O-17 NUCLEAR-QUADRUPOLE RESONANCE STUDY OF PROTON DISORDER IN SOLID BENZOIC-ACID, 4-HYDROXYBENZOIC ACID AND 4-NITROBENZOIC ACID

Temperature dependence of O-17 nuclear quadrupole resonance (NQR) freq uencies has been measured in solid benzoic acid, 4-hydroxy benzoic aci d and 4-nitrobenzoic acid. Only two oxygen sites, O-H and O ... H, are observed in each substance indicating the equivalency of the two hydr ogen bonds in a hydrogen-bonded dimer. A strong temperature variation of the O-17 NQR frequencies is consistent with a proposed model of a c oncerted two-proton jump in slightly asymmetric O-H ... O hydrogen bon ds. The temperature dependence of the proton order parameter is derive d from the NQR data and the energy difference Delta E of the two equil ibrium proton configurations is calculated. The energy difference Delt a E, which is of the order of 10 meV approximate to 1 kJ/mol, is const ant in 4-nitrobenzoic acid whereas it decreases with increasing temper ature in benzoic acid and in 4-hydroxybenzoic acid. A model for the te mperature variation of Delta E is proposed. (C) 1998 Elsevier Science B.V. All rights reserved.