P. Yuan

Raman bandshape analysis of the symmetric bending vibration in liquid chloroform

Abstract In order to determine whether accurate rotational diffusion coefficients in liquids may be determined from the bandshapes of isotopically broadened vibrational peaks, we have investigated the isotropic and anisotropic Raman spectra of the ν3(A1), CCl3 symmetric bending, vibration in CHCl3 as a function of temperature in the liquid phase. The spectral lineshapes were fitted by a model containing four Lorentzian/Gaussian summation bands with relative peak intensities equal to the relative abundances of the four isotopic combinations and frequency displacements constrained to values measured in the matrix infrared spectrum. The calculated room temperature perpendicular diffusion coefficient, D⊥ (25°C) = 8.31010 s−1, was within the range of values reported from Raman measurements on the ν1, symmetric carbon-hydrogen stretching, vibration, but was somewhat lower than published results from NMR relaxation time measurements, T1(2D), on CDCl3, and from dielectric relaxation. The activation energy, Ea(D⊥), determined from the ν3 bandshape measurements was 30% higher than the average value from the NMR and dielectric studies. The deviation is believed to result from the sensitivity of this quantity to the fractional Lorentzian character of the fitting functions.

Temperature dependence of reorientational diffusion in hexafluorobenzene

Abstract Raman bandwidths of the CF stretching [ν 1 (a 1g ) and ν 15 (e 2g ] and ring stretching [ν 2 (a 1g ) and ν(e 2g ] vibrational modes of hexafluorobenzene were analyzed as a function of temperature in the liquid phase. Values of the perpendicular (‘tumbling’) diffusion coefficients, D???, calculated from agreed well with earlier reports in the literature. It was determines both of the e 2g vibrational bandshapes were artificially broadened by presence of closely lying combination modes superposed upon the bandsh of the fundamental vibrations. Following decomposition of the bands in their individual components, the parallel (‘spinning’) diffusion constant, D 1 , calculated from ν 15 and ν 16 were in excellent agreement with each other at all temperatures, and also agreed quite well with values of D 1 predicted by the Free Rotor model of molecular reorientation.

Raman and NMR study of parallel reorientational diffusion in nitromethane

Abstract Raman spectral bandshapes of the ν 7 , antisymmetric CD 3 stretching mode, and deuterium spin-lattice relaxation times in nitromethane- d 3 were measured as a function of temperature in the liquid-phase. Perpendicular diffusion coefficients ( D ⊥ ) calculated from dielectric and 14 N relaxation times in the literature were in excellent agreement at all temperatures. Unlike earlier studies in acetonitrile and iodomethane, parallel diffusion constants ( D z.snfc; ) determined from the Raman lineshape were substantially greater than those calculated from T 1 ( 2 D ), possibly resulting from broadening of the vibrational band due to hydrogen bond interactions. Also in contrast to the earlier investigations, values of D z.snfc; obtained from both methods were substantially lower than coefficients calculated from the Free Rotor model of reorientation, providing clear evidence that intermolecular self-association slows the spinning motion of nitromethane in the liquid-phase.