Vibrational spectroscopy polarisability

Normal vibrational spectroscopy generates information about the molecular frequency of vibration, the intensity of the spectral line and the shape of the associated band. The first of these is related to the strength of the molecular bonds and is the main concern of this section. The intensity of the band is related to the degree to which the polarisability is modulated during the vibration and the band shape provides information about molecular reorientational motion. 

Vibrational spectroscopy (Raman and IR) Energies (with isotope perturbation), intensities and polarisations Identification of ligands coordinated to a metal centre... 

Demonstration of the M = C carbyne vibration by polarised Raman spectroscopy. 

Both infrared and Raman spectra are concerned with measuring molecular vibration and rotational energy changes. However, the selection rules for Raman spectroscopy are very different from those of infrared - a change of polarisability... 

The same ideas extend straightforwardly to deal with property surfaces describing the dependence of a molecular property on geometry, for example, the dipole moment and polarisability derivatives that control the activity of a vibrational mode in IR and Raman spectroscopy. Extension to the case of redundant internal coordinates, the typical situation for polyatomic molecules, is also straightforward. 

Raman spectroscopy seeks to analyse vibrational transitions in biological macromolecnles in a complementary way to IR spectroscopy. The physical basis of the technique is somewhat different as well. Initially, an intense beam of light of frequency Vy is used to irradiate a sample of molecules of interest in order classically to induce oscillating dipoles of equivalent frequency in the polarisable clouds of electrons. The time dependent magnitude of the induced dipole, /rind(t) obeys... 

For a mode to be IR-active the molecule must have an oscillating electric dipole moment, called the transition dipole or transition moment, when it vibrates in this mode. For a vibration to be Raman-active the molecule must have an oscillating electrical polarisability (see section 9.2) when it vibrates in the mode. This means essentially that the shape of the molecule must be different in opposite phases of the vibration. These requirements constitute the selection rules for the two kinds of spectroscopy. Figure 2.10 shows the four modes of vibration of the CO2 molecule and their IR and Raman activities. 

In Table 3 the orientation information which can be obtained from these various structural techniques is summarised. This table also shows the part of the molecular structure which is being characterised, and some of the theoretical and experimental limitations of each method. A further technique, that of polarised fluorescence has been added. This technique is exactly analogous in its orientation aspects to Raman spectroscopy. The distinction between the two techniques lies in the fact that in the Raman effect, the lifetime of the process is of the order of the vibrational period ( 10 s) whereas fluorescence occurs after much longer occupancy of the transition state ( 10 s). 

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). 

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