An NMR relaxation study on the proton transfer in the hydrogen bonded carboxylic acid dimers

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Abstract
We have studied the proton spin‐lattice relaxation times (T1) of a series of benzoic acid (BAC) derivatives and decanoic acid (DAC) over a wide range of temperature and analyzed the results in terms of the double proton switching along the hydrogen bonds. The proton T1 in the high temperature region are analyzed using the classical jump model and the barrier heights for the proton transfer are determined. The thermodynamic parameters for the equilibria between the two configurations in the solid state are also determined by the FT–IR measurements. It is shown that the energetics and dynamics of the proton transfer in DAC and the para‐ and meta‐substituted BAC are all similar, but they are very different in the ortho‐substituted ones. It is suggested that the low temperature behavior of the proton T1 of the dimers of carboxylic acid is due to the tunneling and the asymmetry of the potential brings in a small activation energy.