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E.H. Hardy, P.J. Merkling, R. Witt, A. Dölle

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

The reorientational motion of protonated and deuterated benzene in the neat liquid was investigated by ¹³C NMR relaxation methods. Therefore, the dipolar (DD) spin-lattice relaxation rates were determined by measurement of the spin-lattice relaxation rates and the ¹³C-¹H/²H 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 C₆ 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.

Zeitschrift für Physikalische Chemie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0942-9352
Volume: 214, 12/2000
Pages: 1687

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