Depolarized vibrational band

According to Table 4-3, the former should exhibit two v(PtCl) (Ai(p) and E(dp)) and two v(PtBr) (A p) and E dp)) both in IR and in Raman spectra. Here, p and dp denote polarized and depolarized vibrations, respectively. This was found to be the case (trace 3a of Fig. 4-3). In contrast, the latter should exhibit three v(PtCl) 2A p) and B (dp)) and three v(PtBr) (2A (p) and Biidp)) both in IR and in Raman spectra. Trace 3b of Fig. 4-3 shows that two polarized bands of /li-type are observed for each vibration because fi-type vibrations are weaker than 4-type vibrations. Thus, comparison of Raman spectra of the two isomers should be made using 4-type (polarized) vibrations. 

The general approaches used in the studies considered below for assignment of the observed vibrational bands to the short-lived molecules are analogous to those described in Sections III and IV. The assignment of the revealed bands to normal, or fundamental, vibrational modes has been based on taking into account selection rules, observations of the bands in characteristic regions, observations of isotopic shifts, results of depolarization measurements in the Raman spectra and results of normal coordinate analysis. (It is noteworthy that Raman depolarization measurements can be conducted for matrix isolated species as well see Reference and references cited therein.) Lately, quantum-chemical calculations of vibrational spectra have become an important tool for both identification of CAs and assignment of their vibrational spectra. 

Type of Raman scattering Vibrational band character Polarizability tensor symmetry Depolarization ratio... 

Scattering in the range of lattice vibrations from 280 to 790 cm" was found to persist in the Raman spectrum after melting NaNH2- Deconvolution of the depolarized, broad band gave three peaks which were attributed to hindered rotational modes of the amide ion about its three principal axes [28]. 

The second ring vibration gives rise to a very weak infrared absorption band at 467 cm and to a weak and depolarized Raman line at 470 cm (202, 203) (Table 1-23). 

The frequencies classified in suites IX and X belong to depolarized Raman lines and correspond to vibrations-rotation bands of the C type. They can be assigned to oscillations of A" symmetry. 

The Raman spectrum can be used to give additional information regarding the symmetry properties of vibrations. This information derives from the measurement of the depolarization ratio p for each Raman band. The quantity p is a measure of the degree to which the polarization properties of the incident radiation may be changed after scattering... 

It is experimentally easy to generate Raman spectra using polarized light and to observe the partial depolarization of the spectra. Bands of totally symmetric vibrations are strongly polarized in Hquid or solution spectra. AH other bands in Hquid or solution are depolarized. Polarization effects are essential to elucidate stmctures, but are usuaHy ignored in most other appHcations. Details of the theory and experimental procedure can be found in the Hterature (15,16). 

Besides the vibrational frequency, symmetry is another important feature to describe a vibrational motion. The symmetry of a normal vibration can be determined by measuring the depolarization ration of the corresponding Raman band, which is defined as... 

The carbonyl stretching frequency of both the keto and enol tautomers can be recognized in the vibrational spectrum of pentane-2,4-dione. The enol has v(C=0) at 1618cm" , generally the dominant peak in the spectrum and more intense than the in- and out-of-phase v(C=0) stretching modes of the keto form, which are found at 1727 and 1707 cm" , respectively. These are identified by their Raman counterparts at 1719 cm" (polarized) and 1697 cm" (depolarized) (Ernstbrunner, 1970). The ratio of absorbances of the enol and the out-of-phase keto bands in the ir was used as an early method of analysis of the keto/enol equilibrium in different solvents (Le Fevre and Welsh, 1949). 

Vibrational Raman Scattering of radiation with changed frequency due lo polarizability change during a vibration (A = visible usually) Qualitative for large molecules 10-Ms 10 Pa (100 ion) (v dependent) Useful for characterization. Some structural information from number of bands, position, depolarization ratios, and possibly Isotope effects. All... 

---