Unusual asymmetry of methyl deuterium EFG in thymine: a solid state deuterium NMR and ab initio MO study

Abstract

Solid-state ²H NMR spectroscopy is a powerful method of studying molecular motion in the solid state. The power of this technique derives from the facts that quadrupole coupling is the dominant interaction and the electric field gradient (EFG) tensor is usually axially symmetric for a deuteron bonded to carbon, with the unique axis along the C-D bond. In the case of a methyl group, rapid 3-fold rotation yields an averaged EFG whose symmetry axis is along the rotation axis, and the corresponding powder pattern is axially symmetric (eta less than or equal to 0.01) with a quadrupole splitting of ca. 40 kHz. They are interested in obtaining spectra of thymidine and ribothymidine, deuteriated at the methyl positions, to study motion of nucleotide bases in DNA and tRNA, respectively. In their initial work they are studying the dynamics of the model compound, thymine-methyl-d₃. Inversion-recovery ²H NMR spectra of this compound clearly showed the T₁ anisotropy predicted for 3-fold methyl jumps. The linear Arrhenius plot of correlation time, tau_c, against 1/T (where tau_c=tau₀ exp(E/RT)) yielded an apparent activation energy of 6.9 kJ/mol and a preexponential factor, tau/sub 0/, of 2×10⁻¹³. These results show that the ²H spin-lattice relaxation is determined by the 3-fold methyl motion. However, this motion does not account for the asymmetry in the observed line shape.