Hydrogen bond dynamics An investigation using NMR field-cycling and QENS techniques

The techniques of field-cycling NMR relaxometry and QENS have been employed to study the translational motion of the proton in the hydrogen bonds in a benzoic acid and also a series of para-halo-substituted benzoic acid derivatives. The proton spin-lattice relaxation rate, T_1"-"1, as a function of magnetic field provides a direct measurement of the motional spectrum of the protons in the hydrogen bond (<50 K), from which we can extract the proton correlation time. The broadening of the quasi-elastic line in a neutron scattering spectrum has provided us with direct measurements of the proton correlation time above 100 K, for ring deuterated benzoic acid. For all the carboxylic acids studied, the inverse proton correlation time has been successfully modelled as a function of temperature. At low temperatures, the proton transfer is governed by incoherent tunnelling and is independent of temperature. A the temperature is increased excited state tunnelling contributes until, at higher temperatures the dynamics can be approximated using a classical Arrhenius rate law. There is an exponential relationship between the tunnelling rate and the oxygen to oxygen distance in the hydrogen bonds of carboxylic acids studied. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN028781 / BLDSC - British Library Document Supply CentreGBUnited Kingdo