Structural studies of novel hydrogen bonds by X-ray and neutron diffraction
The first set of measurements in this thesis discusses the hydrogen-bonding behaviour of1,3-dimesitylimidazol-2-ylidene when co-crystallised with organic acids. 1,3-dimesityiimidazol-2-ylidene is an unusual molecule in which it is possible to have a carbon with a valence of two. In the co-crystals of 1,3-dimesitylimidazol-2-ylidene with pentafluorophenol, pentachlorophenol, 2(2-hydroxidophenyl)benzoxazole or 2,6-di-tert-butyl-4-methylpyridine the 1,3-dimesitylimidazol-2-ylidene becomes protonated and uncommonly short C-H - O hydrogen bonds are formed, the shortest has a C-0 separation of 2.800(3)Å. In the 1:1 co-crystal of 1,3-dimesitylimidazol-2-ylidene and diphenylamine the first example of an N-H-C hydrogen bond is observed, with an N-C distance of 3.196(2)Å. The 1:1, 1:2 and 2:1 co-crystals of 4,4'-bipyridine and benzene-1,2,4,5-tetracarboxylic acid(pyromellitic acid) have been studied by X-ray and neutron diffraction and the 1:1 and 1:2 co-crystals of 4,4'-bipyridine and benzene-1,2-dicarboxylic acid (phthalic acid) and the 1:2 co-crystal of bis-1,2-(2-pyridinium)ethane and benzene-1,2-dicarboxylic acid have been studied by X-ray diffraction. In the short N-H- -0 hydrogen bond observed in the 2:1 co-crystal of 4,4'-bipyridine and benzene-1,2,4,5-tetracarboxylic acid (N-0 2.5220(17)Å at 20K) the hydrogen atom was observed to change position with temperature; at 20K the hydrogen lies1.207(3)Å from the nitrogen and 1.325(3)Å from the oxygen and at 296K it has moved across the hydrogen bond to lie 1.240(4)Å from the oxygen atom and 1.302(4)Å from the nitrogen atom. The same phenomenon of temperature dependent proton migration has been observed by neutron diffraction in crystals of pyridine-3,5-dicarboxylic acid and upon deuteration of the hydrogen bonds the magnitude of the effect becomes greater. Solid-state density functional theory calculations have been used to provide theoretical models for the observed behaviour and to predict the inelastic-incoherent neutron-scattering spectra for pyridine-3,5-dicarboxylicacid in different degrees of deuteration.