Proton tunnelling in the hydrogen bonds of halogen- substituted derivatives of benzoic acid studied by NMR relaxometry: the case of large energy asymmetry

The concerted two proton transfer in the hydrogen bonds of para-halogen substituted derivatives of benzoic acid has been investigated using conventional NMR relaxometry combined with field-cycling measurements of the magnetic field dependence of the proton spin-lattice relaxation rate. Thus, the inverse correlation time describing the proton transfer process has been determined over a wide range of temperature. The energy difference between the two proton configurations was determined in all four compounds to be significantly larger than in the prototype model system of benzoic acid dimers. This energy difference exceeds the cut-off frequency of the acoustic phonon spectrum of the crystal as well as the frequency of two lowest modes promoting proton tunnelling. The low temperature limit of the tunnelling rate was found to be one order of magnitude higher than in benzoic acid. For the four compounds studied, this rate and its increase at higher temperature exhibit a different behaviour to benzoic acid which is attributed to the details of the level structure and energy gaps between states corresponding predominantly to one or the other proton configuration. A new adaptation of the phonon-assisted theory is proposed which applies to these cases of high energy asymmetry. To assist in the interpretation of the data, a powder neutron diffraction structure determination of 4-bromobenzoic acid is reported together with DFT calculations on the hydrogen bond structures of all members of the 4-halogen substituted derivatives. (C) 2003 Elsevier Science B.V. All rights reserved.