Hydrogen bond dynamics in isotopically substituted benzoic acid dimers

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Abstract
The hydrogen pair transfer in the hydrogen‐bonded dimers of benzoic acid and its carboxyl‐deuterated species is investigated in the solid. Measurements are reported for the temperature‐dependent NMR relaxation time T1 in single crystals containing dimers with one or two carboxylic deuterons. Combined with previous data, the temperature dependence of the measurements is analyzed in terms of a master‐equation description for a one‐ or two‐dimensional quantum‐mechanical model of the transfer motion. The description by a one‐dimensional model is found to be inadequate as it yields unrealistic isotope effects in the model parameters. On the other hand, reasonable results are obtained for a two‐dimensional model which includes, apart from the transfer motion of the hydrogens, a heavy atom mode with properties suggested by x‐ray structural data. This model explains the thermal activation of the transfer process mainly as a result of promotion of tunneling by heavy atom rocking. Activation energies remain considerably smaller than the barrier height and indicate predominance of tunneling even at room temperature.