Molecular reorientation in acetonitrile. A comparison of diffusion coefficients from Raman bandshapes and nuclear magnetic resonance relaxation times
Abstract
Raman bandshapes of the CH3(CD3) and CN stretching vibrations of acetonitrile and acetonitrile-d3 have been measured as a function of temperature in the liquid phase. Nitrogen-14 spin–spin and deuterium spin–lattice relaxation times have also been measured for CD3CN at the various temperature. Perpendicular diffusion coefficients determined from the vibrational linewidths were approximately equal for the two isotopic species at all temperatures and agreed well with D⊥(NMR) from 14N relaxation times. Parallel diffusion constants calculated from the lineshapes of ν5(E), the antisymmetric CH3(CD3) stretching vibrations, were substantially lower in the deuterated molecule. D∥(Ram) for both isotopes were in near quantitative agreement with coefficients predicted by the Free Rotor model of reorientation, in both magnitude and dependence on temperature. Values of D∥(NMR) deuterium calculated from T1(2D) were found to be equal to those from the Raman bandshape to within the experimental error caused by uncertainties in the deuterium quadrupole coupling constant. The results provide further evidence that accurate parallel diffusion coefficients may be determined from analysis of the Raman bandshapes of degenerate vibrational modes in molecules of C3v symmetry.