32,281 research outputs found
The monoclinic crystal structure of -RuCl and the zigzag antiferromagnetic ground state
The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to
realize a Kitaev spin model with strongly frustrated, bond-dependent,
anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic
moments. Here we report a detailed study of the three-dimensional crystal
structure using x-ray diffraction on untwinned crystals combined with
structural relaxation calculations. We consider several models for the stacking
of honeycomb layers and find evidence for a crystal structure with a monoclinic
unit cell corresponding to a stacking of layers with a unidirectional in-plane
offset, with occasional in-plane sliding stacking faults, in contrast with the
currently-assumed trigonal 3-layer stacking periodicity. We report electronic
band structure calculations for the monoclinic structure, which find support
for the applicability of the jeff=1/2 picture once spin orbit coupling and
electron correlations are included. We propose that differences in the
magnitude of anisotropic exchange along symmetry inequivalent bonds in the
monoclinic cell could provide a natural mechanism to explain the spin gap
observed in powder inelastic neutron scattering, in contrast to spin models
based on the three-fold symmetric trigonal structure, which predict a gapless
spectrum within linear spin wave theory. Our susceptibility measurements on
both powders and stacked crystals, as well as neutron powder diffraction show a
single magnetic transition at TN ~ 13K. The analysis of the neutron data
provides evidence for zigzag magnetic order in the honeycomb layers with an
antiferromagnetic stacking between layers. Magnetization measurements on
stacked single crystals in pulsed field up to 60T show a single transition
around 8T for in-plane fields followed by a gradual, asymptotic approach to
magnetization saturation, as characteristic of strongly anisotropic exchange
interactions.Comment: 13 pages, 9 figures, published in Physical Review
Stability and magnetic properties of Fe double-layers on Ir (111)
We investigate the interplay between the structural reconstruction and the
magnetic properties of Fe doublelayers on Ir (111)-substrate using
first-principles calculations based on density functional theory and mapping of
the total energies on an atomistic spin model. We show that, if a second Fe
monolayer is deposited on Fe/Ir (111), the stacking may change from hexagonal
close-packed to bcc (110)-like accompanied by a reduction of symmetry from
trigonal to centered rectangular. Although the bcc-like surface has a lower
coordination, we find that this is the structural ground state. This
reconstruction has a major impact on the magnetic structure. We investigate in
detail the changes in the magnetic exchange interaction, the magnetocrystalline
anisotropy, and the Dzyaloshinskii Moriya interaction depending on the stacking
sequence of the Fe double-layer. Based on our findings, we suggest a new
technique to engineer Dzyaloshinskii Moriya interactions in multilayer systems
employing symmetry considerations. The resulting anisotropic
Dzyaloshinskii-Moriya interactions may stabilize higher-order skyrmions or
antiskyrmions
Anion directed cation templated synthesis of three ternary copper(II) complexes with a monocondensed N2O donor Schiff base and different pseudohalides
Three copper(II) complexes, [Cu2(L)2(ÎŒ1,1-N3)2] (1), [Cu2(L)2(ÎŒ1,1-NCO)2] (2) and [Cu(L)(ÎŒ1,5-dca)]n (3), where HL is a tridentate mono-condensed Schiff base, 1-(2-aminoethyliminomethyl)naphthalen-2-ol, and dca is dicyanamide, have been prepared and characterized by elemental analysis, IR, UVâVis and fluorescence spectroscopy and single crystal X-ray diffraction studies. The Schiff base ligand was prepared by a counter anion mediated copper(II) templated synthesis. The azide ligand in complex 1 and the cyanate ligand in complex 2 show ÎŒ-1,1 bridging modes, whereas the dca ligand shows the ÎŒ-1,5 bridging mode in complex 3. Three equatorial positions of copper(II) are occupied by the tridentate Schiff bases in all three complexes. The fourth equatorial sites are occupied by nitrogen atoms of azide in 1, cyanate in 2 and dca in 3. Another nitrogen atom from a symmetry related pseudohalide coordinates in the axial position of copper(II) to complete its square pyramidal geometry in each of the complexes. Significant supramolecular interactions are observed in all the complexes. Variable temperature magnetic measurements indicate antiferromagnetic interactions between the copper(II) centres in all three complexes
Inferring DNA sequences from mechanical unzipping data: the large-bandwidth case
The complementary strands of DNA molecules can be separated when stretched
apart by a force; the unzipping signal is correlated to the base content of the
sequence but is affected by thermal and instrumental noise. We consider here
the ideal case where opening events are known to a very good time resolution
(very large bandwidth), and study how the sequence can be reconstructed from
the unzipping data. Our approach relies on the use of statistical Bayesian
inference and of Viterbi decoding algorithm. Performances are studied
numerically on Monte Carlo generated data, and analytically. We show how
multiple unzippings of the same molecule may be exploited to improve the
quality of the prediction, and calculate analytically the number of required
unzippings as a function of the bandwidth, the sequence content, the elasticity
parameters of the unzipped strands
Unveiling interactions between DNA and cytotoxic 2-arylpiperidinyl-1,4-naphthoquinone derivatives: A combined electrochemical and computational study
