Tadashi Sugawara

Tautomerism in the Solid State

Publisher Summary Proton tautomerism occurs readily in solution because there are no restrictions in fluid media on the conformational changes required for the tautomerization to take place. On the other hand, in the solid state, the mobility of molecular skeletons and protons is strictly limited. This is the main reason for tautomerizations being rarely observed in the solid state. One of the important factors for the feasibility of proton tautomerism in the solid state is the presence of an intra- or intermolecular hydrogen bond along which proton transfer takes place, accompanied by a switching of π-bonds. The presence of the hydrogen bond is not sufficient for observing proton tautomerism in the solid state. In fact, the tautomerization is likely to be frozen on the basis of the dipole–dipole interaction between tautomers. Proton tautomerism in the solid state is an ideal system to investigate cooperative phenomena concerning proton transfer, because tautomeric molecules are connected with each other in terms of dipolar and/or hydrogen-bonding interactions. Tautomerization plays a vital role in biological systems as well. The chapter describes the essence of proton tautomerism, focusing on the potential profile of proton transfer along the reaction coordinate of the tautomerization in the solid state.

Manipulating Spin System

Abstract As a building block of organic conducting ferromagnets, TTF-based dimeric donors and spin-polarized TTF donors have been prepared. As an extension of the spin-polarized donor, spin-polarized polyradical donors which afford high spin cation polyradicals upon one-electron oxidation are documented. An idea to realize a manipulating spin system composed of hyper-structured spin-polarized donors is also introduced.

Structure and physical properties of a hydrogen-bonded self-assembled material composed of a carbamoylmethyl substituted TTF derivative

Crystal structures of the carbamoylmethyl substituted TTF derivative AMET are characterized by polymeric hydrogen bonding between amide groups. As a result, the TTF moieties stack in parallel even in the neutral crystal. The nu;NH absorbtions of neutral AMET at 3426 and 3184cm1 show shifts to lower wavenumber, Delta;k, of 37 and 58cm1, respectively, at 4.1GPa. Therefore the shrinkage of the NH‥O distance is estimated to be ca. 0.04 at this pressure. The pressure dependence of the IR spectra of iodine-doped samples at doping ratios of less than 45 was exactly the same as that for a neutral sample, suggesting that the hydrogen bonding pattern is not affected significantly upon doping.Although crystalline AMET is an insulator in the neutral state (sigma;rt=ca. 108 Scm1), the conductivity is enhanced by a factor of 107 upon iodine doping of 45mol (sigma;rt=1.2101 Scm1). Furthermore, the conductivity increases as a function of the external pressure, and the sigma;rt of a 5 iodine-doped sample increased three-fold at 1.0GPa. The enhanced conductivity of iodine-doped samples may be ascribed to the increase in the overlap between the donor moieties based on the shrinkage of the hydrogen bond of the carbamoylmethyl group.

Molecular Magnetism: Present and Future

Abstract Organic functional molecules equipped with an orientation controlling site are found to construct an organic self-assembly exhibiting prominent magnetic properties. Typical examples are a one-dimensional organic ferrimagnetic spin system, a hydrogen-bonded organic ferromagnet, and an approach to high spin charge transfer complexes. Dynamical spin systems respondent to external stimuli are also described.

X-ray diffraction studies on the lock-in phase transition of intramolecular hydrogen-bonded compound d-BrHPLN

The phase transitions of deuterated 5-bromo-9-hydroxyphenalenone (d-BrHPLN), which was characterized as an isolated intramolecular hydrogen-bonded system, have been investigated by low-temperature x-ray diffraction experiments. The temperature dependence of lattice constants showed anomalous behaviour at the phase transition points TC( = 22.8 K) and TI( = 34.5 K). Superlattice reflections were found at (h k+0.5 0) below TC and they deviated from the commensurate position in the intermediate phase (TC<T<TI). The peak shift δ showed a thermal hysteresis and was pinned on the position deviated from the commensurate one after several thermal cycles, even below TC. The cause of the pinning effect was attributed to the creation of an improper hydrogen configuration excited electronically by x-ray radiation.

GROUND STATE TRIPLET CATION DIRADICALS HAVING NON-DEGENERATED SINGLY OCCUPIED MOLECULAR ORBITALS

Abstract Open-shell donors (1, 2) carrying a nitronyl nitroxide group (NN) at the para position of N, N -dimethyl-or diphenylaniline were prepared and thier one-electron oxidation states were investigated by means of the ESR spectroscopic measurement. The cation diradicals of 1 and 2 turned out to be the ground state triplet species based on the non-disjoint character of thier electronic structures consisted of two non-degenerated SOMOs.

Ferromagnetic Spin Ordering Along Intermolecular Hydrogen Bonds of a Hydroquinone Derivative Carrying a Nitronyl Nitroxide

Abstract The crystal structure of an organic ferromagnet, α-HQNN, was characterized by one-dimensional chains constructed through intermolecular hydrogen bonds. Furthermore two parallel hydrogen-bonded chains are connected by bifurcated hydrogen bonds. The analysis of heat capacity data indicates the three-dimensional ferromagnetic intermolecular interaction in this crystal. The results confirm the presence of the ferromagnetic intermolecular interaction along hydrogen bond. The observed isotope effect in the α-HQNN-d2 crystal is consistent with the above interpretation.

Theoretical Studies of Magnetic Interactions in 2′, 5′-Dihydroxyphenyl Nitronyl Nitroxide Crystal

Abstract In order to investigate the ferromagnetic interaction in the α phase of 2′, 5′-dihydroxyphenyl nitronyl nitroxide (HQNN), semiempirical and ab initio molecular orbital calculations were carried out for several pair models of HQNN molecules extracted from the crystal structure. It was shown that the effective exchange interaction (Jab ) for the nearest neighbor molecules is ferromagnetic, being in agreement with the experimental result. The hydroxyl groups and the methyl groups contribute to the ferromagnetic interaction in the α phase of HQNN crystal.

Structure And Property Of Cross-Cyclophane Twin Donors

Abstract Cross-cyclophane twin donors, in which long axes of TTF units are fixed in a cross orientation by four ethylenedithio (x-CPTD (II)) and four trimethylenedithio (x-CPTD (III)) chains, were designed and prepared. Although the donor ability of x-CPTD (II) was low due to a heavy bending of the donor units, that of x-CPTD (III) was the same as BEDT-TTF. The crystal structures of a charge transfer complex, x-CPTD (III) ·TCNQ, and an ion radical salt, x-CPTD (III)·Br (1,1,2-TCE)2, revealed that x-CPTD (III) has an ability to construct characteristic self-assembled structures, recognizing a size and a shape of acceptors or counter ions. The almost isotropic conducting property (σr,t //a = 1.0 × 10−2 S · cm−1, σr,t //a = 5.5 × 10−3 S · cm−1) of the ion radical salt was interpreted based on the unique molecular structure of x-CFTD (III).

Preparation and property of ionic CT complex of dimethylamino nitronyl nitroxide with DDQ

Abstract An ionic charge transfer complex of dimethylamino nitronyl nitroxide (DMANN) was prepared by grinding with DDQ in the solid state. An electronic spectrum of the complex showed a broad band at 0.65 eV. The conductivity of the complex was σ ∼ 10−6 S-cm−1 at room temperature. The measurement of the magnetic susceptibility revealed that this complex behaves paramagnetically.