Search

The structures of the series of N,N′-1,n-phenylenebis(oxamic acid ethyl ester) molecules with n= 2 (H2Et2opba, 1), 3 (H2Et2mpba, 2), and 4 (H2Et2ppba, 3) have been determined by single-crystal X-ray diffraction (XRD) methods. Density functional (DF) calculations have been performed on the simplest model system N-phenyloxamic acid methyl ester (HMepma). Compounds 1–3have either folded (H2Et2opba), bent (H2Et2mpba), or linear (H2Et2ppba) almost planar (periplanar) molecular configurations with the two oxalamide moieties being slightly tilted up and down, respectively, with respect to the benzene ring. The energy calculations as a function of the torsion angle () around the N(amide)–C(benzene) bond for HMepma reveal that the minimum energy synand antiperiplanar conformations of the carboxamide functions are more stable than the corresponding synand antiplanar ones (= 0 and 180°) by 0.18 and 0.13 kcal mol−1, respectively. The calculated values for the synand antiperiplanar minimized conformers of HMepma are 16.0 and 200.0°, respectively, in reasonable agreement with the experimental values for 1–3[= 39.0(4) and 225.0(3) (H2Et2opba), 32.6(5) (H2Et2mpba), and 34.7(2)° (H2Et2ppba)]. This situation likely minimizes the forced repulsive interactions between the amide hydrogen and the nearest benzene hydrogen atoms while it maximizes the attractive interactions between the carbonyl amide oxygen and the nearest benzene hydrogen atoms, which are then implicated in a relatively weak, intramolecular C–H(benzene)OC(amide) hydrogen bond [d(HO) = 2.45(2)–2.57(2) Å]. A supramolecular aggregation of molecules into either a duplex (H2Et2opba) or a brick-wall sheet (H2Et2ppba) occurs for 1and 3, respectively, through moderately strong, intermolecular N–H(amide)OC(amide) hydrogen bonds [d′(HO) = 2.17(2)–2.37(2) Å]. By contrast, moderately weak, intermolecular N–H(amide)OC(ester) hydrogen bonds between the H2Et2mpba molecules are involved in 2to give a meso-helical chain with a unique hydrogen-bonded oxalamide acid ester dimeric unit. The energy calculations as a function of the intermolecular N–H(amide)OC(ester) hydrogen bond distance (d′) for the {HMepma}2dimer show an energy minimum at 2.37 Å, in excellent agreement with the experimental value of 2[d′(HO) = 2.42(4) Å]. The calculated value of the hydrogen bond energy for {HMepma}2(EHB= 4.83 kcal mol−1) is consistent with a partially covalent nature of the interaction between the amide hydrogen and the carbonyl ester oxygen atoms, as confirmed by the existence of a significant electron density delocalization within the resulting four-center H2O2diamond core. [ABSTRACT FROM AUTHOR]

Viadensity functional theory based calculations we show that self-dopingof the surface Dirac cones in three-dimensional Bi2X3(X = Se, Te) topological insulators can be tuned by controllingthe sequence of stacking defects in the crystal. Twin boundaries insidethe Bi2X3bulk drive either n- or p-type self-dopingof the (0001) topological surface states, depending on the preciseorientation of the twin. The surface doping may achieve values upto 300 meV and can be controlled by the number of defects and theirrelative position with respect to the surface. Its origin relies onthe spontaneous polarization generated by the dipole moments associatedwith the lattice defects. Our findings open the route to the fabricationof Bi2X3surfaces with tailored surface chargeand spin densities in the absence of external electric fields. Inaddition, in a thin film geometry two-dimensional electron and holeDirac gases with the same spin-helicity coexist at opposite surfaces. [ABSTRACT FROM AUTHOR]

The suitability of the stilbazole push–pull ligands, 4′-dimethylaminostilbazole (DMAS) and 4′-diethylaminostilbazole (DEAS), for the construction of bimetallic FeII–AgI/AuIcyanide-based coordination polymers that exhibit spin crossover properties is investigated. The structural and physical characterization of four novel two-dimensional FeIIpolymers formulated as {Fe(DMAS)2[Ag(DMAS)(CN)2]2} (1) and {Fe(L)2[M(CN)2]2} (L = DMAS, M = Au (2); DEAS, Ag (3); DEAS, Au (4)) is reported. Polymers 1 and 4 are paramagnetic over the whole range of temperatures studied (5–300 K), whereas 2 and 3 exhibit spin crossover properties. [ABSTRACT FROM AUTHOR]

