Proton transfer dynamics in the hydrogen bond: a direct measurement of the incoherent tunnelling rate by NMR and the quantum-to-classical transition

The incoherent tunnelling rate for proton transfer in the hydrogen bonds of benzoic acid dimers has been measured by field cycling NMR from the dispersion of the proton spin-lattice relaxation time which maps out the spectral density function directly. The intermediate region between quantum and classical dynamics has been studied by measuring the inverse correlation time for proton transfer as a function of temperature and the contribution made by tunnelling in the excited states of the double minimum potential has been determined.