The correlation between hydrogen bond tunneling dynamics and the structure of benzoic acid dimers

We report a correlation between the rate of incoherent tunneling associated with proton transfer in hydrogen bonds and the structure of aromatic carboxylic acid dimers. The compressibility of the hydrogen bond in benzoic acid, specifically the oxygen-oxygen distance r(O··O), has been measured as a function of hydrostatic pressure up to 3.2 kbar using neutron powder diffraction. All data were recorded at a temperature of 5 K. Using previously published pressure dependence NMR measurements, we have investigated the relationship between the dynamics in the quantum regime and r(O··O) in the hydrogen bonds of benzoic acid. The incoherent tunneling rate increases exponentially with decreasing r(O··O). This behavior is attributed to the increase in the tunneling matrix element as the potential wells and the localized eigenfunctions of the double minimum potential which characterize the system are brought into closer proximity. There is a quantitative agreement between this study, in which the hydrogen bonds are compressed by the application of pressure, and the behavior exhibited by two benzoic acid derivatives with different oxygen-oxygen distances at ambient pressure.