Bond polarizability model

Raman intensities of the molecular vibrations as well as of their crystal components have been calculated by means of a bond polarizibility model based on two different intramolecular force fields ([87], the UBFF after Scott et al. [78] and the GVFF after Eysel [83]). Vibrational spectra have also been calculated using velocity autocorrelation functions in MD simulations with respect to the symmetry of intramolecular vibrations [82]. 

Interpretation of the v(CO) Raman intensities of a variety of carbonyls using a bond polarizability model... 

The Raman line shape is calculated with the bond polarizability model as described above. The unpolarized Raman line shape computed from the sum of the VV and VH line shapes is shown in Fig. 3. One again sees fair agreement between theory and experiment, with excellent peak position and evidence of a... 

As can be seen from the precedmg sections, the evaluation of reop for a given molecule is quite a cumbersome and tedious process. In order to simplify the calculation inocedure, Montero and Del Rio have put forward a compact formulation of die valence-optical dieory of Raman intensities known as bond polarizability model [296,297]. The bond polarizability model has been applied in extracting Raman intensity parameters for a number of molecules [263,302-311]. 

In the bond polarizability model the changes of bond lengdi and bond direction during vibrational distortions are described by introducing a displacement vector 1. It is defined in terms of a set of polar coordinates, namely ... 

In zero-order approximation of the bond polarizability model the following three types of electro-optical parameters representing derivatives of the bond polarizability with respect to bond displacement coordinates are defined [296,297]... 

In this section the predictive power of the bond polarizability model is tested in calculating Raman intensities of propyne by transferring parameters from other molecules. 

The application of bond polarizability model at zero-order level, the results obtained are satisfactory. It can be concluded that the set of polarizability parameters transferred from ethane and acetylene is close to the actual set characterizing Raman intensities of propyne. Minor refinement of parameter values is required to obtain a very good fit between observed and calculated spectral curves. 

Calculated spectral characteristics for propyne obtained by transferring bond polarizability parameters are compared with those evaluated by RHF/6-3 lG(d,p) ab initio MO calculations [311] in Table 9.10 and Fig. 9.4. It is seen that the predicted spectrum obtained in applying the bond polarizability model is in better agreement with experiment than the ab initio estimated spectrum. More advanced quantum mechanical computations are, evidently, needed to satisfactorily calculate the Raman intensities of propyne. The computations employing transferable sets of polarizability derivatives are simple and give good results. 

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