The dynamics of hydrogen atoms in the hydrogen bonds of carboxylic acid dimers

Abstract

The techniques of pulsed NMR spectroscopy and inelastic neutron scattering (INS) have been employed to study the coordinated motion of hydrogen atoms in the hydrogen bonds of carboxylic acid dimers. The proton spin-lattice relaxation time T1 has been measured at two frequencies in diglycolic acid and suberic acid and the reorientation rate in diglycolic acid has been measured directly by quasi-elastic neutron scattering. Furthermore T1 data have been collected on benzoic acid, terephthalic acid and malonic acid at different spectrometer frequencies to those in earlier published data. The NMR and INS data have been successfully fitted to a new model proposed by Skinner and Trommsdorff (1988) in which the low temperature dynamics, under the influence of an asymmetric double-minimum potential, are dominated by phonon-assisted tunnelling while that at high temperature follows an Arrhenius rate law. Fitting the T1 data at two different frequencies simultaneously with the INS data restricts the possible values of the parameters in the theory to a narrow range. The values of the parameters, particularly the barrier heights and asymmetry, are discussed in terms of the known molecular structures. Further discussion centres upon some new NMR data of the proton dipolar relaxation time T1D in terephthalic acid and benzoic acid.