Deuterium quadrupole coupling constants in hydrogen bonded dicarboxylic acids

Deuterium quadrupole coupling constants have been determined by distant ENDOR spectroscopy for a series of carboxylic acids in which deuterium is substituted at the H-bond position. The measured coupling constants cover a range of 40 to 175 kHz corresponding to a variation of the bridge (O-H … O) length of 2·45 to 2·70 Å. Features of the electronic and geometrical structure which principally determine e 2 qQ and η of deuterium in these H-bond situations have been assessed. To aid this assessment a simple model is proposed on which theoretical calculations of the INDO MO type are based. Good agreement between experiment and theory is obtained. It is concluded that for calculation of the electric-field gradient tensor two and three-centre integrals in the vicinity of the H-bond cannot be neglected, and further, in the cases of short and symmetrical H-bonds, e 2 qQ can change sign. Our studies also indicate that in non-ionic systems the asymmetry parameter η is primarily determined by the molecule itself, rather than by the crystal field.