IndexaciĂłn: Scopus.Three 2-arylpiperidinyl-1,4-naphthoquinone derivatives were synthesized and evaluated in vitro to determine their cytotoxicity on cancer and normal cell lines. In order to establish their possible action mechanism, the electrochemical behaviour of these quinones was examined using cyclic voltammetry (CV) as technique by using a three-electrode setup: a glassy carbon, Ag/AgCl (in 3 M KCl), and platinum wire as working, reference, and counter electrodes, respectively. Kinetic studies were done to determine the control of the reduction reaction and the number of transferred electrons in the process. Furthermore, the addition of dsDNA to the quinone solutions allowed for the observation of an interaction between each quinone and dsDNA as the current-peaks became lower in presence of dsDNA. Otherwise, motivated to support the aforementioned results, electronic structure calculations at the TPSS-D3/6-31+G(d,p) level of theory were carried out in order to find the most favourable noncovalently bonded complexes between quinones and DNA. Noncovalent complexes formed between DNA and 2-arylpiperidinyl-1,4-naphthoquinones and stabilized by Ï-stacking interactions along with the well-known hydrogen-bonded complexes were found, with the former being more stable than the latter. These results suggest that the intercalation of these quinone derivatives in DNA is the most likely action mechanism. © 2018 King Saud Universityhttps://www.sciencedirect.com/science/article/pii/S1878535218300893?via%3Dihu
Concentration dependent tautomerism in green [Cu(HL1)(L2)] and brown [Cu(L1)(HL2)] with H2L1 = (E)-Nâ-(2-hydroxy-3-methoxybenzylidene)- benzoylhydrazone and HL2 = pyridine-4-carboxylic (isonicotinic) acid
The in situ formed hydrazone Schiff base ligand (E)-Nâ-(2-hydroxy-3-methoxybenzylidene)-benzoylhydrazone (H2L1) reacts with copper(II) acetate in ethanol in the presence of pyridine-4-carboxylic acid (isonicotinic acid, HL2) to green-[Cu(HL1)(L2)]ă»H2Oă»C2H5OH (1) and brown-[Cu(L1)(HL2)] (2) complexes which crystallize as concomitant tautomers where either the mono-anion (HL1)- or di-anion (L1)2- of the Schiff base and simultaneously the pyridine-carboxylate (L2)- or the acid (HL2) (both through the pyridine nitrogen atom) function as ligands. The square-planar molecular copper(II) complexes differ in only a localized proton position either on the amide nitrogen of the hydrazone Schiff base in 1 or on the carboxyl group of the isonicotin ligand in 2. The proportion of the tautomeric forms in the crystalline solid-state can be controlled over a wide range from 1:2 = 95 : 5 to ~2 : 98 by increasing the solution concentration. UV/Vis spectral studies show both tautomers to be kinetically stable (inert), that is, with no apparent tautomerization, in acetonitrile solution. The UB3LYP/6-31+G* level optimized structures of the two complexes are in close agreement with experimental findings. The solid-state structures feature 1D hydrogen-bonded chain from charge-assisted O(-) ⊠HâN and OâH ⊠(-)N hydrogen bonding in 1 and 2, respectively. In 1 pyridine-4-carboxylate also assumes a metal-bridging action by coordinating a weakly bound carboxylate group as a fifth ligand to a Cu axial site. Neighboring chains in 1 and 2 are connected by strong Ï-stacking interactions involving also the five- and six-membered, presumably metalloaromatic Cu-chelate rings
Coordination polymers and isomerism; a study using silver(I) and a â-stacked ligand
The ligand 2,5-bis(2-pyridylmethylsulfanylmethyl)pyrazine (L) was prepared by the base coupling of 2-(sulfanylmethyl)pyridine and 2,5-bis(chloromethyl)pyrazine. This new ligand was treated with AgClOâ in a 1 1 metal-to-ligand ratio and with AgNOâin a 2 1 metal-to-ligand ratio to give coordination polymers. The crystal structures of {[Ag(L)]ClOâ}â ( 1) and {[Agâ(L)](NOâ)â}â ( 2) were determined. The Ag(I) ions in the one-dimensional polymeric chains of 1 adopted square-pyramidal geometries with the pyridine and pyrazine N donors coordinated in an extremely bent fashion. The structure of 2 revealed two isomeric polymer chains in the one crystal forming a single supramolecular array. The isomeric polymers differed in the donor atoms about the Ag(I) ions and in the arrangement of adjacent ligands along the chain. A feature of both structures was that L adopted a three-layer â-stacked arrangement
J-factors of short DNA molecules
The propensity of short DNA sequences to convert to the circular form is
studied by a mesoscopic Hamiltonian method which incorporates both the bending
of the molecule axis and the intrinsic twist of the DNA strands. The base pair
fluctuations with respect to the helix diameter are treated as path
trajectories in the imaginary time path integral formalism. The partition
function for the sub-ensemble of closed molecules is computed by imposing chain
ends boundary conditions both on the radial fluctuations and on the angular
degrees of freedom. The cyclization probability, the J-factor, proves to be
highly sensitive to the stacking potential, mostly to its nonlinear parameters.
We find that the J-factor generally decreases by reducing the sequence length (
N ) and, more significantly, below N = 100 base pairs. However, even for very
small molecules, the J-factors remain sizeable in line with recent experimental
indications. Large bending angles between adjacent base pairs and anharmonic
stacking appear as the causes of the helix flexibility at short length scales.Comment: The Journal of Chemical Physics - May 2016 ; 9 page
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