Symmetric stretch, infrared forbidden

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 characteristic band of the C104 ion, namely, the triply degenerate Cl—O stretching mode, occurs at about 1110 cm -1 and is usually observed as a very broad band. A weak band is often found at about 980 cm-1 due to the infrared-forbidden totally symmetric stretching frequency, which, as in the case of the sulfate ion, can acquire a little intensity due to perturbation by an unsymmetrical environment. In compounds such as the last two mentioned above, which contain unidentate C104, there are three bands, at about 1120 cm-1, 1040 cm-1 and 920 cm-1, in accord with expectation for C3v symmetry. 

With a little group theory, we can determine whether or not the vibration has a dipole derivative. The same symmetry selection rules apply to vibrations as to electronic transitions for a transition to be allowed, the direct product of the representations for the initial and final states must be one of the representations for the transition moment. The transition moments for electric dipole or infrared selection rules correspond to the functions x, y, and z. For Raman transitions, the transition moments correspond to any of the second-order functions of x, y, and z, such as xz or -I- y. The representation of the ground vibrational state is always the totally symmetric representation, so F, F is equal to Fy for fundamental transitions. Therefore, the selection rule for fundamental transitions is F, (x) Fy = F = F. For example, the group theory predicts that for CO2 the transitions V2 = 0 1 and V3 = 0 1 are infrared-allowed, because those vibrational modes have TTu (x,y) and (z) symmetry, respectively. On the other hand, the symmetric stretch transition Vj = 0 1 is forbidden by infrared selection rules but allowed by Raman selection rules, because that vibrational mode has (x + y, z ) symmetry. Here are the relevant rows from the character table in Table 6.4 ... 

From a spectroscopic viewpoint, although the two fundamental absorption bands of CO2 do not present a major problem for flow-cell-based SFC/FT-IR measurements, a broad doublet in the region near 1400 cm does. These bands are assigned to the first overtone of the bend and the (infrared forbidden) symmetric stretch, which exhibit second-order coupling, or Fermi resonance (see Section 1.2). In the supercritical state, this doublet becomes weakly allowed (see Figure 23.8). 

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 ". ... 

Extensive compilations of IR and Raman frequencies are available, in some cases with FTIR and Raman spectra plotted together for comparison (6-9). A few frequencies for organic compounds are listed in Table 2.1, in part to illustrate differences in IR and Raman intensities. Symmetric vibrations such as the acetylenic C — C stretch, the —S — S— stretch, and ring breathing modes are generally strong in the Raman but forbidden in the infrared, while... 

There are a number of weaker bands in both spectra which cannot be due TiCl4 the symmetric TiCLt stretching mode at 388 cm is forbidden in the IR and the TiCl deformation modes are below 200 cm and cannot be observed [12]. On the other hand, the surface products of reactions (3) and (4) are expected to have TiCl stretching modes which lie between 500 and 388 cm and which would be allowed in the infrared, and some of the weak bands observed in this spectral region are undoubtedly due to these modes. 

Symmetrically substituted 5-tetrazines have a center of symmetry, so the quadrant ring stretching band at 1600-1500 cm is forbidden in the infrared. When the two substituents are electronically different, a strong band may result in this region. The semicircle ring stretching vibration (two components) remains active regardless of symmetry and results in absorption at 1495-1320 cm . Another tetrazine band is found at 970-880 cm . ... 

The N=N stretching vibration of the tram symmetrically substituted azo group is forbidden in the infrared spectrum as this is a symmetrical vibration in a molecule having a center of symmetry. It is active in the Raman effect, however... 

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