Single-crystal ESR Spectroscopy Research Articles

Determining the Geometry and Magnetic Parameters of Fluorinated Radicals by Simulation of Powder ESR Spectra and DFT Calculations: The Case of the Radical RCF2CF2• in Nafion Perfluorinated Ionomers

The ESR spectrum of the chain-end radical RCF2CF2* detected in Nafion perfluorinated membranes exposed to the photo-Fenton reagent was accurately simulated by an automatic fitting procedure, using as input the hyperfine coupling tensors of the two F alpha and two F beta nuclei as well as the corresponding directions of the principal values from density functional theory (DFT) calculations. An accurate fit was obtained only for different orientations of the hyperfine coupling tensors for the two F alpha nuclei, indicating a nonplanar structure about the C alpha radical center. The fitted isotropic hyperfine splittings for the two F beta nuclei in the Nafion radical, 24.9 and 27.5 G, are significantly larger than those for the chain-end radical in Teflon (15 G), implying different radical conformations in the two systems. The excellent fit indicated that the geometry and electronic structure of free radicals can be obtained not only from single-crystal ESR spectroscopy, but also, in certain cases, from powder spectra, by combination with data from DFT calculations. The optimized structures obtained by DFT calculations for the CF3CF2CF2CF2* or CF3OCF2CF2* radicals as models provided additional support for the pyramidal structure determined from the spectral fit. Comparison and analysis of calculated and fitted values for the hyperfine splittings of the two F beta nuclei suggested that the radical detected by ESR in Nafion is ROCF2CF2*, which originates from attack of oxygen radicals on the Nafion side chain. The combination of spectrum fitting and DFT is considered important in terms of understanding the hyperfine splittings from 19F nuclei and the different conformations of fluorinated chain-end-type radicals RCF2CF2* in different systems, and also for elucidating the mechanism of Nafion fragmentation when exposed to oxygen radicals in fuel cell conditions.

Ferromagnetic Exchange Interactions for Cu612+and Mn612+Hexagons Sandwiched by Two B-α-[XW9O33]9-(X = AsIIIand SbIII) Ligands inD3d-Symmetric Polyoxotungstates

Two novel Cu6 and Mn6 hexagon sandwiched polyoxometalates, [(CuCl)6(AsW9O33)2]12- (1a) and [(MnCl)6(SbW9O33)2]12- (2a), have been synthesized and characterized by X-ray single-crystal analysis and magnetic measurements. These complexes are D3d symmetric and were isolated as [n-BuNH3]+ salts from aqueous solutions: (n-BuNH3)12[(CuCl)6(AsW9O33)2].6H2O (1), rhombohedral, R, a = 20.33(1) A, c = 26.35(2) A, Z = 3 and (n-BuNH3)12[(MnCl)6(SbW9O33)2].6H2O (2). Six Cu (or Mn) atoms, each of which shows 5-fold coordination, make an approximately equatorial hexagon with a first-neighboring Cu...Cu (Mn...Mn) distance of 2.913(2) A (3.248(1) A) and a Cu-O-Cu (Mn-O-Mn) bond angle of 94.5(2) degrees (100.4(2) degrees ). The magnetic behavior investigated by magnetic susceptibility measurements shows the ferromagnetic exchange interactions with J/k = +12.7 K (J/hc = +8.82 cm(-1)) and a S = 3 ground state for 1 and J/k = +0.20 K (J/hc = +0.14 cm(-1)) and a S = 15 ground state for 2, when only J refers to the isotropic magnetic-exchange interactions for first-neighbor atoms of the approximately equilateral Cu6(12+) and Mn6(12+) hexagons. The single-crystal ESR spectroscopy of 1 under the orientation of the magnetic field along a Cu6 hexagon's nearly 6-fold axis equal to the c axis on the variation of temperature supports the S = 3 ground state and allows an estimate of the zero-field (fine-structure) energy separation between Sz = 0 and Sz = +/-1 of D = -0.182 K to be obtained.

