Karplus-type relationship for quadrupole coupling constants and asymmetry parameters for substituted acetic acids

Abstract

The deuterium quadrupole coupling constant for the methyl group in acetic acid dimer was calculated as a function of the torsion angle about the carbon-carbon bond. The results show that when a deuteron approaches either of the negatively charged oxygen atoms, the quadrupole coupling constant for the deuteron is reduced by as much as 4.2 kHz. Both the calculated quadrupole coupling constant, e/sup 2/q/sub zz/Q/h, and asymmetry parameter, eta, are fitted with a Karplus-type equation: e/sup 2/q/sub zz/Q/h = A - 0.5491 cos theta - 1.7859 cos 2theta; eta = 0.0491 + 0.0058 cos theta - 0.0081 cos 2theta. Adiabatic demagnetization in the laboratory frame spectroscopy at 77 K for (4-chlorophenyl)(2,2-/sup 2/H/sub 2/)acetic acid showed two inequivalent deuteron sites that, on the basis of deuterium double transitions, are demonstrated to be due to two deuterium sites bound to the same carbon atom. The solid-state structure of (4-chlorophenyl)acetic acid was determined by single-crystal X-ray diffraction. The ADLF and structural data for (4-chlorophenyl)(2,2-/sup 2/H/sub 2/) acetic acid were used to obtain a preliminary value for the A parameter as 170.767 kHz.