Three novel three-dimensional coordination polymers, {Fe II(azpy)[(M II(CN) 4)]}· nH 2O (azpy = 4,4′-azopyridine; M = Ni, Pd, and Pt), have been synthesized as polycrystalline bulk materials and as continuous as well as nanopatterned thin films on gold substrates. Single crystals of the Pt derivative have also been isolated and the corresponding X-ray diffraction analysis at 140 K indicates that it crystallizes in the tetragonal P4/ mmmspace group with a= b= 7.1670(5) Å, c= 13.0330(13) Å, V= 669.40(9) Å 3and Z= 1. Four square-planar [Pt(CN) 4] 2−anions occupy the equatorial positions of [Fe IIN 6] octahedron and four Fe(II) atoms are coordinated to each [Pt(CN) 4] 2−anion, thereby defining an infinite set of 2D planar layers pillared by the azpy ligands which bridge through the axial positions consecutive Fe(II) atoms. Magnetic susceptibility, calorimetric, and Mössbauer measurements reveal the occurrence of a thermal spin crossover phenomenon in the powder samples, which depends strongly on the water-content of the lattice in the case of Pd and Pt derivatives. The spin crossover is less cooperative with Tc= 245 K in the case of Ni, but the Pd (Pt) analogues display almost complete spin transitions with a hysteresis loop centered around 292 (280) and 191 (183) K, in the hydrated and dehydrated forms, respectively. Thin films of the three compounds were grown via a sequential assembly method using coordination reactions. Raman spectroscopic study proves that thermal spin crossover, similar to the bulk forms, occurs in the films. With the exception of the bulk Ni complex, a virtually complete light-induced spin conversion (LIESST effect) is detected by Raman spectroscopy in the thin films as well as in the powders below ca. 130 K. [ABSTRACT FROM AUTHOR]

A complete structural, calorimetric, and magnetic characterisation of the 2D coordination spin crossover polymer {Fe(pmd)2[Cu(CN)2]2} is reported. The crystal structure has been investigated below room temperature at 180 K and 90 K, and at 30 K after irradiating the sample at low temperature with green light (λ = 532 nm). The volume cell contraction through the thermal spin transition is only 18 Å3 which is lower than the usually observed value of around 25–30 Å3 while the average Fe–N bond distances decrease by the typical value of about 0.19 Å. The structural data of the irradiated state indicate that the high spin state is well induced since the cell parameters are consistent with the data at 180 K. Calorimetric and photo-calorimetric experiments have also been performed. The entropy content for the thermal spin transition, ΔS = 35–37 J mol−1 K−1 lies in the lowest range of the typical values and correlates with the low volume cell contraction. The combination of the crystallographic and calorimetric data predicts, in accordance with a mean-field approach, a linear pressure dependence of the critical temperature with a slope of 302 K GPa−1. Magnetic measurements under pressure reveal an anomalous behaviour since the critical temperature and hysteresis do not change up to 0.22 GPa but an apparent linear dependence is obtained for higher pressures (up to 0.8 GPa) with a slope two times higher than the mean-field estimation. [ABSTRACT FROM AUTHOR]

This study was designed to evaluate the effect of melatonin on cholestatic oxidative stress under constant light exposure. Cholestasis was induced by double ligature and section of the extra-hepatic bile duct. Melatonin was injected i.p.(1000 μg kg-1 day-1). Malondialdehyde, reduced glutathione, catalase, superoxide dismutase, glutathione reductase, peroxidase and transferase were determined in liver. After bile-duct obstruction and under constant light exposure, an increase in malondialdehyde (p < 0.05) and a slight decrease in reduced glutathione were seen. Enzyme activity, with the exception of glutathione reductase, had significantly diminished. After melatonin administration, malondialdehyde fell (p < 0.001), whereas there was an increase in reduced glutathione (p < 0.0001) compared with untreated controls. Constant light exposure was associated with an increase in hepatic oxidative stress. Treatment with melatonin decreased lipid peroxide synthesis, and permitted a recovery of both reduced glutathione and scavenger enzyme activity. Copyright © 2003 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]