Abstract
Temperature dependence of 17 O nuclear quadrupole resonance (NQR) frequencies has been measured in solid benzoic acid, 4-hydroxy benzoic acid and 4-nitrobenzoic acid. Only two oxygen sites, O–H and O⋯H, are observed in each substance indicating the equivalency of the two hydrogen bonds in a hydrogen-bonded dimer. A strong temperature variation of the 17 O NQR frequencies is consistent with a proposed model of a concerted two-proton jump in slightly asymmetric O–H⋯O hydrogen bonds. The temperature dependence of the proton order parameter is derived from the NQR data and the energy difference Δ E of the two equilibrium proton configurations is calculated. The energy difference Δ E, which is of the order of 10 meV≈1 kJ/mol, is constant in 4-nitrobenzoic acid whereas it decreases with increasing temperature in benzoic acid and in 4-hydroxybenzoic acid. A model for the temperature variation of Δ E is proposed.
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