THE QUANTUM DYNAMICS OF PROTON-TRANSFER IN THE HYDROGEN-BOND

Double proton transfer in the hydrogen bonds of carboxylic acid dimers has been investigated via measurements of the dispersion of the proto n spin-lattice relaxation time, T-1, using magnetic field-cycling NMR. A plot of the spin-lattice relaxation rate, T-1(-1), as a function of magnetic field, provides a direct measurement of the motional spectru m and hence of the proton transfer rate. In the materials studied, the se NMR experiments are optimised for the low temperature region where the dynamics are dominated by incoherent quantum tunnelling. Measureme nts of the proton transfer rate at higher temperatures have been made using quasi-elastic neutron scattering (QENS) and these facilitate the characterisation of the transition between quantum and classical dyna mics in the intermediate temperature region. The role of tunnelling in the excited vibrational states of the double minimum potential (DMP) which characterises the system is revealed in these measurements and h as been analysed according to the behaviour of a particle in a DMP whi ch is coupled to a bath of phonons. The complementarity between the NM R and QENS techniques is emphasised and discussed, and the dynamic ran ge accessible to the field-cycling NMR technique is explored through s tudying a range of related materials.