Vibrational Raman spectra

Figure 2.28 Comparison of the Raman and ROA bands of - -)- P)-1,4-dimethylenespiro-pentane (a) in substance and (b) as a 20% by volume solution in trideuterioacetoni-trile measured in backscattering for the 1650 to 1830cm 1 region. From bottom to top Raman, ROA, degree of circularity. The relative scattering intensities in substance and trideu-terioacetonitrile solution were normalized so that the largest peak in the measured Raman spectra, vibration 8 at 609 cm-1, has the same height. The experimental parameters are as in Figure 2.27.

E/cR,cRj Force constants fundamental vibrational frequencies infrared and Raman spectra vibrational amplitudes vibration-rotation coupling constants... 

The vibrational-rotational spectra of a polyatomic molecule contains information about its main structural properties. Information about the bonds and structures formed by the atoms in the molecule can be extracted from the IR and Raman spectra. Vibration frequencies that characterize a definite bond are nevertheless very sensitive to the rest of the molecular components. This circumstance provides potentially high selectivity in the effect that resonance laser radiation can have on molecular vibrations, as well as the possibility of implementing laser control of molecules via their vibrations. The following tasks have been achieved ... 

In other words, in both infrared and Raman spectra, vibrational coordinates along these particular directions induce charge redistributions in the electronic cloud that are reflected in the vibrational intensities. Vibrational hyperpolarizabilities, which according to Eqs. (22) and (23) are expressed in terms of vibrational intensities, are obviously greatly affected. 

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

CAHRS and CSHRS) [145, 146 and 147]. These 6WM spectroscopies depend on (Im for HRS) and obey the tlnee-photon selection rules. Their signals are always to the blue of the incident beam(s), thus avoiding fluorescence problems. The selection ndes allow one to probe, with optical frequencies, the usual IR spectrum (one photon), not the conventional Raman active vibrations (two photon), but also new vibrations that are synnnetry forbidden in both IR and conventional Raman methods. 

Figure 6.7 Rotational transitions accompanying a vibrational transition in (a) an infrared spectrum and (b) a Raman spectrum of a diatomic molecule...

Figure 6.9 The 1-0 Stokes vibrational Raman spectrum of CO showing the 0-, Q-, and 5-branch rotational structure...

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... 

Because of fhe presence of heavy atoms in many inorganic molecules fhere may be several low-wavenumber vibrations. For tins reason if is generally more importanf fhan for organic molecules fo obfain fhe far-infrared or Raman spectrum. 

Fig. 23. Experimental room temperature Raman spectrum from a sample consisting primarily of bundles or ropes of single-wall nanotubes with diameters near that of the (10,10) nanotube. The excitation laser wavelength is 514.5 nm. The inset shows the lineshape analysis of the vibrational modes near 1580 cm . SWNT refers to singlewall carbon nanotubes [195].

The Raman spectrum of aqueous mer-cury(I) nitrate has, in addition to lines characteristic of the N03 ion, a strong absorption at 171.7 cm which is not found in the spectra of other metal nitrates and is not active in the infrared it is therefore diagnostic of the Hg-Hg stretching vibration since homonuclear diatomic vibrations are Raman active not infrared active. Similar data have subsequently been produced for a number of other compounds in the solid state and in solution. 

The room temperature Raman spectrum excited in pre-resonance conditions [351 indeed shows bands at 169 cm-1 and 306 cm, which are in agreement with the modes observed in the fluorescence spectrum and that have been assigned by ab initio calculations to totally symmetric vibrations jl3). 

Tabic 6-5. Comparison of (he aK vibrational modes in the ground and excited states. The totally symmetric vibrations of the ground stale measured in tire Raman spectrum excited in pre-resonance conditions 3S] and in the fluorescence spectrum ]62 ate compared with the results of ab initio calculations [131- The corresponding vibrations in the excited stale arc measured in die absorption spectrum. 

The short Os-Os bonds in Os2X correspond to triple bonds and give rise to stretching vibrations associated with bands around 280 cm-1 in the Raman spectrum (Table 1.6). 

For a vibration to be observable in the Raman spectrum there must be a change in molecular polarizability during the vibration. 

Hence we may conclude for a vibration to be active in the Raman spectrum it must have the same symmetry properties (i.e. transform in the same way),... 

The example of COj discussed previously, which has no vibrations which are active in both the Raman and infrared spectra, is an illustration of the Principle of Mutual Exclusion For a centrosymmetric molecule every Raman active vibration is inactive in the infrared and any infrared active vibration is inactive in the Raman spectrum. A centrosymmetric molecule is one which possesses a center of symmetry. A center of symmetry is a point in a molecule about which the atoms are arranged in conjugate pairs. That is, taking the center of inversion as the origin (0, 0, 0), for every atom positioned at (au, yi, z ) there will be an identical atom at (-a ,-, —y%, —z,). A square planar molecule XY4 has a center of symmetry at atom X, whereas a trigonal planar molecule XYS does not possess a center of symmetry. 

Clouter M. J., Kiefte H, Ali N. Anomalous behaviour in the vibrational Raman spectrum of oxygen under near-critical conditions, Phys. Rev. 

Fig. 4 Raman spectrum of the external and torsional vibrations of a single crystal of a- Sg (resolution <0.1 cm ), after [110]...

Figures 8 and 9 shows a part of the bending region at low temperature containing the components of Vg (150-160 cm ) and Vs (190-200 cm ). The Vg vibration, IR active in the free molecule, has weak components in the Raman spectrum. According to theoretically calculated Raman intensities, which almost perfectly fit the experimental spectrum, the big component has a very low scattering cross-section [87] and is accidentally degenerate with the b2g component at ca. 188 cm. The IR active components of Vg cause strong absorptions in the IR spectrum even if the crystalline sample used for transmission studies is as thin as 400 pm [107, 109].

Fig. 16 Raman spectrum of two-phonon processes in single crystalline a- 8g range 500-1000 cm , after [109]. The strong bands in the range 800-950 cm result from combinations of components of the stretching vibrations...

First attempts to model the vibrational spectrum of polymeric sulfur have been reported by Dultz et al. who assumed a planar zig-zag chain structure [172]. The calculated vibrational DOS was in qualitative agreement with the observed Raman spectrum of fibrous sulfur. However, some details of the spectrum like the relative intensities of the modes as well as the size of the gap between stretching and bending vibrations could not be reproduced exactly by this simplified model [172]. 

Fig. 29 Raman spectrum of p-S at high pressure and room temperature [109]. The wavenumbers indicated are given for the actual pressure. No signals of other allotropes have been detected. The line at 48 cm (ca. 25 cm atp 0 GPa) may arise from lattice vibrations, while the other lines resemble the typical pattern of internal vibrations of sulfur molecules...

Since the vibrational spectra of sulfur allotropes are characteristic for their molecular and crystalline structure, vibrational spectroscopy has become a valuable tool in structural studies besides X-ray diffraction techniques. In particular, Raman spectroscopy on sulfur samples at high pressures is much easier to perform than IR spectroscopical studies due to technical demands (e.g., throughput of the IR beam, spectral range in the far-infrared). On the other hand, application of laser radiation for exciting the Raman spectrum may cause photo-induced structural changes. High-pressure phase transitions and structures of elemental sulfur at high pressures were already discussed in [1]. 

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