H-2 AND O-17 NMR-STUDIES OF HYDROGEN-BOND EXCHANGE AND MOLECULAR-MOTION IN DILUTED PIVALIC ACID

H-2 and O-17 NMR spin-lattice relaxation times, linewidths and chemical shifts have been measured over a wide temperature region for pivalic acid dissolved in inert solvents. The observed singularities in temperature dependences of the NMR parameters at 250-280 K have been interpreted as the second-order phase transition in an ensemble of the cyclic dimer, i.e. as the change of their average symmetry due to a decrease in the rate of hydrogen-bond exchange. The theoretical approach has been developed on the basis of a stochastic model of an overdamped oscillator in a double-well potential whose harmonic part is noise-modulated. The activation energies for proton exchange have been evaluated and their relationship with other parameters of the association processes examined.