Deuterium quadrupole coupling tensors in methyl halides: Ab initio effective core potential and liquid crystal nuclear magnetic resonance study

Abstract

The deuterium quadrupole coupling tensors were calculated for the methyl halides CD3X (X=F, Cl, Br, and I) using ab initio effective core potential (ECP) methods up to the QCISD level. The effects of basis set quality and electron correlation were investigated, and a comparison with all-electron calculations was made. We also determined a new experimental value for methyl chloride by the liquid crystal nuclear magnetic resonance (LC NMR) method. Locally dense basis sets were used for the deuterium atoms and ECPs for the halogens. The Br and I ECPs were scalar relativistic. Computations at the equilibrium (re) geometry result in slightly too negative tensor elements along the molecular C3 symmetry axis for all X, as compared to the experimental LC NMR and microwave spectroscopic results. Values corresponding to the rα(300 K) geometry are in a very good agreement with experiment. As the tensors are sensitive to the length of the associated CD bond, we applied approximate vibrational corrections to the theoretical results. The current corrected values for the deuterium quadrupole coupling constants and asymmetry parameters are likely to be the most reliable available for the present set of molecules, despite that residual computational errors still remain. The common approximation of a cylindrically symmetric electric field gradient tensor at the deuterium site is shown to lead to an underestimation of the corresponding quadrupole coupling constant in methyl halides.