Forbidden Raman bands

Application of Raman spectroscopy to a study of catalyst surfaces is increasing. Until recently, this technique had been limited to observing distortions in adsorbed organic molecules by the appearance of forbidden Raman bands and giant Raman effects of silver surfaces with chemisorbed species. However, the development of laser Raman instrumentation and modern computerization techniques for control and data reduction have expanded these applications to studies of acid sites and oxide structures. For example The oxidation-reduction cycle occurring in bismuth molybdate catalysts for oxidation of ammonia and propylene to acrylonitrile has been studied in situ by this technique. And new and valuable information on the interaction of oxides, such as tungsten oxide and cerium oxide, with the surface of an alumina support, has been obtained. 

W. Holzer and R. Ouillon. Forbidden Raman bands of SF Collision induced Raman scattering. Chem. Phys. Lett., 24 589-593 (1974). 

Curiously, however, spectra due to some of the intermediates were not observed using both spectroscopic techniques, and the authors discuss possible causes for this. As with the previously discussed acetylene-zeolite systems (30,31), very strong Raman bands were observed for the v(ChC) modes which are very weak in the infrared (and forbidden in C2H2). 

Before discussing other examples, we note here that, for a centrosymmet-ric molecule (one with an inversion center), rx, ry, and rz are u (from the German word ungerade, meaning odd) species, while binary products of x, y, and z have g (gerade, meaning even) symmetry. Thus infrared active modes will be Raman forbidden, and Raman active modes will be infrared forbidden. In other words, there are no coincident infrared and Raman bands for a centrosymmetric molecule. This relationship is known as the mle of mutual exclusion. 

Stretching vibrations of atoms with different electronegativity modulate the molecular dipole moment, thus, they show strong infrared bands. Vibrations of bonds between equal atoms show infrared bands of very low intensity, however, they modulate the molecular polarizability and therefore show strong Raman bands. The intensity of the bands in the infrared spectrum is zero - the bands are forbidden in the infi ared spectrum - if the environment of both atoms is equivalent by symmetry. 

All iy(C=C) vibrations give rise to sharp and narrow Raman bands of high intensity. IR spectra, on the other hand, frequently exhibit comparatively weak C=C vibrations. A symmetrical substitution in trans position introduces a center of inversion at the center of the C=C bond due to the rule of mutual exclusion (Sec. 2.7.3.4) the C=C vibration must then be forbidden in the infrared spectrum. 

The N=N linkage has only very weak absorptions in the infra-red and if the molecule is in the trans form and is symmetrically substituted the band is forbidden. This band is therefore better identified in the Raman spectrum where it is strong. Herzberg [21] has identified the... 

The Rayleigh and Raman spectra discussed have backgrounds due to interaction induced effects. For some vibrations, (the u-s3nnmetry modes in CS2), the entire band is interaction induced. The DID mechanism cannot give rise to these Raman bands since 8a/3q = 0 (v2> V3, In a similar way the near infra-red spectrum of vi(z ) band is interaction induced. We shall use this as an example of the calculation of the spectrum of a forbidden band. The induced dipole in i due to the quadrupole on j is given by... 

Raman spectroscopy is a form of vibrational spectroscopy, much like infrared (IR) spectroscopy. It is to be noted, whereas IR bands arise from a change in the dipole moment of a molecule, Raman bands arise from a change in the polarizability. In many cases, transitions that are allowed in Raman are forbidden in IR, and therefore, these techniques are often complementary. 

Polymer chains are typically present as random coils in the amorphous regions, but are packed together in a regular three-dimensional lattice in the crystalline domains. In order to determine the crystallinity it is therefore necessary to identify the appropriate Raman bands that originate due to scatter from these three-dimensional crystalline domains. The inter-chain interactions within the crystals, lowers the symmetry and leads to band splitting or the activation of previously forbidden vibrational bands. These are tme crystallinity bands i.e., they require three... 

This broad band at 1500 cm was ascribed by Kaufman. Metin, and Saper-stein [10], to an IR observation of the amorphous carbon Raman D and G bands. This is forbidden by the selection rules, and has been attributed to the symmetry breaking introduced by the presence of CN bonds in the amorphous network. As carbon and nitrogen have different electronegativities, the formation of CN bonds gives the necessary charge polarity to allow the IR observation of the collective C=C vibrations in the IR spectrum. This conclusion was stated by the comparison of spectra taken from films deposited from N2 and N2. In the N2-film spectrum, no shift was observed for the 1500-cm band, whereas all other bands shifted as expected from the mass difference of the isotopes. Figure 25 compares... 

