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Tuning Proton Disorder in 3,5-Dinitrobenzoic Acid Dimers: the Effect of Local Environment

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posted on 19.02.2016 by Andrew O. F. Jones, Nicholas Blagden, Garry J. McIntyre, Andrew Parkin, Colin C. Seaton, Lynne H. Thomas, Chick C. Wilson
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.