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.
Jones, A. O. F., Blagden, N., McIntyre, G. J., Parkin, A., Seaton, C. C., Thomas, L. H., & Wilson, C. C. (2013). Tuning proton disorder in 3,5-dinitrobenzoic acid dimers: The effect of local environment. Crystal Growth and Design, 13(2), 497-509. https://doi.org/10.1021/cg300906j