A field-cycling NMR relaxometry investigation of proton tunnelling in a partially disordered system of hydrogen bonds

Abstract

Double-proton transfer in the hydrogen bonds of the benzoic acid dimer is described by an asymmetric double-well potential. At low temperature, the hydrogen bond dynamics are dominated by phonon-assisted tunnelling and the correlation time for proton transfer is independent of temperature. We report proton transfer measurements on samples that incorporate a small concentration of thioindigo dye molecules as a substitutional impurity. The potential energy surface, particularly the energy asymmetry of the double-well potential, of dimers within a sphere of influence of the guest molecule is perturbed. These dimers exhibit a distribution of energy asymmetries and correlation times. The system is analogous to disordered systems such as glasses and provides insight into low-frequency excitations that are invoked to explain the molecular dynamics in those systems. Field-cycling NMR relaxometry was used to make a direct measurement of the spectral density of the sample at low temperatures and to identify the contribution made by the proton transfer dynamics of the hydrogen bonds in the vicinity of the guest molecules. The mean proton transfer rate and asymmetry of these dimers have been measured and are compared with the values characteristic of dimers that are remote from impurity centres. The proficiency of field-cycling NMR relaxometry, and the advantages over conventional narrow band spin–lattice relaxation measurements, is discussed.