Hydrogen bonding in solids correlation of spectroscopic and crystallographic data

Publisher Summary This chapter discusses crystal structures of metal–peptide complexes. Most of the crystal-structure analyses of metal–amino acid and metal–peptide complexes have been carried out on the assumption that such complexes act as models for the metal-binding sites on proteins. Crystal-structure analyses show that the geometrical features of metal complexes of amino acids, peptides and imidazole are related in systematic ways to the chemical structure, which the complexes have in the crystalline state. Moreover, the transfer of geometrical information from crystal-structure analyses to species that exist in solution depends on the assumption that the complexes found in crystals are present also in the solutions from which the crystals grow. This chapter illustrates that, the existence of cis-trans isomers coordination isomers, optical isomers, and dimeric species in the crystalline complexes emphasizes the variety of species, which must be considered when equations are written to represent metal–peptide equilibria in solution.

Crystal structures of metal-peptide complexes.

Imidazole single crystal has a highly anisotropic conductivity; the ratio of conductivity in the c and a crystallographic directions is ∼103. In the c crystallographic direction, the hydrogen bonded direction, protonic carriers are discharged as hydrogen at the negative electrode. The limiting conduction process is possibly due to L‐defect injection and transport back down the hydrogen bonded chain after the proton is discharged. Conductivity in the a direction is probably electronic and extrinsic.

Protonic Conductivity in Imidazole Single Crystal

The x‐ray photoelectron spectra of tetraphenylporphin, phthalocyanine and their copper complexes were measured. The copper complexes show a single nitrogen 1s photoline, whereas the metal free bases give rise to a doublet nitrogen 1s photoline. The central pyrrole and aza nitrogens in the metal free bases could be identified, whereas the central and meso‐bridging aza nitrogens in phthalocyanine could not be discriminated. Each of the central two protons in the metal free bases is localized on one of the central four porphinato nitrogens. The shifts in N 1s binding energy observed in the present work could be reproduced by the shifts in the charge density on nitrogen obtained for the bonded structure by an extended Huckel molecular orbital calculation.

X‐ray photoelectron spectroscopy of tetraphenylporphin and phthalocyanine

Hydrogen-bond interaction in donor−acceptor charge-transfer complexes of TTF-imidazole demonstrated the electronic effects in terms of control of component ratio and redox activation. These unprecedented effects of hydrogen bonds renewed the criteria giving “a high probability of being organic metals” and produced a number of highly conductive complexes with various acceptors having a wide range of electron-accepting ability. In p-chloranil complex, both molecules were linked by hydrogen bonds and formed a D−A−D triad, regulating the donor−acceptor composition to be 2:1. Theoretical calculations have revealed that the polarizability of hydrogen bonds controls the redox ability of the donor and p-benzoquinone-type acceptors and afforded different ionicity in complexes from those expected by the difference of redox potentials between donor and acceptors. In the p-chloranil complex, this electronic and structural regulation by hydrogen bond realized the first metallic donor−acceptor charge-transfer complex b...

Hydrogen-bond interaction in organic conductors: redox activation, molecular recognition, structural regulation, and proton transfer in donor-acceptor charge-transfer complexes of TTF-imidazole.

/pdf/the-crystal-structure-of-imidazole-at-150-c-1uxuiwxfd2.pdf

S. Martinez-Carrera

Papers

The crystal structure of imidazole at –150°C