17O nuclear quadrupole resonance study of proton disorder in solid benzoic acid, 4-hydroxybenzoic acid and 4-nitrobenzoic acid Original Research Article

Temperature dependence of 17O 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 17O 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 ΔE of the two equilibrium proton configurations is calculated. The energy difference ΔE, 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 ΔE is proposed.