Vibrational spectroscopy symmetry considerations

Raman spectroscopy is a useful probe for detecting transannular S - S interactions in bicyclic or cage S-N molecules or ions. The strongly Raman active vibrations occur at frequencies in the range 180-300 cm-1, and for S- -S distances in the range 2.4-2.7 A. On the basis of symmetry considerations, the Raman spectrum of the mixed sulfur-selenium nitride S2Se2N4 was assigned to the 1,5- rather than the 1,3- isomer.37... 

Since polynuclear carbonyls take a variety of structures, elucidation of their structures by vibrational spectroscopy has been a subject of considerable interest in the past. The principles involved in these structure determinations were described in Sec. 1-10. However, the structures of some polynuclear complexes are too complicated to allow elucidation by simple application of selection rules based on symmetry. Thus the results are often ambiguous. In these cases, one must resort to X-ray analysis to obtain definitive and accurate structural information. However, vibrational spectroscopy is still useful in elucidating the structures of metal carbonyls in solution. 

Combining DFT/B3LYP theoretical results, normal coordinate analysis with symmetry considerations from a SQM force-field approach, IR and Raman data, a first-time complete, accurate vibrational frequency assignment was carried out on Dimethoate The error over all modes was about 1.1% for the IR and 1.4% for the Raman frequencies. DFT/B3LYP calculations have also supported the IR, Raman and surface-enhanced Raman scattering (SERS) spectroscopy characterization of L-leucine- and L-valine phosphonate analogues. ... 

Considerable vibrational studies have been made, with Raman spectroscopy being a major contributor. Most of the materials have a center of symmetry and thus the gerade, low-lying phonon modes are only observed in the Raman experiment (see Section 1.17). A typical application is presented herewith. 

The considerations on the symmetries of the ground and excited states and the above conditions lead to the selection rule for infrared spectroscopy A fundamental vibration will be infrared active if the corresponding normal mode belongs to the same irreducible representation as one or more of the Cartesian coordinates. 

An electron diffraction analysis of / has been published 5D as a valuable supplement to the X-ray data. The best accord with experiment is obtained with a model assuming Czv symmetry as shown in Fig. lb. Remarkably, the vibrational amplitude of the Sj—Sea bond is foimd to be considerably larger than the Si—Se amplitude. On the whole these data are consistent with the ones obtained from X-ray analysis and ESCA spectroscopy (see Chapter IV 4.). 

As already noted, whether a mode is IR or Raman active can be determined in advance (although its intensity cannot) by group theoretical considerations and depends on its symmetry type. For trans-l,2-dichloroethane (because ofits point group symmetry) each mode is eif/ier IR active or Raman active, but not both, and so IR and Raman spectroscopy excite completely different vibrational fundamentals. In the Raman spectrum, only bands of Ug and bg symmetry will appear. These can be distinguished experimentally by measuring the depolarization ratio according to theory (Wilson et al. 1955), the depolarization ratio oftotaUy symmetric modes ag under 2/1) is less than 0.75 (these are called polarized bands) whereas that of aU other modes is 0.75 (depolarized bands). (See the depolarization ratios calculated for trans-l,2-dichloroethane in O Fig. 10-5.)... 

Low-wavenumber FT-IR spectroscopy is among the most generally applicable methods used in the study of the conformers for certain types of small molecules with few substituents. Infrared spectra can, and variable-temperature experiments may, be investigated in all phases. The gas-phase band contours observed for the infrared spectra, along with Raman depolarization data, provide considerable information on the molecular symmetry of the conformers. The limitation of the vibrational spectroscopic technique is that it is best applied to relatively simple molecules that contain at least one element of symmetry, and one or perhaps no more than two portions of the molecule capable of producing different conformations upon internal rotation. 

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