ELECTRONIC CHARGE FLOW AND RAMAN TRACE SCATTERING INTENSITIES FOR CH STRETCHING VIBRATIONS IN N-PENTANE

The gas phase Raman trace scattering spectrum of the isolated CH stret ching modes in n-pentane-d(11) has been recorded. Relative differentia l trace scattering cross sections for unique CH bond types have been d etermined from curve-fit analysis. The magnitudes of the differential scattering cross sections for the various CH bonds are found to depend on the position and orientation of a bond in the molecule as well as on the overall chain conformation: trans-trans, gauche-trans, or gauch e-gauche. The change in the mean molecular polarizability associated w ith the stretch of a single CH bond is found to be a maximum for the t erminal CH bond which lies in the plane of the carbon chain in the all -trans conformer. Ab initio molecular orbital calculations employing a split-valence basis set confirm this structural and conformational de pendence. The wave functions from these calculations have been analyze d with the theory of atoms in molecules. Molecular polarizability and its derivative during a molecular vibration are analyzed in terms of c harge flow within the molecule. It is found that (1) a molecule is mos t polarizable along the chain and (2) the polarizability along the cha in is the most sensitive to changes in dimension, hence the observed d ependence on structure and conformation. These results are contrary to the conventional models of bond polarizability and electrooptical par ameters.