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.