HYDROGEN-BOND DYNAMICS IN TETRAFLUOROTEREPHTHALIC ACID STUDIED BY NMRAND INS

The quantum and classical dynamics of hydrogen atoms in the hydrogen b onds of tetrafluoroterephthalic acid have been studied using nuclear m agnetic resonance (NMR), and inelastic neutron scattering (INS). The t emperature 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 s cattering measurements above 250 K. It is shown that the spin-lattice relaxation is governed by modulations of both homonuclear (H-1-H-1) an d heteronuclear (H-1-F-19) dipolar interactions and the magnetization recovery is multi-exponential. Expressions for the elements of the spi n-lattice relaxation matrix are derived and, together with a simple th eory for the hydrogen bond dynamics, a good fit and satisfactory accou nt of the experimental NMR and INS data are obtained. The low temperat ure dynamics are governed by incoherent tunnelling, and a comparison i s made with the behaviour of the closely related molecules, benzoic ac id and terephthalic acid. There is an exponential dependence of the tu nnelling rate with oxygen-oxygen distance in the hydrogen bond in thes e three molecules, and this provides strong supporting evidence for tu nnelling at low temperatures.