Isotope effects on the degree of order and the deuterium quadrupole coupling constants, as measured by N.M.R. of oriented benzene-d1, 1,4-benzene-d2and 1,3,5-benzene-d3

Experimental and quantum-chemical determination of the2H quadrupole coupling tensor in deuterated benzenes

Deuterium Quadrupole Coupling Constant (DQCC) in benzene was determined both experimentally by Nuclear Magnetic Resonance spectroscopy in Liquid Crystalline solutions (LC NMR) and theoretically by ab initio electronic structure calculations. DQCCs were measured for benzene-d(1) and 1,3,5-benzene-d(3) using several different liquid crystalline solvents and taking vibrational and deformational corrections into account in the analysis of experimental dipolar couplings, used to determine the orientational order parameter of the dissolved benzene. The experimental DQCC results for the isotopomers benzene-d(1) and 1,3,5-benzene-d(3) are found to be 187.7 kHz and 187.3 kHz, respectively, which are essentially equal within the experimental accuracy (+/-0.4 kHz). Theoretical results were obtained at different C-D bond lengths, and by applying corrections for electron correlation and rovibrational motion on top of large-basis-set Hartree-Fock results. The computations give a consistent DQCC of ca. 189 kHz for three different isotopomers; benzene-d(1), 1,3,5-benzene-d(3), and benzene-d(6), revealing that isotope effects are not detectable within the present experimental accuracy. Calculations carried out using a continuum solvation model to account for intermolecular interaction effects result in very small changes as compared to the data obtained in vacuo. The comparison of theoretical and experimental results points out the selection of the underlying molecular geometry as the most likely source of the remaining discrepancy of less than 2 kHz. Such an agreement between the calculated and the experimental DQCC results can only be achieved if rovibrational effects are considered on one hand in the experimental direct dipolar coupling data, and on the other hand in the theoretical property calculation, as is done presently.

A new method for the determination of the dynamic isotope effect and the deuterium quadrupole coupling constant in liquids

Using a self-consistent NMR method, it is now possible to determine both the deuterium quadrupole coupling constant as well as the rotational dynamic isotope effect in liquids. We successfully tested the method on benzene for temperatures between 280 K and 293 K. The average value of 185(3) kHz for the coupling constant is compared with measurements in solid state and gas phase. As might be expected for liquids without hydrogen bonds, no difference can be detected. A rotational dynamic isotope effect of 6(3)% is observed. This value is significantly smaller than 20(5)%, the only other result reported in the literature. The results are corrected for the influence of vibrational motion. Copyright 1998 Academic Press.