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 C_3v symmetry.