Solvent effects Raman intensities

The intensities are plotted vs. v, the final vibrational quantum number of the transition. The CSP results (which for this property are almost identical with CI-CSP) are compared with experimental results for h in a low-temperature Ar matrix. The agreement is excellent. Also shown is the comparison with gas-phase, isolated I. The solvent effect on the Raman intensities is clearly very large and qualitative. These show that CSP calculations for short timescales can be extremely useful, although for later times the method breaks down, and CTCSP should be used. 

The use of laser Raman spectrometry in order to quantitatively investigate the urea synthesis under process conditions has been reported by Van Eck et al. (1983). Only Raman spectroscopy seems to suit the problem, since the visible radiation which is used to excite and detect Raman transitions can easily be directed to a measuring cell. Furthermore, water, which is an acceptable solvent, and all compounds involved in the synthesis show characteristic Raman bands. In order to compensate for many of the instrumental factors relative intensities were used instead of absolute intensities. Reproducible window mountings are a necessity. The effect of pressure and temperature on the Raman intensity have to be taken into account if measurements are to be carried out in situ (Sec. 6.8). The effect of the temperature is moderated by using an internal standard. 

The two most effective modes, either in vacuo and in solution, contain a main component related to C=C and C-C stretching of the chain, respectively. The changes induced by the solvent on these modes are evident both on frequencies and IR intensities what we observe is a significative decrease of frequency ( 117 cm ) and an important increase in IR intensity (by a factor of 2.5) for mode 1, and, for mode 2, a small increase of frequency ( 6 cm" ) accompanied by a very large increase in IR intensity (by a factor 8.5). Let us try to analyze these data in a more detailed way, eis they can give useful informations for a more physical description of a" and The direct relation between polarizability and spectroscopic data is explicit if we recall that IR intensity is proportional to (dn/dQ) , the parallel relation for /3 is a little more complex as it also involves Raman intensities, but for our scopes we can limit to consider the still present proportionality between yS and dfi/dQ, and then IR intensities. 

Comi, S., Cappelli, C., Del Zoppo, M., Tomasi, J. (2003). Prediction of solvent effects on vibrational absorption intensities and Raman activities in solution within the polarizable continuum model a study on push-pull molecules. J. Phys. Chem. A 107(48), 10261-10271. 

GAMES S is a program for ab initio molecular quantum chemistiy which can compute self-consistent field (SCF) wave functions ranging from restricted Hartree-Fock (RHF), ROHF, UHF, GVB, and MCSCF [47], Computation of the Hessian energy permits prediction of vibrational frequencies with IR or Raman intensities. Solvent effects may be modeled by the discrete Effective Fragment potentials or continuum models such as the polarizable continuum model [48]. Numerous relativistic computations are available, including infinite order two component scalar corrections, with various spin-orbit coupling options [49]. 

In order to observe the effect of the electric field on the orientation of the solvent, DMSO, Raman spectroscopy was employed. The molecule has a strong dipole moment, and can be expected to orient along the field direction (see Fig. 2.21). It is oriented very efficiently even in relatively low electric fields, but the orientation decreases over the maximum field intensity (see Fig. 2.22). The deformation of the gel becomes greater in the region of the higher field than that of the maximum orientation, suggesting that the solvent orientation is not directly related to the deformation of the gel. 

The term A plays the opposite role the polarization by the end groups induces a reduction of the bond alternation in both the ground and excited states [151,1521, thus producing more similar geometrical structures in the two relevant electronic states. The fact that the intensities are even larger with respect to those of the apolar species indicates that the structure is far from a perfectly equalized CC chain even in the ca.se of very efficient end groups. Notice, however, that the zwitterionic structure can be stabilized by solvent effects [151-153]. It is shown in Section X.C that the chain structure of compounds VI and Vll can be equalized by the choice of a suitable polar solvent in this case a marked decrease of the Raman cross section (by virtue of the reduction of AQk ) is observed. This finding is very relevant, especially if compared with the experimental data from Raman spectra of heavily doped polyacetylene, where a drastic reduction in the Raman cross section was observed (Section V). 

More recently, the PCM has been amply extended to the treatment of vibrational spectroscopies, by taking into account not only solvent-induced vibrational frequency shifts, but also vibrational intensities in a unified and coherent formulation. Thus, models to treat IR [8], Raman [9], IR linear dichroism [10], VCD [11] and VROA [12] have been proposed and tested, by including in the formulation local field effects, as well as an incomplete solute-solvent regime (nonequilibrium) and, when necessary, by extending the model to the treatment of specific solute-solvent (or solute-solute) effects. 

Shimada R, Kano H, Hamaguchi H (2008) Intensity enhancement and selective detection of proximate solvent molecules by molecular near-field effect in resonance hyper-Raman scattering. J Chem Phys 129 024505... 

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