17O and 2H quadrupole double resonance in some carboxylic acid dimers

Abstract

¹⁷ O quadrupole coupling constants and asymmetry parameters for the C¹⁷O and C—¹⁷OH nuclei in benzoic acid, eleven substituted derivatives and β-oxalic acid have been determined at 291 K by double resonance techniques together with the ²H values for the hydrogen-bonding deuteron. Many of these compounds form planar, or nearly planar, cyclic hydrogen-bonded units in the solid state, and the maximum principal components of the ¹⁷O quadrupole coupling tensor, e²q_zzQ/h, lie probably in this plane, with the minimum component, e²q_xxQ/h, perpendicular to the plane. The changes in these two components, Δq_xx and Δq_zz, due to substituents display an almost linear relationship to each other with a ratio Δq_zz/Δq_xx of ca.–0.5 for C—¹⁷OH. This relationship can be understood if the changes in the ¹⁷O quadrupole coupling constants are predominantly due to variations in the p_π oxygen populations, with smaller but proportional changes in p_σ. Such changes can be brought about by a variety of effects, including molecular vibration, and their special effects on the C—¹⁷OH and C¹⁷O quadrupole coupling tensors may be related to the fixed geometry which these tensors tend to assume in the bonding system.