Molecular reorientational motion in liquid acetonitrile: Raman band shapes, diffusion constants and activation energy of reorientation

Isotropic I α(ω) and anisotropic I β(ω) Raman band shape analyses of the ν1(a1) fundamentals in liquid CH3CN and CD3CN are reported. The ν2 and ν4 fundamentals are complicated by secondary structure and therefore were not studied in detail. Values of the perpendicular rotational diffusion constants D⊥, derived from the ν1(a1) fundamentals of both isotopic species are in good agreement with those obtained using NMR relaxation and dielectric methods and in addition agree closely with values calculated from microviscosity theory. The latter is based upon a model which involves a connection between the rotational and translational motions in liquids. In this model, reorientations perpendicular to the C3 axis proceed by small step Brownian diffusion and D⊥ (Raman) for CD3CN yields an activation energy for liquid state molecular reorientations perpendicular to the C3 symmetry axis of 2.0 ± 0.3 kcal/mole, in good agreement with existing NMR and microviscosity values. Comparison of Raman data and results reported here with available infrared results reveals that the D⊥(ir) values are too high by a factor of 2. The source of the infrared errors is discussed.