Tuning proton disorder in 3,5-dinitrobenzoic acid dimers: The effect of local environment

The carboxylic acid dimer is a frequently observed intermolecular association used in crystal engineering and design, which can show proton disorder across its hydrogen bonds. Proton disorder in benzoic acid dimers is a dynamic, temperature-dependent process whose reported occurrence is still relatively rare. A combination of variable temperature X-ray and neutron diffraction has been applied to demonstrate the effect of local crystalline environment on both the degree and onset of proton disorder in 3,5-dinitrobenzoic acid dimers. Dimers which have significantly asymmetric local intermolecular interactions are found to have a higher onset temperature for occupation of a second hydrogen atom site to be observed, indicating a greater energy asymmetry between the two configurations. Direct visualization of the electron density of hydrogen atoms within these dimers using high resolution X-ray diffraction data to characterize this disorder is shown to provide remarkably good agreement with that derived from neutron data.