Investigating the Proton Transferring Route in a Heteroaromatic Compound Part I: A Trial to Develop Di- and Trifunctional Benzimidazole Model Compounds Inducing the Molecular Packing Structure with a Hydrogen Bond Network

pH-induced conformational changes in histamine in the solid state

Histamine is one of the most basic biogenic amino-compounds, which is composed of imidazole and a flexible ethylamine side chain moiety. Histamine is known to take the form of various types of cations, free base, monocation and dication form, where its conformational change is highly sensitively to the pH conditions. The details of these changes are still controversial due to a lack of detailed information on its crystal structures. Thus, in this study, the molecular packing structures of histidine at various pH were analyzed via X-ray diffraction in combination with vibrational spectroscopy and energy calculations. A variety of molecular conformations including the tautomeric phenomenon was found to be intimately related with intra- and intermolecular hydrogen bonds. The role of the hydrogen bonds was studied also to check the possibility of high proton conductivity of histamine, as predicted by computer simulation. Consequently, the thus-predicted proton conductivity was confirmed for the first time experimentally. During the heating process, the conductivity showed the relatively high maximum value of 10-4 S cm-1 at around 60 °C, which is related to the effective proton transfer between the amino NH group of one histamine unit and the imidazole ring of another.

Structure determination of three furan-substituted benzimidazoles and calculation of π-π and C-H···π interaction energies

The synthesis and structural characterization of 2-(furan-2-yl)-1-(furan-2-ylmethyl)-1H-benzimidazole [C16H12N2O2, (I)], 2-(furan-2-yl)-1-(furan-2-ylmethyl)-1H-benzimidazol-3-ium chloride monohydrate [C16H13N2O2(+)·Cl(-)·H2O, (II)] and the hydrobromide salt 5,6-dimethyl-2-(furan-2-yl)-1-(furan-2-ylmethyl)-1H-benzimidazol-3-ium bromide [C18H17N2O2(+)·Br(-), (III)] are described. Benzimidazole (I) displays two sets of aromatic interactions, each of which involves pairs of molecules in a head-to-tail arrangement. The first, denoted set (Ia), exhibits both intermolecular C-H···π interactions between the 2-(furan-2-yl) (abbreviated as Fn) and 1-(furan-2-ylmethyl) (abbreviated as MeFn) substituents, and π-π interactions involving the Fn substituents between inversion-center-related molecules. The second, denoted set (Ib), involves π-π interactions involving both the benzene ring (Bz) and the imidazole ring (Im) of benzimidazole. Hydrated salt (II) exhibits N-H···OH2···Cl hydrogen bonding that results in chains of molecules parallel to the a axis. There is also a head-to-head aromatic stacking of the protonated benzimidazole cations in which the Bz and Im rings of one molecule interact with the Im and Fn rings of adjacent molecules in the chain. Salt (III) displays N-H···Br hydrogen bonding and π-π interactions involving inversion-center-related benzimidazole rings in a head-to-tail arrangement. In all of the π-π interactions observed, the interacting moieties are shifted with respect to each other along the major molecular axis. Basis set superposition energy-corrected (counterpoise method) interaction energies were calculated for each interaction [DFT, M06-2X/6-31+G(d)] employing atomic coordinates obtained in the crystallographic analyses for heavy atoms and optimized H-atom coordinates. The calculated interaction energies are -43.0, -39.8, -48.5, and -55.0 kJ mol(-1) for (Ia), (Ib), (II), and (III), respectively. For (Ia), the analysis was used to partition the interaction energies into the C-H···π and π-π components, which are 9.4 and 24.1 kJ mol(-1), respectively. Energy-minimized structures were used to determine the optimal interplanar spacing, the slip distance along the major molecular axis, and the slip distance along the minor molecular axis for 2-(furan-2-yl)-1H-benzimidazole.

Kinetic Monte Carlo simulations of proton conductivity

The kinetic Monte Carlo method is used to model the dynamic properties of proton diffusion in anhydrous proton conductors. The results have been discussed with reference to a two-step process called the Grotthuss mechanism. There is a widespread belief that this mechanism is responsible for fast proton mobility. We showed in detail that the relative frequency of reorientation and diffusion processes is crucial for the conductivity. Moreover, the current dependence on proton concentration has been analyzed. In order to test our microscopic model the proton transport in polymer electrolyte membranes based on benzimidazole C(7)H(6)N(2) molecules is studied.

3-[2-(1H-1,3-Benzodiazol-2-yl)eth-yl]-1,3-oxazolidin-2-one

In the title compound, C₁₂H₁₃N₃O₂, the dihedral angle between the oxazolone ring and the benzimidazole unit is 45.0 (5)°, exhibiting a staggered conformation at the Cα—Cβ bond. In the crystal, a strong N—H⋯N hydrogen bond links the mol­ecules into a C(4) chain along the c axis while a C—H⋯O hydrogen-bonding inter­action generates a C(5) chain along the a axis, i.e. perpendicular to the other chain.