Temperature Dependence of 2H Nuclear Quadrupole Interaction in Very Short Hydrogen Bonds in Some Organic Acidic Salt Crystals

Abstract

The 2H nuclear quadrupole interaction parameters, e2Qq/h and η, are closely related to the shape of the potential energy surface at hydrogen bonds and depend sensitively on their geometry. We measured the temperature dependence of the 2H NMR spectra of the crystalline acidic salts KDCO3, KD cetylenedicarboxylate, RbD acetylenedicarboxylate, and KD maleate, which contain very short O-D···O type hydrogen bonds. The temperature coefficient decreases with increase in the O···O distance in the hydrogen bond. Ab inito molecular orbital calculations of the electric field gradient tensor based on the temperature dependent structure of each crystal indicate that thermal expansion of the hydrogen bond geometry is not responsible for this tendency. Ab initio calculations also predict that a fictitious off center shift of the hydrogen position in a symmetric hydrogen bond causes very high e2 Qq/h values. This suggests that low energy vibrational excitation may be responsible for large positive d (e2 Qq/h)/dT values in symmetric hydrogen bonds.