Search results for "chiral"
showing 10 items of 752 documents
From Symmetry Breaking to Unraveling the Origin of the Chirality of Ligated Au13 Cu2 Nanoclusters
2018
A general method, using mixed ligands (here diphosphines and thiolates) is devised to turn an achiral metal cluster, Au13 Cu2 , into an enantiomeric pair by breaking (lowering) the overall molecular symmetry with the ligands. Using an achiral diphosphine, a racemic [Au13 Cu2 (DPPP)3 (SPy)6 ]+ was prepared which crystallizes in centrosymmetric space groups. Using chiral diphosphines, enantioselective synthesis of an optically pure, enantiomeric pair of [Au13 Cu2 ((2r,4r)/(2s,4s)-BDPP)3 (SPy)6 ]+ was achieved in one pot. Their circular dichroism (CD) spectra give perfect mirror images in the range of 250-500 nm with maximum anisotropy factors of 1.2×10-3 . DFT calculations provided good corre…
Synthesis of a chiral rod-like metal–organic framework from a preformed amino acid-based hexanuclear wheel
2019
We report the two-step synthesis of a chiral rod-like metal-organic framework (MOF). The chemical approach consists on the use of a previously prepared oxamato-based homochiral hexanuclear wheel, the ligand being a derivative of the natural amino acid l-alanine, with formula (Me4N)6{CuII6[(S)-alama])6}·10H2O (1) [where (S)-alama=(S)-N-(ethyl oxoacetate)alanine]. The anionic hexacopper(II) wheels, stabilized by the presence of templating tetramethylammonium counter-cations, disassemble in the presence of cationic square-planar [Ni(cyclam)]2+ complexes to yield, after a supramolecular reorganization process that involves axial coordination of the [Ni(cyclam)]2+ cations through the free carbon…
Inherently Chiral Calixarenes
1994
Due to the nonplanarity of the basic 1 n -metacyclophane system, calixarenes and resorcarenes can be transformed into molecules with inherent chirality. Various attempts to achieve this goal are reviewed. Special emphasis is given to derivatives with C n -symmetry, including derivatives of spherand calixarenes and other calixarene-like macrocycles.
Chiral symmetry breaking condensates from baryonic sum rules
1984
We analyse baryonic sum rules in order to determine chiral symmetry breaking condensates. We especially investigate the influence of the choice of the interpolating field and of the factorization hypothesis for the four quark condensate. Our results are consistent with those obtained from pseudoscalar sum rules and PCAC.
Textures in hexatic films of nonchiral liquid crystals: Symmetry breaking and modulated phases
1994
Novel modulated textures, such as stripes and multiarmed star defects, have been observed in freely suspended films of nonchiral liquid crystals just below the smectic-$C$ to hexatic phase transition. Detailed studies using depolarized reflection microscopy suggest that the stripes are locally chiral surface splay domains of the smectic-$L$ phase, a tilted hexatic not previously identified in thermotropic liquid crystals. Line defects which form additional domain walls in the hexatic lattice lead to characteristic modulations of the basic one-dimensional stripe pattern. Inside thick circular islands, for example, stripes form circumferentially and the lines form centered 12-armed stars, res…
An Na8 Cluster in the Structure of a Novel oxamato-bridged Na'Cu'' three-Dimensional Coordination Polymer
1999
[EN] The new heterometallic sodium(I)¿copper(II) compound Na4Cu2 (2) · 10.5 H2O (3), where H8[2] stands for N,N',N'',N'''-methanetetrayltetrakismethylenetetrakis(oxamic acid), has been synthesized and its crystal structure determined by single-crystal X-ray diffraction. The structure of 3 consists of cationic [Cu2(¿4:¿4-2)]4- dinuclear units, coordinated sodium cations, and water molecules. In the crystal, the dinuclear copper entities are joined through discrete aggregates of eight sodium atoms linked by oxamato and water bridges, leading to a three-dimensional polymeric network.
Electromagnetic mass difference of pions at low temperature
1999
We compute low temperature corrections to the electromagnetic mass difference of pions in the chiral limit. The computation is done in a model independent way in the framework of chiral perturbation theory, using the background field method and the hard thermal loop approximation. We also generalize at low temperature the sum rule of Das et al. We find that the mass difference between the charged and neutral pions decreases at low temperature $T$ with respect to the T=0 value. This is so in spite of the fact that charged particles always get a thermal correction to their masses of order $\sim eT$, where $e$ is the gauge coupling constant. Our result can be understood as a consequence of the…
Infrared renormalization of two-loop integrals and the chiral expansion of the nucleon mass
2007
We describe details of the renormalization of two-loop integrals relevant to the calculation of the nucleon mass in the framework of manifestly Lorentz-invariant chiral perturbation theory using infrared renormalization. It is shown that the renormalization can be performed while preserving all relevant symmetries, in particular chiral symmetry, and that renormalized diagrams respect the standard power counting rules. As an application we calculate the chiral expansion of the nucleon mass to order O(q^6).
Contribution of the $a_1$ meson to the axial nucleon-to-$\Delta$ transition form factors
2018
We analyze the low-$Q^2$ behavior of the axial form factor $G_A(Q^2)$, the induced pseudoscalar form factor $G_P(Q^2)$, and the axial nucleon-to-$\Delta$ transition form factors $C^A_5(Q^2)$ and $C^A_6(Q^2)$. Building on the results of chiral perturbation theory, we first discuss $G_A(Q^2)$ in a chiral effective-Lagrangian model including the $a_1$ meson and determine the relevant coupling parameters from a fit to experimental data. With this information, the form factor $G_P(Q^2)$ can be predicted. For the determination of the transition form factor $C^A_5(Q^2)$ we make use of an SU(6) spin-flavor quark-model relation to fix two coupling constants such that only one free parameter is left.…
Determination of the axial coupling constant $g_{A}$ in the linear representations of chiral symmetry
2001
If a baryon field belongs to a certain linear representation of chiral symmetry of $SU(2) \otimes SU(2)$, the axial coupling constant $g_{A}$ can be determined algebraically from the commutation relations derived from the superconvergence property of pion-nucleon scattering amplitudes. This establishes an algebraic explanation for the values of $g_{A}$ of such as the non-relativistic quark model, large-$N_{c}$ limit and the mirror assignment for two chiral partner nucleons. For the mirror assignment, the axial charges of the positive and negative parity nucleons have opposite signs. Experiments of eta and pion productions are proposed in which the sign difference of the axial charges can be…