Tumbling and spinning diffusions of acetonitrile in water and organic solvents

The spin-lattice relaxation times T-1 for H-2 and N-14 of acetonitrile-d(3) (CD3CN) are measured in pure liquid, n-hexane, carbon tetrachloride, aceton e, methanol, water, and heavy water as functions of temperature and concent ration. From the T-1 values, the rotational diffusion coefficients for the tumbling and spinning motions are separately obtained. The tumbling motion is strongly dependent on the solvent and temperature and its diffusion coef ficient is qualitatively explained by a simple dielectric friction model. T he spinning motion is, on the other hand, weakly dependent on the solvent a nd temperature. The observed large anisotropy ratio (similar to 9) of the s pinning to the tumbling modes represents the anisotropy of the solvation sh ell and is explained by neither the free rotor model nor the hydrodynamic c ontinuum model. The tumbling motion is more strongly hindered than the spin ning motion due to the dipolar interaction between the solute and solvent. (C) 2000 American Institute of Physics. [S0021-9606(00)51203-3].