Hydrogen transfer in hydrogen bonded solid state materials

Abstract

The investigation of strongly hydrogen bonded solid state materials and the hydrogen transfer processes therein are the subject of the present work. Strong hydrogen bonds are found whenever the hydrogen bonded species compete for the hydrogen atom, and are thereby on the verge of showing hydrogen transfer. Consequently, the strongly hydrogen bonded solid state materials investigated in this work are synthesised by co-crystallising chemical compounds which have a similar affinity for the proton. The molecular complexes of isonicotinamide with oxalic acid crystallise in two hydrogenous polymorphs and, upon substituting the acidic hydrogen for deuterium, in two deuterated polymorphs, neither being isostructural to the hydrogenous forms. This phenomenon is known as isotopomeric polymorphism and is rarely observed in molecular materials. The four polymorphic forms are found to exhibit different degrees of hydron transfer. The hydrogenous forms show strong hydrogen bonding between the acid and the pyridine base. The nature of these strong hydrogen bonds is characterised by combined X ray charge density and single crystal neutron diffraction studies and found to be covalent in nature. The covalent hydroxyl bonds are considerably elongated, to an extent that in one polymorph the hydrogen atom occupies a near central position in the strong hydrogen bond. The structural work has been complemented by ab-initio computational studies, using the plane wave and localised atomic orbital methods, to evaluate the nature and the dynamics of the strong hydrogen bonds, and to establish an energy scale for polymorphism. It is found that the atomic orbital calculations yield results in good agreement with the experiment, while the plane wave calculations fail to reproduce the experimental hydrogen bond geometries. A strong electronic delocalisation is observed in the difference electron densities of strong acid – pyridine base hydrogen bonds. The major contribution to the delocalisation is found to originate from the nitrogen lone pair density which in this type of strong hydrogen bond is found to be observed to low experimental resolutions in standard X-ray diffraction experiments. As a consequence, such hydrogen bonds are susceptible to misinterpretation, and can be misinterpreted as hydrogen bonds with a disordered hydrogen, altering the descriptive character of materials significantly from being neutral to being ionic. It is shown that a careful examination of the difference electron densities, with the knowledge of the presence of the nitrogen lone pair density, allows a reasonably accurate determination of nuclear hydrogen positions from X-ray diffraction experiments alone. The hydrogen transfer behaviour in a series of strongly hydrogen bonded materials has been studied. For the molecular complexes of pentachlorophenol with the series of dimethylpyridines, a correlation is established between the dissociation constants determined in solution and the degree of hydrogen transfer from phenol to the pyridine bases in the solid state. The influence of additional strong and weak hydrogen bonding interactions in the vicinity of the strong hydrogen bonds on the hydrogen transfer behaviour is rationalised. Similar studies have been carried out on the molecular complexes of oxalic acid and fumaric acid with the dimethylpyridines, and on the molecular complexes of pentachlorophenol with 1,4-diazabicyclo[2.2.2]octane. The design approach leading to these materials and the hydrogen transfer behaviour observed in these materials is critically analysed