg-S vibrational modes is unclear, for the reasons described above. 

Rey-Lafon et al. 171) analyzed the solid state vibrational spectrum of... 

Both solution and solid-state vibrational spectroscopy have been employed to characterize the reaction shown above. In addition. X-ray powder diffraction of the recovered solids from the uranium experiment shows the presence of both (HjO" ") (SbFg) and UF4 -2SbF5. Raman spectroscopy of the solid mixture after vacuum removal of the solvent features bands at 667,561, and 295 cm due to the Vj, V2, and Vj stretching and bending modes of SbF g, respectively. The infrared spectrum of the solid also shows a broad band at 3058 cm (vj) and a sharp band at 1623 cm" (V4) due to the (HjO ) moiety. Little difference between the spectra reported for pure (HjO" ") (SbFg) (Christe et al. 1984) and our heterogeneous mixtures was observed. Therefore,... 

The Raman spectrum of aqueous mer-cury(I) nitrate has, in addition to lines characteristic of the N03 ion, a strong absorption at 171.7 cm which is not found in the spectra of other metal nitrates and is not active in the infrared it is therefore diagnostic of the Hg-Hg stretching vibration since homonuclear diatomic vibrations are Raman active not infrared active. Similar data have subsequently been produced for a number of other compounds in the solid state and in solution. 

There are 78 vibrational degrees of freedom for TgHg and it has been shown that the molecule has 33 different fundamental modes under Oh symmetry, 6 are IR active, 13 are Raman active, and 14 vibrations are inactive. The experimental fundamental IR active vibrational frequencies have been assigned as follows 2277 (v Si-H), 1141 (vas Si-O-Si), 881 5 O-Si-H), 566 ( s O-Si-O), 465 (v O-Si-O), and 399 cm ( s O-Si-O). These generally agree well with calculated values The IR spectrum recorded in the solid state shows bands at 2300 and 2293 cm ... 

The specific requirement for a vibration to give rise to an absorption in the infrared spectrum is that there should be a change in the dipole moment as that vibration occurs. In practice, this means that vibrations which are not centrosymmetric are the ones of interest, and since the symmetry properties of a molecule in the solid state may be different from those of the same molecule in solution, the presence of bands may depend on the physical state of the specimen. This may be an important phenomenon in applying infrared spectroscopy to the study of AB cements. 

The wavenumbers of the observed bands are identical with those of the spontaneous Raman spectrum of the solution and oxazine solid [27]. The impulsive stimulated Raman transition may initiate coherent vibrations in the electronic excited state. However, there was no sign of the excited-state vibrations superimposed on the ground-state bands in the spectrum of Figure 6.3. 

The bands due to Fe(CO)4 are shown in Fig. 8. This spectrum (68) was particularly important because it showed that in the gas phase Fe(CO)4 had at least two vq—o vibrations. Although metal carbonyls have broad vC—o absorptions in the gas phase, much more overlapped than in solution or in a matrix, the presence of the two Vc—o bands of Fe(CO)4 was clear. These two bands show that in the gas phase Fe(CO)4 has a distorted non-tetrahedral structure. The frequencies of these bands were close to those of Fe(CO)4 isolated in a Ne matrix at 4 K (86). Previous matrix, isolation experiments (15) (see Section I,A) has shown that Fe(CO)4 in the matrix had a distorted C2v structure (Scheme 1) and a paramagnetic ground state. This conclusion has since been supported by both approximate (17,18) and ab initio (19) molecular orbital calculations for Fe(CO)4 with a 3B2 ground state. The observation of a distorted structure for Fe(CO)4 in the gas phase proved that the distortion of matrix-isolated Fe(CO)4 was not an artifact introduced by the solid state. 

As the interpretation of the vibrational spectrum of sohd state samples can provide information concerning the cation-anion interactions, it is of special interest to the chemist. In some cases information about the crystal structure can be obtained from the interpretation of spectra of the solid state when the substance possesses an anion of high symmetry. 

An appreciation of the crystal field effect on the vibrations of the Bravais cell which is repeated to build the crystal is extremely important when interpreting the vibrational spectra of many substances, since in the presence of a crystal field influence the number of observed bands in the spectrum cannot be directly determined from the formula unit which goes to make up the unit cell. In other words, there is almost always a larger number of bands to account for when investigating solid state samples. The solid state effects often cause degenerate bands to split in the same degree as symmetric and antisymmetric stretching modes split. 

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