Home About us Contact
|
Home About us Contact | ||
|
|
||
Strong Antiferromagnetic Interaction (strong + antiferromagnetic_interaction)
Selected AbstractsDicopper(II) Complexes with the Enantiomers of a Bidentate Chiral Reduced Schiff Base: Inclusion of Chlorinated Solvents and Chiral Recognition of1,2-Dichloroethane Rotamers in the Crystal LatticeEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2006Vamsee Krishna Muppidi Abstract Bisphenoxo-bridged dicopper(II) complexes [Cu2Ln2Cl2] {1 (n = 1) and 2 (n = 2)} with the N,O-donor reduced Schiff bases N -(2-hydroxybenzyl)-(R)-,-methylbenzylamine (HL1) and N -(2-hydroxybenzyl)-(S)-,-methylbenzylamine (HL2) have been synthesised and characterised. In both 1 and 2, the bidentate chiral ligands coordinate the metal centres through the secondary amine N atom and the bridging phenolate O atom. The chloride ion occupies the fourth coordination site and completes a slightly distorted square-planar NO2Cl environment around each copper(II) centre. Magnetic susceptibility measurements in the solid state suggest a strong antiferromagnetic interaction between the metal centres in both complexes. Both 1 and 2 readily form 1:1 host-guest compounds with chlorinated solvents such as CH2Cl2, CHCl3 and Cl(CH2)2Cl. All the host-guest compounds crystallise in noncentrosymmetric space groups. 1·CH2Cl2 and 2·CH2Cl2 crystallise in the P21 space group while 1·CHCl3, 2·CHCl3, 1·Cl(CH2)2Cl and 2·Cl(CH2)2Cl crystallise in the P212121 space group. In these inclusion crystals, the C,H···Cl interactions between the guest and the host molecules are primarily responsible for enclatheration of the chloroalkane molecules. In the case of CH2Cl2, one of its Cl atoms acts as the acceptor. On the other hand, for CHCl3 and Cl(CH2)2Cl, the metal coordinated Cl atom of the host complex acts as the acceptor. The structures of 1·(P)-Cl(CH2)2Cl and 2·(M)-Cl(CH2)2Cl provide rare examples for chiral recognition of the right handed (P) and the left handed (M) gauche forms of Cl(CH2)2Cl in molecular assemblies. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Supramolecular Photomagnetic Materials: Photoinduced Dimerization of Ferrocene-Based Polychlorotriphenylmethyl RadicalsCHEMISTRY - A EUROPEAN JOURNAL, Issue 3 2004Imma Ratera Abstract New ferrocenyl Schiff-base polychlorotriphenylmethyl radicals have been synthesized and characterized. The imino group of one such radical undergoes an irreversible trans to cis structural isomerization induced by light. Such photoinduced isomerization has been monitored by UV/Vis and ESR spectroscopy and also monitored by HPLC. ESR frozen solution experiments at low temperature revealed that the cis isomer dimerizes, showing a strong antiferromagnetic interaction. Although numerous photochromic supramolecular systems have been described, such a photoinduced self-assembly process represents the first example of a one-way photoswitchable magnetic system in which a conversion between a doublet and a singlet ground state species is promoted by a photoinduced dimerization process driven by the formation of hydrogen bonds. DFT calculations on the minimized structure and on the rotational barriers have been performed to establish the origin of such behavior. The effect of the substituents and the media polarity on the photoisomerization of this imine chromophore have also been studied. It has been observed that the efficiency of the process is markedly dependent on the presence and characteristics of electron-donor and electron-acceptor substituents of the ferrocenyl Schiff-base polychlorotriphenylmethyl radicals as well as on the polarity of the solvent. Han estat sintetitzats i caracteritzats nous radicals policlorotrifenilmetílics connectats a una unitat de ferrocè mitjançant una base de Schiff. El grup imino d,un d,aquests radicals experimenta una isomerització estructural irreversible de trans a cis mitjançant la llum. Aquesta isomerització fotoinduïda ha estat seguida amb les tècniques d, UV/Vis, espectroscòpia de RPE així com també mitjançant cromatografia d, HPLC. L,espectre de RPE d,una solució congelada revel.la que L,isòmer cis dimeritza mostrant una forta interacció antiferromagnètica entre monòmers. Malgrat els nombrosos sistemes supramoleculars fotocròmics que han estat descrits fins ara, aquest procés d,autoassemblatge fotoinduït representa el primer exemple d,interruptor magnètic fotoinduït d,un sol sentit en el qual té lloc una conversió