Raman spectroscopy overtone

Equations (6.5) and (6.12) contain terms in x to the second and higher powers. If the expressions for the dipole moment /i and the polarizability a were linear in x, then /i and ot would be said to vary harmonically with x. The effect of higher terms is known as anharmonicity and, because this particular kind of anharmonicity is concerned with electrical properties of a molecule, it is referred to as electrical anharmonicity. One effect of it is to cause the vibrational selection mle Au = 1 in infrared and Raman spectroscopy to be modified to Au = 1, 2, 3,. However, since electrical anharmonicity is usually small, the effect is to make only a very small contribution to the intensities of Av = 2, 3,. .. transitions, which are known as vibrational overtones. 

In Section 6.1.3 it was noted that vibrational overtone transitions, whether observed by infrared or Raman spectroscopy, are very weak. They become even weaker as the vibrational quantum number increases. The high sensitivity of CRDS makes it an ideal technique for attempting to observe such transitions. 

Raman spectroscopy can in principle be applied to this problem in much the same manner as infrared spectroscopy. The primary difference is that the selection rules are not the same as for the infrared. In a number of molecules, frequencies have been assigned to combinations or overtones of the fundamental frequency of the... 

The chemistry of cluster complexes, e.g. of the sort [FeitSi, (SR) i,] 2, is of particular interest since such complexes are known to be close representations or synthetic analogues of the redox centres present in various iron-sulphur proteins. It is important to know whether the valence electrons are localized or delocalized in such complexes - in fact several studies by e.s.r., n.m.r., and, more recently, resonance Raman spectroscopy have shown that such clusters are delocalized rather than trapped-valence species. This result is linked with the most important biophysical property of iron-sulphur proteins, viz. that of electron transfer. Rapid electron transfer is possible if any consequential geometric rearrangements around the metal atom sites are small, as implied by many resonance Raman results on such cluster complexes (cf. the small-displacement approximation, which provides a basis for enhancement to fundamental but not to overtone bands) (22). Initial studies of [MSi,]2- ions (M = Mo or W) (23,24) have since been supplemented by studies of dinuclear species e.g. [(PhS)2FeS2MS2]2 (25) and cluster species... 

Infrared (IR) and Raman spectroscopy (71PMH(4)265) are of limited value in the conformational analysis of more complex molecules, since it is usually impossible to identify the bands, and to distinguish between fundamentals and overtones and combination tones. 

In the region of NIR FT Raman spectroscopy, at F = 5000...10000 cm , substances with X-H bonds (X = any element) show overtones and combinations of the normal frequencies. They may have considerable intensity as demonstrated by Fig. 3.5-3 with the NIR absorption spectra of liquid H2O, D2O, ethanol, and cyclohexane. The linear decadic absorption coefficient a of water at a Raman shift of about 2500 cm is of the order of 10. The transmission of a layer of d = 1 cm is given by ... 

For the discussion of stretching vibrations of all types of bonds the aforementioned tables are recommended (Weidlein et al., 1981 and 1986). Only one topic in inorganic chemistry should be mentioned here metal-metal bonds are often identified by their characteristic vibrations. They are usually observed in the Raman spectrum or in the Resonance Raman (RR) spectrum. In this way a variety of polynuclear metal species were detected in solid noble gases (Moskovits, 1986). In addition to the frequency range of these vibrations, which allow the characterization of certain species, overtones observed in the RR spectrum are important for the calculation of dissociation energies. Raman. spectroscopy was used successfully to characterize metal-metal bonds in new compounds which are stable at room temperature the first compound with an Al-Al bond was detected in this way (Uhl, 1988). 

Also linear chain complexes such as Pt(etn)4Cl3 which is known colloquially as Wolf-fram s red (etn being an abbreviation for ethylamine) have been studied successfully by resonance Raman spectroscopy (Clark, 1984). As example, we show in Fig. 6.1-12 the resonance Raman spectrum of a related halogen-bridged linear-chain species, [Pt(pn)2] [Pt(pn)2Br2] [Cu3Br5]2 (Clark et al., 1980). Although this species contains a complicated copper bromine chain, the resonance Raman spectrum (Fig. 6.1-12) is completely dominated by bands attributed to the v fundamental and its overtones n U[ of the platinum-bromine chain. 

Compared to the measurement of VCD the measurement of optical activity in the Raman spectrum offers all the well known advantages that Raman spectroscopy has over infrared spectroscopy the use of the inexpensive glass as the sample cell, and the occu-rance of fewer bands, overtones and combination bands are reduced in intensity, thereby diminishing the possibility of overlap. Very important also is the fact that water is usable as solvent. 

Fig. 7.2 Most important Raman lines of single-wall carbon nanotubes as excited with three different laser lines. RBM radial breathing mode, D defect-induced line, G graphitic line, D2 overtone of D-line, G2 overtone of G-line. The thin straight lines indicate the dispersion of the modes. All spectra in one slot were normalized to unit height (Reprinted with permission from Kuzmany H, Plank W, Schaman CH, Pfeifer R, Hasi F, Simon F, Rotas G, Pagona G, Tagmatarchis N (2007) Raman scattering from nanomaterials encapsulated into single-wall carbon nanotubes. Journal of Raman Spectroscopy 38 (6) 704—713, John Wiley Sons, Ltd.)...

The final two examples of the determination of excited state distortions are large bimetallic compounds whose electronic absorption spectra are broad and featureless. We must turn entirely to resonance Raman spectroscopy to measure the distortions because all of the information in the electronic spectrum is buried under the envelope. Fortunately, the resonance Raman profiles contain a great deal of information. These molecules were chosen as illustrative examples precisely because the resonance Raman spectra are so rich. The spectrum contains long overtone progressions and combination bands. Excitation profiles of not only the fundamentals but also of overtones and combination bands will be used to determine the distortions. The power of time-dependent theory from Section III.F and experimental examples of the effects of A on fundamentals, overtones, and combination bands are shown. The calculated distortions provide new insight about the orbitals involved in the electronic transition. 

Near-infrared (NIR) absorption spectroscopy is another technique of importance to the context of the development of analytical Raman spectroscopy. The method is generally referred to as NIR, despite the unfortunate confusion with NIR-Raman. NIR absorption is based on overtone and combination bands of mid-IR transitions, as shown in Figure 1.1. Such transitions are quantum mechanically forbidden and significantly weaker than mid-IR fundamentals. However, the higher energy photons involved in NIR absorption are transmitted by fiber optics and common optical materials, and the method has... 

An example of a molecule displaying Fermi resonance is carbon dioxide. The O-C-O bending mode is seen at 667 cm-1 in the IR. The overtone (in Raman spectroscopy) is expected near 1330 cm1. However, the symmetric C-0 stretching... 

My talk will not be technique-oriented, but it is appropriate to mention briefly how high overtone spectra are observed. They are of course much weaker than the fundamental vibrations when they are observed directly by the methods of Infrared or Raman spectroscopy. However they may be observed as conventional Infrared absorption spectra... 

This holds true especially for vibronically coupled systems. In absorption spectra, vibronic coupling effects are often hidden by Franck-Condon effects. Resonance Raman spectroscopy makes it possible to selectively enhance such hidden vibronic coupling effects by measuring the REPs of inducing modes, in particular those that are nontotally symmetric. A proper experimental study should include both the intensity and the depolarization ratio across the absorption band and should cover not only the fundamental but also overtones. 

---