Experimental investigations of the quantum dynamics of the hydrogen bond using single crystal NMR

Double proton transfer in the hydrogen bonds of the benzoic acid dimer has been studied using proton NMR measurements on a single crystal. Both the NMR absorption spectrum and the spin-lattice relaxation time, T-1, are dominated by the dipolar interaction between the two protons in the dimer and measurements have been made in the temperature range from 300 down to 10 K. The spectrum exhibits motional narrowing and T-1 is sensitive to the dynamics over the whole range. Quantitative analysis of the data has enabled measurements of the inter-proton distance, the asymmetry energy of the double minimum potential and the correlation time, Incoherent quantum tunnelling characterises the dynamics al low temperature (T < 50 K).