HYDROGEN-BOND DYNAMICS IN TETRAFLUOROTEREPHTHALIC ACID STUDIED BY NMR AND INS

The quantum and classical dynamics of hydrogen atoms in the hydrogen bonds of tetrafluoroterephthalic acid have been studied using nuclear magnetic resonance (NMR), and inelastic neutron scattering (INS). The temperature dependence of the correlation time for the motion has been investigated using measurements of the spin-lattice relaxation of both hydrogen and fluorine nuclei in the temperature range 20 K less-than-or-equal-to T less-than-or-equal-to 300 K, and quasi-elastic neutron scattering measurements above 250 K. It is shown that the spin-lattice relaxation is governed by modulations of both homonuclear (H-1-H-1) and heteronuclear (H-1-F-19) dipolar interactions and the magnetization recovery is multi-exponential. Expressions for the elements of the spin-lattice relaxation matrix are derived and, together with a simple theory for the hydrogen bond dynamics, a good fit and satisfactory account of the experimental NMR and INS data are obtained. The low temperature dynamics are governed by incoherent tunnelling, and a comparison is made with the behaviour of the closely related molecules, benzoic acid and terephthalic acid. There is an exponential dependence of the tunnelling rate with oxygen-oxygen distance in the hydrogen bond in these three molecules, and this provides strong supporting evidence for tunnelling at low temperatures.