de doblet a singlet de L,espècie en estat fonamental promoguda per un procés d,autoassemblatge mitjançant la formació d,enllaços d,hidrogen. S,han realitzat càlculs de DFT de les estructures minimitzades i de les barreres rotacionals per tal de poder establir L,origen d,aquest comportament. També s,ha estudiat L,efecte dels substituents i de la polaritat del medi en la fotoisomerizació d,aquests cromòfors d,imina. S,ha observat que L,eficiència del procés té una dependència molt marcada amb la presència de substituents electro donadors així com també amb la polaritat del dissolvent. [source] Theory of chemical bonds in metalloenzymes III: Full geometry optimization and vibration analysis of ferredoxin-type [2Fe,2S] clusterINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2007Mitsuo Shoji Abstract The nature of chemical bonds in a ferredoxin-type [2Fe,2S] cluster has been investigated on the basis of natural orbitals and several bond indices developed in Parts I and II of this study. The broken-symmetry hybrid density functional theory (BS-HDFT) with spin projection approach has been applied to elucidate the natural orbitals and occupation numbers for a model compound [Fe2S2(SCH3)4] (1), which is used to calculate the indices. The molecular structure, vibration frequencies, electronic structures, and magnetic properties in both oxidized and reduced forms of 1 have been calculated and compared with the experimental values. The optimized molecular structures after approximate spin projection have been in good agreement with experimental data. The structure changes upon one-electron reduction have been slight (<0.1 Å) and only limited around one side of the Fe atom. Raman and infrared (IR) spectra have been calculated, and their vibration modes have been assigned using the bridging 34S isotope substitution. Their magnetic properties have been examined in terms of spin Hamiltonians that contain exchange interactions and double exchange interactions. The BS-HDFT methods have provided the magnetic parameters; i.e., effective exchange integral (J) values and valence delocalization (B) values, which agree with the experimental results. It is found that large charge transfer (CT) from the bridging sulfur to the iron atoms has led to the strong antiferromagnetic interactions between iron atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Compressed Octahedral Coordination in Chain Compounds Containing Divalent Copper: Structure and Magnetic Properties of CuFAsF6 and CsCuAlF6CHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2004Zoran Mazej Dr. Abstract Crystal structures and magnetic investigations of CuFAsF6 and CsCuAlF6 are reported. Together with KCuAlF6, these appear to be the only examples of Jahn,Teller pure CuII compounds containing only one type of ligand that exhibits a compressed octahedral coordination geometry. The Rietveld method has been used for refining the CsCuAlF6 structure based on neutron powder diffraction data at 4 K. The compound crystallizes in space group Pnma (no. 62) with a=7.055(1), b=7.112(1), c=10.153(1) Å and Z=4 at 4 K. The structure is built from infinite [CuF5]n3n, chains of [CuF6]4, octahedra running along the [1,0,0] direction and (AlF6)3, octahedra connected by corners in the trans position, thus giving rise to chains oriented along the [0,1,0] direction. Single crystals of CuFAsF6 were prepared under solvothermal conditions in AsF5 above its critical temperature. The structure was determined from single-crystal data. CuFAsF6 crystallises in the orthorhombic space group Imma (No. 74) with a=10.732(5), b=6.941(3), c=6.814(3) Å and Z=4 at 200 K. The structure can also be described in terms of one-dimensional infinite [CuF5]n3n, chains of tilted [CuF6]4, octahedra linked by trans -vertices running along the b axis. The [CuF5]n3n, chains are connected through [AsF6], units sharing joint vertices. The compressed octahedral coordination of CuII atoms in CuFAsF6 and CsCuAlF6 compounds at room temperature is confirmed by Cu K-edge EXAFS (extended x-ray absorption fine structure) analysis. For both compounds strong antiferromagnetic interactions within the [CuF5]n3n, chains were observed (,p=,290±10 K and ,p=,390±10 K for CuFAsF6 and CsCuAlF6, respectively). The peculiar magnetic behaviour of chain compounds containing divalent copper at low temperature could be related to uncompensated magnetic moments in the one-dimensional network. [source] | ||||||||||