Raman and IR activity

There are differences between the kinds of groups that absorb in the IR and those that are Raman active. Parts of Raman and IR spectra are complementary, each being associated with a different set of vibrational modes within a molecule. Other vibrational modes may be both Raman and IR active. The intensity or power of a Raman peak depends in a complex way on the polarizability of the molecule, the intensity of the source, and the concentration of the active group, as well as other factors. Raman intensities are usually directly proportional to the concentration of the active species. 

We see that the ax fundamental vibrations of BC13 transform as A, A2, and 2E. Each representation describes two vibrational modes of equal energy. Thus the 2E notation refers to four different vibrations, two of one energy and two of another. The a mode is Raman active, the A2 is IR active, and the modes are both Raman and IR active. 

Both A - and Ei-modes are Raman and IR active. The two nonpolar E2-modes E and E are Raman active only. The Bi-modes are IR and Raman inactive (silent modes). Phonon dispersion curves of wurtzite-structure and rocksalt-structure ZnO throughout the Brillouin Zone were reported in [106-108]. For crystals with wurtzite crystal structure, pure longitudinal or... 

Yoshida and coworkers reported the Ai (Raman active) and (Raman and IR active) vibrations of a number of cyclopropenium ions (Table 19). The nature of the substituent effects led to the suggestion that the variation in frequency is due to an electronic effect of the substituent, since if a mass effect of the substituent were dominant the sequence of shifts to higher frequency would be H < Me < NMe2 < CP. Some IR and Raman spectra of cyclopropenium ions are shown in a review by Schrader ... 

The optically active modes of a-(Ala) are classified into A (8 = 0°), E (8 = 99.6°), and Eg (8 = 199.1°) species, where 8 is the phase difference between motions in adjacent units of the helix. The A and Ej species are both Raman- and IR-active, with the former exhibiting parallel and the latter perpendicular dichroism in the IR. The E2 species is active only in the Raman. There are 28 A species modes, 29 doubly degenerate Ei species modes, and 30 doubly degenerate E2 species modes (Fanconi et al., 1969). 

An isolated n-atom molecule has 3n degrees of freedom and in—6 vibration degrees of freedom. The collective motions of atoms, moving with the same frequency and which in phase with all other atoms, give rise to normal modes of vibration. In principle, the determination of the form of normal modes for any molecule requires the solution of equation of motion appropriate to the n-symmetry. Methods of group theory are important in deriving the symmetry properties of the normal modes. With the aid of the character tables for point groups and the symmetry properties of the normal modes, the selection rules for Raman and IR activity can be derived. For a molecule with a center of symmetry, e.g. AXe, octahedral molecule, a non-Raman active mode is also IR active, whereas for the BX4 tetrahedral molecule, some modes are simultaneously IR and Raman active. 

The selection rules for Raman and IR active vibrations are different. A vibrational mode is Raman active if the polarizability of the molecule changes during the vibration. Changes in polarizability (for Raman spectra) are not as easy to visualize as changes in electric dipole moments (for IR spectra) and in most cases it is necessary to use group theory to determine whether or not a mode will be Raman active. 

Both Raman and IR spectra have been observed for all the XF cations. These spectra are rather simple ones since both the cations and MF anions are of the highly symmetrical octahedral structure, Oh, with a center of symmetry. Therefore the selection rules for Raman and IR active vibrations are mutually exclusive. Only three Raman lines, vi, V2 stretchings and 1 5 bending mode are expected, whereas two non-coinciding IR lines, stretching and P4 bendingmode are expected. Indeed, the imported spec-... 

The SFG technique probes the second-order nonhnear hyperpolarizability tensor this tensor includes the Raman and IR susceptibihty, which requires that the molecular vibrational modes are both Raman and IR active. Since Raman- and IR-dipole moment transition selection rules for molecules with a center of symmetry indicate that a vibrational mode is either Raman or IR active but not both, only molecules in a non-centrosymmetric environment on the surface interact with the electric fields molecules in the isotropic bulk phase show inversion symmetry where the third rank hyperpolarizability tensor goes to zero [25-27]. 

It is convenient to plot the Raman spectrum as intensity versus shift in wavenumbers in cm", because these can be related directly to IR spectra. The Raman shift in cm is identical to the IR absorption peak in cm" for a given vibration, because both processes are exciting the same vibration. That is, if a vibration is both Raman and IR active, it will be seen at the same position in wavenumbers in both spectra, although with different intensity. The Raman spectra for benzene and ethanol are shown in Figure 4.63, along with the related IR transmission spectra. 

Why is the fundamental stretching vibration of NO both Raman and IR active ... 

Since the Raman and IR activity are subject to different selection rules, as exposed above, the techniques are used as complementary characterization tools. For example, in the case of a homonuclear diatomic molecule, its stretching mode will be Raman active, but since there is no change of the dipole moment, it will be IR inactive, while a heteronuclear diatomic molecule will be both, Raman and IR active. 

If the local structure deviates from the average structure, the crystal symmetry is usually broken and the symmetry is lowered. For instance, the local symmetry may be rhombohedral or tetragonal but if the direction of the axes vary randomly from site to site, the system would appear cubic as a whole. Then, even though the crystal structure is cubic, the solid would have many properties reflecting the lower local symmetry, such as the mixing of the Raman and IR active modes and non-cubic crystal-field effects. Many high-symmetry solids are of this type as we will see later. 

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