Chemistry Letters
Online ISSN : 1348-0715
Print ISSN : 0366-7022
ISSN-L : 0366-7022
Volume 42 , Issue 12
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  • Yasuhiro Nakazawa, Satoshi Yamashita
    2013 Volume 42 Issue 12 Pages 1446-1454
    Published: December 05, 2013
    Released: December 05, 2013
    [Advance publication] Released: September 18, 2013
    JOURNALS FREE ACCESS
    We review novel thermodynamic properties of κ-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, where BEDT-TTF stands for bisethylenedithiotetrathiafulvalene and dmit stands for 1,3-dithiole-2-thione-4,5-dithiolate unveiled via single-crystal calorimetry. These compounds are organic dimer-based Mott insulators with a two-dimensional triangular lattice, where electron correlations produce localized radical spins on each molecular dimer. Néel-type magnetic orderings are prohibited by geometric frustrations and strong quantum mechanical fluctuations. The spin orientation of localized π electron fluctuates like a liquid even at zero energy, and the so-called spin-liquid (SL) ground state appears in them. We have performed heat capacity measurements down to extremely low temperatures and observed that the low-temperature heat capacities show a gapless character, evidenced by the finite electronic heat capacity coefficient, γ. Although the charge-transport properties of these compounds are insulating, the thermodynamic parameters, which reflect the low-energy excitations from the liquid ground state, resemble those of typical metallic compounds with Fermi-liquid characteristics. The magnitude of the γ was scaled with the magnetic susceptibility extrapolated down to T = 0. The realization of the unusual magnetic state coupled with the charge degree of freedom is suggested by several other experiments in addition to the heat capacity measurements. Herein we review thermodynamic discussions based on the experiments reported up to now to clarify the novelty of this magnetic ground state.
    Liquid-like states of electron spins in two-dimensional triangle systems of κ-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2 are investigated by single-crystal calorimetry. Heat capacity measurements revealed that these compounds have a gapless character in their thermal excitations evidenced by the finite electronic heat capacity coefficient, γ. Although the charge-transport properties of these compounds show insulating characters, the thermodynamic parameters resemble those of typical metallic compounds with Fermi-liquid characters. Anomalous enhancement of γ by the deuteration of cation sites in EtMe3Sb[Pd(dmit)2]2 is also discussed from thermodynamic viewpoints. Fullsize Image
     
  • Ying Zhang, Takumi Yamaguchi, Koichi Kato
    2013 Volume 42 Issue 12 Pages 1455-1462
    Published: December 05, 2013
    Released: December 05, 2013
    [Advance publication] Released: September 28, 2013
    JOURNALS FREE ACCESS
    Oligosaccharides play pivotal roles in various molecular recognition events on cell surfaces and in intracellular environments. Although information about the three-dimensional structure of oligosaccharides is crucial for understanding the mechanisms underlying their biological functions and for designing drugs targeting carbohydrate recognition systems, their inherent flexibility hampers detailed conformational analysis. NMR spectroscopy has immense potential for providing atomic details of oligosaccharide structures in solution as well as in complexes with other biomolecules. However, the traditional NMR approach based on local conformational information provided by the nuclear Overhauser effect (NOE) is often precluded by insufficient information about long interatomic distances and hydrogen bonds involving hydroxy groups. To address these issues, new NMR techniques have recently been developed, exploiting the effects of introduced paramagnetic probes and stable isotopes to target oligosaccharides. Lanthanoid tagging and the spin labeling of oligosaccharides induce paramagnetic effects such as pseudocontact shift and paramagnetic relaxation enhancement, offering NOE-independent sources of long-distance information. Deuterium-induced isotope shifts of neighboring 13C resonances provide an experimental tool for the identification of hydrogen bonds involving oligosaccharide hydroxy groups. The uniform and position-selective 13C enrichment of oligosaccharides can be achieved by metabolic labeling using genetically engineered yeast cells. Furthermore, small ganglioside-embedding bicelles have been used for detailed NMR analyses of intermolecular interactions involving glycolipids in membrane environments. These NMR approaches, in conjunction with computational simulation, have opened up new vistas for the analysis of oligosaccharide conformations and interactions at atomic level.
    NMR spectroscopy has immense potential for providing atomic details of oligosaccharide structures in solution as well as in complexes with other biomolecules. New carbohydrate NMR techniques along with improved oligosaccharide preparation techniques have recently been developed, exploiting the effects of introduced paramagnetic probes and stable isotopes to target oligosaccharides. These NMR approaches, in conjunction with computational simulation, have opened up new vistas for the analysis of oligosaccharide conformations and interactions at atomic level. Fullsize Image
     
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