CONFORMATION AND LOW-FREQUENCY ISOTROPIC RAMAN-SPECTRA OF THE LIQUID NORMAL-ALKANES C4-C9

An analysis of the low-frequency (0-600 cm-1) vibrations of the n-alkanes C4 through C9 in the liquid state is reported. Emphasis is placed on the relation between conformation and isotropic Raman spectra, since Raman spectra are commonly used to characterize chain conformation in polymethylene (PM) assemblies. Isotropic Raman spectra of liquid n-alkanes were simulated through calculation with the use of a known valence force and two adjustable parameters that relate to conformer energy differences and Raman intensities. Good frequency and integrated intensity agreement were found between the observed and calculated spectra. The widths of single nonoverlapped bands in the observed spectra varied, however, as a result of large thermal fluctuations in the dihedral angles that define chain conformation. In general, conformers having a greater number of gauche bonds tend to have broader bands. Since band congestion in the spectra of the longer n-alkanes precludes detailed assignments, observed bands were assigned to sets of conformers, each set consisting of all conformers that have exactly the same number of gauche bonds. Two bands, unique because they appear in Raman spectra at constant frequencies for chains longer than C-8, were found to be associated with specific conformational sequences: t3g-, -gt3g-, and -gt3g'- give rise to the 343-cm-1 band and t2g-to the 405-cm-1 band. The vibrations of liquid C-9, characterized through a standing-wave analysis of the normal modes, are found to be similar to the vibrations of the ordered (all-trans) chain with respect to the internal-coordinate representation of the normal coordinates and the frequency dispersion of the vibrations. Our calculations indicate why the measurement of the low-frequency Raman spectra of PM chain assemblies becomes more difficult with increasing disorder. With increasing gauche bond concentration, the total integrated isotropic Raman intensity in the 0-600-cm-1 region decreases markedly, while the width of the distribution of intensity increases, also markedly.