Abstract

Abstract

The chemical literature presents evidence for the nonexistence of the intramolecular hydrogen bond in neutral 2-guanidinobenzimidazole, a result that defies chemical intuition. In the current study, analyses of substituted 2-guanidinobenzimidazoles by dynamic ¹H NMR, IR, and X-ray diffraction unveiled the contribution of the intramolecular hydrogen bond to the overall structure and conformational equilibria. The presence of the intramolecular hydrogen bond in this work and its absence in previous studies of the unsubstituted parent compound is reconciled by the fact that intramolecular hydrogen bonds between the imidazole moieties and guanidino NH₂ protons were weak. The intramolecular hydrogen bonds were more apparent in derivatives with guanidino NHR. The behavior of the latter indicated competition between and coexistence of inter- and intramolecular hydrogen bonding. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

This study aimed to elucidate the structural nature of the polydisperse, nanoscopic components in the solution and the solid states of partially reduced polyoxomolybdate derived from the {Mo₁₃₂} keplerate, {(Mo)Mo₅}₁₂-{Mo₂ acetate}₃₀. Designer tripodal hexamine-tris-crown ethers and nanoscopic molybdate coprecipitated from aqueous solution. These microcrystalline solids distributed particle radii between 2–30 nm as assayed by transmission electron microscopy (TEM). The solid materials and their particle size distributions were snap shots of the solution phase. The mother liquor of the preparation of the {Mo₁₃₂} keplerate after three days revealed large species (r=20–30 nm) in the coprecipitate, whereas {Mo₁₃₂} keplerate redissolved in water revealed small species (3–7 nm) in the coprecipitate. Nanoparticles of coprecipitate were more stable than solids derived solely from partially reduced molybdate. The TEM features of all material analyzed lacked facets on the nanometer length scale; however, the structures diffracted electrons and appeared to be defect-free as evidenced by Moiré patterns in the TEM images. Moiré patterns and size-invariant optical densities of the features in the micrographs suggested that the molybdate nanoparticles were vesicular.

A novel, multi-state, conformational analysis based on the magnetic anisotropy of molecules undergoing fast dynamic exchange is described. Calculated chemical shift tensors combined with experimental data from proton NMR studies were used to quantify conformational distributions as a function of solvent and temperature for a hydrocarbon and two fluorocarbon derivatives of N,N′-[1,3-phenylenebis(methylene)]bis(2-phenylpyridinium) dibromide. Inspection for adequate analyses involved confirming that the mathematical expressions conserved magnetization (mass). Enthalpic parameters from VT-NMR gave some indication of the nature of the impact of solvent on conformation. Results indicated that electrostatic interactions between aromatic rings can strongly impact organic conformation in solution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)