Anisotropies of the reorientational motion of deuterated and protonated benzene by C-13 nuclear magnetic relaxation

The reorientational motion of protonated and deuterated benzene in the neat liquid was investigated by C-13 NMR relaxation methods. Therefore, the dip olar (DD) spin-lattice relaxation rates were determined by measurement of t he spin-lattice relaxation rates and the C-13-H-1/H-2 nuclear Overhauser fa ctors at a temperature of 293 K. The contribution of the chemical-shift ani sotropy (CSA) mechanism to the spin-lattice relaxation rate was obtained at 293 K by measurements at different magnetic flux densities. From the NMR d ata the rotational diffusion constants for rotations about the C-o axis (R- parallel to) and perpendicular to it (R-perpendicular to) were evaluated. T he rotational anisotropies, defined as the ratio R-parallel to/R-perpendicu lar to, for the protonated and deuterated benzene were determined to be 3.0 and 1.7, respectively. While for R-parallel to, a dynamic isotope effect, defined as the ratio of the rotational diffusion constants of the protonate d to the deuterated benzene, of approximately 30% was found, unexpectedly a negative isotope effect of approximately -25% was observed for R-perpendic ular to. Furthermore, the influence of C-H vibrations on the DD and CSA cou pling constants and thus on the isotope effects was discussed.