The α and β forms of oxalic acid di-hydrate at high pressure: a theoretical simulation and a neutron diffraction studyElectronic supplementary information (ESI) available: Results of the neutron diffraction data and simulated X-ray powder diffraction files of relevant phases discussed in the paper; theoretical unit cell data for 1β. This work was supported by the Swiss National Science Foundation, project 200021_126788/1. We thank ISIS and the STFC for the provision of neutron beam time and D. J. Francis for his technical assistance during the neutron diffraction experiment. See DOI: 10.1039/c002471f

The high pressure structure of α-oxalic acid di-hydrate shows a prototypical behaviour of acid to base proton transfer reaction. The solid state transformation has been characterized by neutron and X-ray diffraction as well as by quantum chemical calculations with periodic boundary conditions. The two resonant configurations playing the most important role in the hydrogen bond mechanism, i.e.a neutral and a charge transfer configuration, are interchanged by modifying the pressure on the system. A different behaviour is predicted for the less common β form, despite an apparently similar packing motif. Implications for the characterization of hydrogen bonds in the solid state are discussed as well as the possibility of estimating internal energy variations from experimental measurements. [ABSTRACT FROM AUTHOR]