Raman spectrum, overtone

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). 

As in the infrared spectrum, overtone bands with Ac > 1 are possible, but have much weaker intensity and are usually not observed.) The A/= -2, 0, and +2 branches of a vibration-rotation Raman band are called O, Q, and S branches, respectively, in an extension of the P, Q, R notation used in infrared spectra. 

As well as shifting the expected positions of vibrational bands, Fermi resonance affects their intensities. For example, the wave function of the (02°0) level of C02 has a considerable contribution from the harmonic-oscillator function corresponding to the (10°0) level since the latter is a fundamental level, the transition from the ground level (00°0) to the (02°0) level is much more intense than would be expected for an overtone band. This transition is Raman active, but IR inactive the Raman spectrum of C02 shows two very strong bands at 1285 and 1388 cm-1, corresponding to transitions to the (02°0) and (10°0) levels from the ground level. 

In the actual case of the CO molecule, which provides an excellent example, the three fundamental transitions have frequencies of 667, 1300, and 2350 cm-1. The first overtone of the 667-cm-1 vibration, which is doubly degenerate, has a frequency of 1334 cm 1, which is quite close to that of the 1300-cm"1 fundamental. Now it can be shown that the excited state for the 1300-cmfundamental and one component of the representation generated by the excited state corresponding to the first overtone of the 667-cm"1 vibration do belong to the same representation of the group ),, and hence a Fermi resonance occurs. Thus, in the Raman spectrum of CO two strong bands at 1285 and 1388 cm 1 are observed, instead of just one at —1300 cm-1. 

The macrocyclic phthalocyanine ligand will form a complex Pt(phthalocyanine).1106 The crystal structure shows two polymorphs present because of molecular packing.1107 The platinum is in a square planar coordination geometry with a mean Pt—N distance of 1.98 A. The complex can be partially oxidized with iodine to give conducting mixed valence solids.1108 Eighteen fundamental and overtone combination bands are observed in the resonance Raman spectrum of platinum phthalocyanine, and from this data the symmetry of the excited singlets are found to be Dy.. Qlv or D2.1109... 

The normal Raman spectrum obtained with 647.1 nm excitation serves as a comparison for the Raman spectra obtained with excitation frequencies of 488.0 and 514.5 nm, which lie within the 5- 5 absorption band. The tremendous enhancement of the i>,(Mo-Mo) alg mode, the high overtone progression in v, the increase in overtone bandwidth with increasing vibrational quantum number, and the increased intensity of the overtones relative to the fundamental as the excitation frequency approaches the electronic absorption maximum are all attributable to the resonance Raman effect. Polarization... 

Interpretable Laser Raman and Laser mass spectra have been obtained from certain other microstructures ( Ramsaysphaera ) (Fig. 32). The Laser mass spectra are characterized by CN and CNO ions (Fig. 33). Raman lines (Fig. 32) appear at 1360, 1600, 2720, 2960 cm-1 within the organic range of the spectrum. The strong line at 1360 cm-1 may be atrributed to a symmetric N—O vibration of the N02 group, the weaker line at 1600 cm-1 is characteristic of aromatic double bonds C = C. The first overtone of the 1360 cm-1 line is observed at 2720 cm-1. The spectrum has the features of a resonant Raman spectrum. It is very often obtained with this type of product in which a large delocalisation of electrons is possible. 

The resonance Raman spectrum of K4[Mo2C18] has been reinvestigated using 488.0 and 514.5 nm excitation. An enormous enhancement of the intensity of the Mo—Mo stretching mode relative to the intensity of other fundamentals was observed and an overtone progression in Vj to 5vj identified. From these data the harmonic frequency and anharmonicity constant X, were calculated as 347.1 + 0.5 cm -1... 

The resonance Raman spectrum of a thin film of selenium exhibits 10 signals in the region 115-1400 cm-1, which have been assigned to the fundamentals P2(ai) and vio(e3) of Seg and to their overtones and combination vibrations 44). 

In the specific example of ACN (46) (point group C3V), there is one C-H stretch and one C-H bend of ai-symmetry and a pair of doubly degenerate stretches and bends of e-symmetry (84). In Fig. 5, the ACN Raman spectrum in the C-H bending region can be seen. The spectrum consists of a sharper ai-symmetry bend 1372 cm-1 and a broader e-symmetry bend at 1440 cm 1. The e-symmetry bend is broadened by Fermi resonance because the e-bend overtones (2 x 1440 cm 1) are degenerate with the e-stretches ( 3000 cm-1). The ai-bend and stretch have no Fermi resonance because the bend overtone (2 x 1372 cm 1) is not degenerate with the stretch (2943 cm-1). In the gas phase, the anharmonicity of the e-bend is 25 cm-1 [85], For an e-bend v = 1 0 transition at 1440 cm-1, the... 

Evaluated from overtone observed in the Raman spectrum. 

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). 

A further aid in assigning a band is its observation in the Resonance Raman (/ / ) spectrum. In RR spectroscopy, the exiting frequency coincides with a symmetry-allowed electronic transition. In general, only the totally symmetric vibrations are therefore enhanced in the RR spectrum (c.f. Sec. 6.1). The entire RR spectrum consists only of a few bands and their overtones. In the following highly symmetric species the totally symmetric breathing modes are easily identified ... 

Ethylene has no dipole moment and a center of symmetry and therefore the Raman spectrum is an important source of structural information. After the early work on the rotational (Dowling and Stoicheff, 1959) and rovibrational Raman spectrum (Feldman et ah, 1956) these spectra were thoroughly studied in a series of publications (Hills and Jones, 1975 Hills et ah, 1977 Foster et ah, 1977). Overtones and combination bands were measured in an intracavity Raman experiment by Knippers et ah (1985). The Q-branch of the U2 band was resolved by pulsed CARS spectroscopy in a molecular beam experiment (Byer et ah, 1981). 

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. 

Raman spectrum of I2 is observed. A solution of the Br2 cation also gives a resonance Raman spectrum with a fundamental of 360 cm and strong overtones 14). Edwards and Jones (17) reported that solid Br2" Sb3Pie has a Raman band at 368 cm which they attributed to the Brg cation. Table I shows the stretching frequencies, absorption maxima, and bond lengths of the halogens and the diatomic halogen... 

The calculated intensities for all of the observed bands attributable to fundamentals, overtones, and combination tones at the excitation wavelength 363.8 nm (to, = 27,488cm ) are compared to the experimental resonance Raman data in Figure 31. The set of cross-sections of excitation profiles for all modes at the experimental excitation wavelength 363.8 nm forms the resonance Raman spectrum. The intensities at 363.8 nm excitation in the Raman excitation profiles are represented by vertical broken lines in Figure... 

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