An investigation of hydrogen bonded molecular systems using X-ray and neutron diffraction

The main focuses of this project have been investigations of a variety of hydrogen bonding systems for unusual behaviour such as disordered or migrating hydrogens/protons with both single crystal X-ray and neutron diffraction, crystallisation of a large number of molecular complexes of the chloranilic acid molecule, and examining the bifurcated hydrogen bond motif found in many of the chloranilic acid co-crystals discovered. The neutron single crystal diffraction instruments SXD and VIVALDI have been used to provide conclusive results in cases of suspected unusual hydrogen bond behaviour in molecular materials. 2,4-dihydroxybenzoic acid and its isomer 2,5-dihydroxybenzoic acid have been examined using X-ray and neutron diffraction to investigate possible disordered cooperative hydrogen bond systems. The energy difference of the three possible tautomers of 2,4-dihydroxybenzoic acid in different environments have been calculated in theoretical computations which concur with the neutron results. Single crystal neutron diffraction experiments were also carried out on isonicotinamidium formate, 2-iodoaniliium picrate, chloranilic acid 2,4-lutidine and malonic acid, where unusual behaviour in the hydrogen systems was also suspected. The molecular complexes of chloranilic acid with various pyridine-based molecules have been the main focus of the X-ray diffraction work of this thesis. Multiple crystallisations over a range of different conditions were set up for chloranilic acid with various series of molecules including lutidines (dimethylpyridines) and picolines (methylpyridines). This resulted in a large number of new crystal structures, determined by X-ray diffraction and all found to contain a bifurcated hydrogen bond motif producing two robust hydrogen-bonded supramolecular synthons. The investigation examines the bifurcated hydrogen bond interactions and the suitability of chloranilic acid complexes for crystal engineering. The two related supramolecular synthon units are discussed and difference Fourier maps and Hirshfeld surfaces used to examine the hydrogen bond architecture. Bromanilic acid co-crystals are also studied to examine the effect of the halogen in the crystal structures