Electronic Charge Flow and Raman Trace Scattering Intensities for CH Stretching Vibrations inn-Pentane

The gas phase Raman trace scattering spectrum of the isolated CH stretching modes in n -pentane- d 11 has been recorded. Relative differential trace scattering cross sections for unique CH bond types have been determined 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 gauche − gauche . The change in the mean molecular polarizability associated with the stretch of a single CH bond is found to be a maximum for the terminal 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 dependence. The wave functions from these calculations have been analyzed with the theory of atoms in molecules. Molecular polarizability and its derivative during a molecular vibration are analyzed in terms of charge flow within the molecule. It is found that (1) a molecule is most polarizable along the chain and (2) the polarizability along the chain is the most sensitive to changes in dimension, hence the observed dependence on structure and conformation. These results are contrary to the conventional models of bond polarizability and electrooptical parameters.