Raman-active band

Vapor-phase intensity studies of the Raman-active bands of SeF6 and TeFg yielded, via electrooptical parameters, a Pauling covalent bond character of Se—F = 0.60 and Te—F = 0.47, and Pauling elec-... 

As the laser beam can be focused to a small diameter, the Raman technique can be used to analyze materials as small as one micron in diameter. This technique has been often used with high performance fibers for composite applications in recent years. This technique is proven to be a powerful tool to probe the deformation behavior of high molecular polymer fibers (e.g. aramid and polyphenylene benzobisthiazole (PBT) fibers) at the molecular level (Robinson et al., 1986 Day et al., 1987). This work stems from the principle established earlier by Tuinstra and Koenig (1970) that the peak frequencies of the Raman-active bands of certain fibers are sensitive to the level of applied stress or strain. The rate of frequency shift is found to be proportional to the fiber modulus, which is a direct reflection of the high degree of stress experienced by the longitudinally oriented polymer chains in the stiff fibers. 

The selection rule for Raman spectroscopy requires a change in the induced dipole moment or polarizability of the molecule, and so it is a complementary technique to infrared which requires a change in the permanent dipole moment. For molecules having a center of inversion, all Raman-active bands are infrared inactive and vice versa. As the symmetry of the molecule is lowered, the coincidences between Raman-active and infrared-... 

M. L. Walker and J. L. Mills (37), and later M. Jansen and M. Moebs (79), investigated the vibrational spectra of P407 (see Table XIV). In Ref. 79, a scheme correlating the Raman-active bands of P407 with those observed in the Raman spectra of P406 and P4O10 is given (see Fig. 9). 

Figure 2.7-4 Examples of tetra-atomic molecules belonging to different point groups. They can be distinguished by the number of polarized and depolarized Raman active vibrations, the infrared active vibrations and the coincidences of the frequencies of infrared and Raman active bands.

The IR spectrum of complexes such as these will therefore show a single band (the mode). If Raman data are unavailable, and the supposedly IR-inactive bands are not observed as weak features, reasonably accurate force constants can be obtained from equation (18). More usually, asymmetry in the ligand or distortion of the force field due to solvent-CO interactions makes both the Raman-active bands very slightly allowed in the IR. Indeed, trans-[ i-PrO)3P]2Mo(CO)4 shows pronounced effects due to the irregular packing of the phosphite ligands. Not only are the a g and big bands readily observable, but the band is rather broader than is usually found. In the absence of such data, isotopic spectra will provide enough information to resolve the problem. The monosnbstitnted molecnle is present at 4.4% (natural abundance). The force fields of this type of molecule invariably support Cotton s approximation... 

The XeFs anion has 12 normal vibrations, which are classified into A ](R)+A2(IR)+2 j(IR)+2 2(R)+ 2 (inactive) under D5/, symmetry. Thus, only three vibrations (A 2 and 2Il ) are IR-active and only three vibrations (A +2 2) are Raman-active. In agreement with this prediction, the observed Raman spectmm of (CH3)4N[XeF5] exhibits three bands at 502 (symmetric stretch, A j), 423 (asymmetric stretch, E 2), and 377 cm (in-plane bending, As discussed in Sec. 1.11, the trigonal-bipyramidal and tetragonal-pyramidal structures can be ruled out since many more IR- and Raman-active bands are expected for these structures. 

Raman spectroscopy does not suffer interference from atmospheric water vapor or carbon dioxide, as does IR. Gases do not scatter well, so even though Raman-active bands occur for these gases, the contribution to the Raman signal from air in the optical path is insignificant. Materials in the optical path outside of the laser focus also have negligible scattering. 

Free Nj has a Raman-active band at 2331 cm". Upon coordination to a metal, would you expect this band to shift to higher or lower frequencies Explain your answer and briefly explain what MOs are involved in the bonding of / -N2 to the metal. 

Raman studies by Loos and Jones [355] have, however, clearly shown that the two Raman active bands are due to different polyhalide species rather than to asymmetry in Presently it thus appears that l in aqueous solution is indeed symmetric and linear. 

It has been shown that the peak positions of the Raman-active bands of carbon fibres are strain-sensitive and that Raman Microscopy can be used to follow the deformation of carbon fibres both in air and in a thermoplastic PEEK matrix. It has been demonstrated that the fibres near the surface in the carbon-fibre/PEEK composite examined are subject to a residual compressive strain of the order of 0.287o which is of the same order as that expected (19) from matrix shrinkage due to crystallisation and thermal contraction on cooling from the processing temperature. It is found that when the composite is subject to an externally-applied tensile deformation then, as expected, the change in fibre strain is similar to the applied strain as expected from simple composite theory. 

Low Frequency Observations for Amorphous Polymers. Many amorphous materials studied by Raman exhibit an extremely broad band in the low frequency region. This is true for polycarbonate, poly(methyl methacrylate), and polystyrene (58). Low frequency Raman bands can potentially provide much information regarding the density of states (directly related to the intensity distribution of the broad Raman active band) as well as anomalous behavior observed for the specific heat (58). Often the separation of main-chain and side-chain bands is important in modeling of specific heat, making careful band assignment extremely meaningful (136,137). This separation is analogous to inclusion of optical... 

For polymethylene chains, the origin of these complexities may be described in terms of the appropriate binary combinations involving methylene bending modes interacting with the infrared or Raman-active symmetric stretching fimda-mental (154,155). Two levels of Fermi resonance interactions, intramolecular and intermolecular, need be distinguished, however. Unexpectedly, the Raman-active bands observed for different intermolecular packings are quite different (Fig. 16). 

Several Raman-active bands of pol5miers exhibit a frequency shift when the sample is stressed. 

Since the molecular symmetry is important in vibrational spectroscopy few explanations will be given shortly with regards to comments within the tables. For more details the reader is referred to basic literatures on the theory of molecular symmetry. Vibrations of functional groups of a polymer chain can be described by the so-called point groups. The point group theory means that during the vibration at least one point in the molecule remains unaffected. Furthermore, for molecular units with some symmetry, point group theory can be used to predict how many vibrational modes should appear in the IR and Raman spectra. As different molecular symmetries lead to different numbers of IR and Raman active bands this is a... 

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