Temperature dependence of proton transfer in 4-chlorobenzoic acid

The energy asymmetry associated with hydrogen atom transfer in the O–H⋯O hydrogen bond of crystalline 4-chlorobenzoic acid has been determined from the temperature dependence of the hydrogen atom site occupancy factors, refined using single-crystal neutron diffraction data; the energy asymmetry (ΔH°/R) is estimated to be 200 ± 15 K, a factor of ca. 2.6 greater than the value obtained for benzoic acid.