Raman spectroscopy forbidden spectra

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

Infrared spectroscopy is not as inherently informative with regard to metal interactions in highly symmetrical metal-metal bound dimers as is Raman spectroscopy, since the totally symmetric metal-metal stretch is a forbidden absorption in the infrared experiment. Oldham and Ketteringham have prepared mixed-halide dimers of the type Re2ClxBr 2xto lower the symmetry and hence introduce some infrared allowedness into the Re-Re stretching mode (206). Indeed, the appearance of a medium-intensity band at 274 cm 1 in the infrared spectrum of the mixed-halo species was considered to be the result of absorption by the metal—metal stretching vibration, which was also observed in the Raman spectrum at 274 cm ". ... 

Vibrational sum-frequency spectroscopy (VSFS) is a second-order non-linear optical technique that can directly measure the vibrational spectrum of molecules at an interface. Under the dipole approximation, this second-order non-linear optical technique is uniquely suited to the study of surfaces because it is forbidden in media possessing inversion symmetry. At the interface between two centrosymmetric media there is no inversion centre and sum-frequency generation is allowed. Thus the asynunetric nature of the interface allows a selectivity for interfacial properties at a molecular level that is not inherent in other, linear, surface vibrational spectroscopies such as infrared or Raman spectroscopy. VSFS is related to the more common but optically simpler second harmonic generation process in which both beams are of the same fixed frequency and is also surface-specific. 

Infrared spectroscopy is now nearly 100 years old, Raman spectroscopy more than 60. These methods provide us with complementary images of molecular vibrations Vibrations which modulate the molecular dipole moment are visible in the infrared spectrum, while those which modulate the polarizability appear in the Raman spectrum. Other vibrations may be forbidden, silent , in both spectra. It is therefore appropriate to evaluate infrared and Raman spectra jointly. Ideally, both techniques should be available in a well-equipped analytical laboratory. However, infrared and Raman spectroscopy have developed separately. Infrared spectroscopy became the work-horse of vibrational spectroscopy in industrial analytical laboratories as well as in research institutes, whereas Raman spectroscopy up until recently was essentially restricted to academic purposes. 

Infrared and Raman spectroscopy are complementary in structural determinations because some molecular vibrations that are inactive in the infrared (that is, do not result in a change in dipole moment and therefore do not cause an absorption band) do have a strong Raman line. The reverse is also true. Some bands that are weak or forbidden in the Raman spectrum are strong in the infrared spectrum. With the combined use of these techniques, the vibrational energies of a molecule can be fully described. 

One finds that, in molecules of high symmetry, both IR and Raman spectroscopy are needed to observe the vibrational modes. Even with both techniques, there may still be some vibrations that are totally forbidden. The best known selection rule for IR and Raman spectroscopy is known as the Rule of Mutual Exclusion , which states that if a molecule has a centre of symmetry, vibrations cannot be active in both IR and Raman spectroscopy. This rule has often been applied in molecular structure investigations to determine whether a centre of symmetry is present. In general, vibrations that do not distort the molecular symmetry, symmetric vibrations , are intense in the Raman spectrum while those that maximize the distortion are most intense in the IR spectrum. If the atoms involved in these vibrations are highly polarizable (e.g., sulfur or iodine) then the Raman intensity is high. Some examples of... 

Mid-IR absorption and Stokes Raman deal with the same vibrations but are subject to different selection rules (and consequently the spectra differ). IR and RS provide complementary images of molecular vibrations. Vibrations which modulate the molecular dipole moment are visible in the IR spectrum, while those which modulate the polarisability appear in the Raman spectrum. Compositions that do not absorb in the IR range generally give a Raman spectrum and strong IR absorbers will produce a weak spectrum by Raman. Examples of silent Raman vibrational modes are specific point groups (e.g. C(, De, Cev, C4h, D, >3h. Den, etc.). Other vibrations may be forbidden in both spectra. Raman spectroscopy complements IR spectroscopy, particularly for the study of non-polar bonds and functional groups e.g. C=C, C—S, S—S, metal-metal bonds). 

CAHRS and CSHRS) [145, 146 and 147]. These 6WM spectroscopies depend on (Im for HRS) and obey the tlnee-photon selection rules. Their signals are always to the blue of the incident beam(s), thus avoiding fluorescence problems. The selection ndes allow one to probe, with optical frequencies, the usual IR spectrum (one photon), not the conventional Raman active vibrations (two photon), but also new vibrations that are synnnetry forbidden in both IR and conventional Raman methods. 

To gain information about the active sites in a catalyst, a common approach is to use probe molecules. In the case of IINS spectroscopy, the molecule of choice is dihydrogen, H2. Because this is also often a reactant, it may provide some assurance of the relevance of what is measured. The low mass, 2amu, and short homonuclear bond, 0.746 A, of dihydrogen provide this molecule with a uniquely high rotational constant, B — 59.3 cm Figure 2a shows the IINS spectrum of solid H2 at 13 K. The intense, resolution-limited line at 118.6cm is the IR- and Raman-forbidden... 

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