4,4'-, 5,5'- and 6,6' -dimethyl-2,2'-bipyridyls: the structures, phase transitions, vibrations and methyl group tunneling of their complexes with chloranilic acid

The crystal and molecular structures of 4,4(')- and 6,6(')-dimethyl-2,2(')-bipyridyl complexes with 2,5-dichloro-3,6-dihydroxy-p-benzoquinone (chloranilic acid, CLA) have been determined and compared with those of the complex with the 5,5(')-derivative, which is known to possess interesting antiferroelectric properties. In the crystalline state, all three compounds form hydrogen bonded chains with N(+)-H···O(-) and O-H···N bridges on both sides of the bipyridyl constituent. The comparison of three derivatives indicates that the N(+)-H···O(-) hydrogen bonds are shortest for the 5,5(')-dimethyl complex. The 4,4(')- and 6,6(')-derivatives do not show any ferroelectric feature. The 6,6(')-one is, however, characterized by a continuous phase transition, revealed in the differential scanning calorimetry, dilatometric, and dielectric characteristics. The tunneling splitting measured by neutron backscattering in the energy range ±30 μeV for the neat dimethyl bipyridyls and their complexes with CLA indicates that the different splittings are primarily due to the crystal packing effect and that charge transfer between interacting compounds plays only a minor role

The (2:1) complex of picric acid with tetramethylpyrazine. The structure,l IR spectra and tunnel splitting of methyl groups

The crystal structure of 2:1 picric acid (PAH) with tetramethylpyrazine (TMP) was determined by using X-ray diffraction studies. Two equivalent N+-H center dot center dot center dot O- hydrogen bonds are formed with the length of 2.601 angstrom (calculated 2.640 angstrom). In the IR spectra very broad doublet at ca. 2000 and 2400 cm(-1) is observed, which can be interpreted as due to symmetric and asymmetric N+-H vibrations. In neutron backscattering two tunnel splittings are observed, in agreement with the symmetry of 2:1 assemblies. At 4 K the tunnel peaks are located at 3.17 and 4.24 mu eV. (C) 2010 Elsevier B.V. All rights reserved

Elastic, quasielastic and inelastic neutron scattering studies on the CT hexamethylbenzene-acyanoquinodimethane complex

The 1:1 hexamethylbenzene (HMB)-tetracyanoquinodimethane (TCNQ) complex shows a first-order phase transition at 230/218 K (heating/cooling) with no change of the space group. The neutron-diffraction studies reveal that this transition is related to a freezing of the rotation of methyl groups. The results for 100 K enabled precise determination of configuration of HMB center dot TCNQ complexes. The planes of HMB and TCNQ molecules from small angle (6 degrees) so that the dicyanomethylene group approaches the HMB molecule to a distance of 3.34 angstrom. The conformation of methyl groups was exactly determined. The quasielastic neutron-scattering spectra can be interpreted in terms of 120 degrees jumps with different activation barrier in low- and high-temperature phases, equal to 3.7 and 1.8 kJ/mol, respectively. These values are lower than that for neat HMB (6 kJ/mol). The conclusion can be drawn that the methyl groups can reorient more freely in the complex. This conclusion is in agreement with the results of inelastic neutron-scattering studies of low-frequency modes assigned to torsional vibrations of methyl groups. These frequencies are lower than those for neat HMB. The analyzed increase of frequencies of these modes as compared with free molecules can be interpreted as due to formation of unconventional C-(HY)-Y-... hydrogen bonds which are more pronounced in crystals of neat HMB than in those of HMB(.)TCNQ. The low-frequency librational modes can be treated as a sensitive measure of unconventional hydrogen bonds formed by the CH3 groups. (c) 2005 American Institute of Physics