The number of fundamental vibrational modes of a molecule is equal to the number of degrees of vibrational freedom. For a nonlinear molecule of N atoms, 3N - 6 degrees of vibrational freedom exist. Hence, 3N - 6 fundamental vibrational modes. Six degrees of freedom are subtracted from a nonlinear molecule since (1) three coordinates are required to locate the molecule in space, and (2) an additional three coordinates are required to describe the orientation of the molecule based upon the three coordinates defining the position of the molecule in space. For a linear molecule, 3N - 5 fundamental vibrational modes are possible since only two degrees of rotational freedom exist. Thus, in a total vibrational analysis of a molecule by complementary IR and Raman techniques, 31V - 6 or 3N - 5 vibrational frequencies should be observed. It must be kept in mind that the fundamental modes of vibration of a molecule are described as transitions from one vibration state (energy level) to another (n = 1 in Eq. (2), Fig. 2). Sometimes, additional vibrational frequencies are detected in an IR and/or Raman spectrum. These additional absorption bands are due to forbidden transitions that occur and are described in the section on near-IR theory. Additionally, not all vibrational bands may be observed since some fundamental vibrations may be too weak to observe or give rise to overtone and/or combination bands (discussed later in the chapter). 

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. 

A set of SER spectra for adsorbed azide on silver, obtained for the same surface and solution conditions and for a similar sequence of electrode potentials as for the PDIR spectra in Figure 1, is shown in Figure 2. (See the figure caption and reference 7 for experimental details.) Inspection of these SER spectra in comparison with the PDIR results illustrate some characteristic differences in the information provided by the two techniques. Most prominently, in addition to the Nj" j/as band around 2060 cm"1, the former spectra exhibit three other features at lower frequencies attributable to adsorbed azide vibrations. By analogy with bulk-phase spectra for free and coordinated azide (15), the 1330 cm"1 SERS band is attributed to the N-N-N symmetric stretch, vt (2). The observation of both i/a and j/aa features in the SER spectra differs from the surface infrared results in that only the v band is obtained in the latter (2). The appearance of the vn band in SERS is of interest since this feature is symmetry forbidden in the solution azide Raman spectrum. 

The spectra of methane, adsorbed at 90° K., showed a weak band at 2,899 cm.", in addition to a strong band (vt) at 3,006 cm. h This weak band was assigned to the I l symmetrical breathing frequency of methane, which is normally observed only in the bulk state in the Raman spectrum at 2,916 cm. h No over-all dipole change is associated with the vi vibration consequently, it is forbidden in the infrared spectra of liquid and gaseous methane. The appearance of this band is a direct measure of the... 

Both the above forbidden bands lie at frequencies close to that of a normally allowed one. Once a molecule is distorted, the new band may gain considerably in intensity by resonance with the allowed one. When hydrogen was adsorbed, at a coverage of 0.2, a band appeared at 4,131 cm., the corresponding Raman frequency being 4,160 cm.. There are no allowed bands for hydrogen, and the appearance of this new band is entirely due to the effect of surface forces. 

For structure type Ila ligands both the v(8-8) and the totally symmetric v(M-8) vibrations are practically forbidden in the infrared spectrum, but in the Raman spectrum the corresponding bands (intense and strongly polarized) can easily be observed. 

Different hexaoxometallates have been studied by Hawk (149—152) and by Corsmit et al. (153). Baran and Muller (154) have clearly found by means of IR spectra that Ba5(ReOe)2 contains isolated ReOi octahedra. The known data are collected in Table 20 for different hexaoxometallates however, most of these measurements are incomplete. The IR data for V3 and V4 are only partly correct, since these bands are very broad for some of the lithium salts so that it is often difficult to differentiate between V3, V4, and the v(Li—O) vibrations (755). In most cases it is doubtful whether the IR bands forbidden for the free ion, vi, V2, vs are correctly assigned. Surprisingly enough V3 and V4 do not split in most of the IR spectra even though there is enough perturbation to allow the appearance of normally forbidden transitions. Several hthium hexaoxometallates have been investigated in the Raman effect (755). 

The N=N stretching vibration of a symmetrical trans-azo compound is forbidden in the IR but absorbs in the 1576 cm-1 region of the Raman spectrum. Unsymmet-rical para-substituted azobenzenes in which the substituent is an electron-donating group absorb near 1429 cm1. The bands are weak because of the nonpolar nature of the bond. 

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