NMR Study of Dynamics of Carboxyl Proton in Crystals of Acetylenedicarboxylic Acid and <i>p</i>-Nitrobenzoic Acid

Abstract

Abstract Two kinds of motions of proton in dimeric carboxyl group were distinguished by the temperature dependence of spin-lattice relaxation times of dipolar system (T1d) and of Zeeman system (T1z) on acethylenedicarboxylic acid (ADCA) and on p-nitrobenzoic acid (PNBA) in the A2⁄a form. High temperature motion of ADCA is 180° rotation of the eight-membered ring of dimeric carboxyl group about the axis nearly parallel to the hydrogen bonds. The activation energy of this motion of ADCA is 58 kJ mol−1. The low temperature motion of ADCA and PNBA is the proton translation along the hydrogen bonds in a dimeric carboxyl group, which induces the minima of T1z and T1d at the same temperature. The activation energy derived from the high temperature slope of T1z curve is 6.9 kJ mol−1 for ADCA and 2.5 kJ mol−1 for PNBA. The low temperature slope gives a small activation energy of 1.9 kJ mol−1 for ADCA and 1.0 kJ mol−1 for PNBA. The frequency dependence of T1z in the low temperature limit was constant at low Larmor frequency. This result indicates that the translational motion of protons along the hydrogen bonds is not classical but quantum-mechanical.