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The reorientational motion of protonated and deuterated benzene in the neat liquid was investigated by  13 C NMR relaxation methods. Therefore, the dipolar (DD) spin-lattice relaxation rates were determined by measurement of the spin-lattice relaxation rates and the  13 C- 1 H/ 2 H nuclear Overhauser factors at a temperature of 293 K. The contribution of the chemical-shift anisotropy (CSA) mechanism to the spin-lattice relaxation rate was obtained at 293 K by measurements at different magnetic flux densities. From the NMR data the rotational diffusion constants for rotations about the  C  6  axis ( R  II ) and perpendicular to it ( R  ⊥ ) were evaluated. The rotational anisotropies, defined as the ratio  R  II / R  ⊥ , for the protonated and deuterated benzene were determined to be 3.0 and 1.7, respectively. While for  R  II  a dynamic isotope effect, defined as the ratio of the rotational diffusion constants of the protonated to the deuterated benzene, of approximately 30% was found, unexpectedly a negative isotope effect of approximately -25% was observed for  R  ⊥ . Furthermore, the influence of C-H vibrations on the DD and CSA coupling constants and thus on the isotope effects was discussed.

Anisotropies of the Reorientational Motion of Deuterated and Protonated Benzene by 13 C Nuclear Magnetic Relaxation