ESR Study of High-Spin Nitrenes with s-Triazine Skeleton

Electronic and molecular structures of mononitrene 1 and bisnitrene 2 derived from 2,4,6-triazide- s-triazine 4 by photolysis have been studied by powder-pattern and single-crystal ESR spectroscopy. The experimentally determined fine-structure parameters D and E were 1.402 and 0.011 cm −1 for 1 ( S = 1), and 0.280 and 0.058 cm −1 for 2 ( S = 2), respectively. These values are much larger than those of phenylnitrenes ( S = 1) and m-phenylenebisnitrenes ( S= 2). The fine-structure parameters of bisnitrene 2 were interpreted by a semiempirical model considering dominant one-center spin-spin interactions at the monovalent nitrogen atoms, indicating that the π-electron spins are more localized on the monovalent nitrogen atoms and that the π-delocalization effect is hampered by introducing heteroatoms into the π-conjugation system

Crystal Structure and Magnetic Characterization of a Building Block for Quasi One-Dimensional Heterobimetallic Polymeric Complexes Bridged by 2,2′-Bibenzimidazolate Ligands

We have synthesized 63Cu(bbim)2](nBu4N)2(1)by 2,2′bibenzimidazolate ligands (bbim) with multi-proton donor property as a building block of models for quasi one-dimensional heterobimetallic polymeric complexes with dp-pp interaction. The study of the molecular and electronic structures of 1 and the non-deprotonated form of 1, in terms of crystal structure analysis and magnetic characterization by single-crystal ESR spectroscopy, has been carried out. The angular dependence of the Cu(II) hyperfine ESR spectra of 1 showed that the parallel orientation (B0 || c axis) gave both the maximum copper hyperfine splitting (84 × 10−4cm−1) and the maximum anisotropic g value (g || =2.218). The findings show that the copper(II) ion is bound at the site of a tetragonally distorted tetrahedral field, being in accord with the molecular structure of the donor site with the distorted tetrahedral symmetry as obtained by the X-ray crystal analysis. Superhyperfine structures due to the nitrogen atoms of bbim2− dianion were detected in the g1 region of the ESR spectrum of [63Cu(bbim)2](nBu4N)2 in a frozen glass.

Crystal structure and magnetic characterization of a building block for quasi one-dimensional heterobimetallic polymeric complexes bridged by 2,2′-bibenzimidazolate ligands

We have synthesized [ 63Cu(bbim) 2] ( nBu 4N) 2( 1)by 2,2′-bibenzimidazolate ligands (bbim) with multi-proton donor property as a building block of models for quasi one-dimensional heterobimetallic polymeric complexes with dπ-pπ interaction. The study of the molecular and electronic structures of 1 and the non-deprotonated form of 1, in terms of crystal structure analysis and magnetic characterization by single-crystal ESR spectroscopy, has been carried out. The angular dependence of the Cu(II) hyperfine ESR spectra of 1 showed that the parallel orientation ( B 0 // c axis) gave both the maximum copper hyperfine splitting (84 × 10 −4cm −1) and the maximum anisotropic g value (g // =2.218). The substantially isotropic behavior of the g value (g ⊥=2.05) was obtained in the perpendicular orientation. The findings show that the copper(II) ion is bound at the site of a tetragonally distorted tetrahedral field, being in accord with the molecular structure of the donor site with the distorted tetrahedral symmetry as obtained by the X-ray crystal analysis. The results from the ESR measurements of 1 indicate that the ground-state electronic configuration for the copper(II) ion is (z 2)( 2)…(yz)( yz)(xy)¦ in terms of d-orbitals. Superhyperfine structures due to the nitrogen atoms of bbim 2− dianion were detected in the g ⊥ region of the ESR spectrum of [ 63Cu(bbim) 2]( nBu 4N) 2 in a frozen glass.

Crystal Structure and Magnetic Characterization of a Building Block For One-Dimensional Heterobimetallic Poly- Meric Complexes Bridged by 2,2′-Bibenzimidazolate Ligands

Abstract We have designed a bidentate ligand with multi-proton donor property as a building block of models for one-dimensional heterobimetallic complexes. We have synthesized [Cu(bbim)2](nBu4N)2 (1) as a candidate for the building block of such one-dimensional complexes. The crystal structure analysis of 1 and magnetic characterization by single-crystal ESR spectroscopy have been carried out. The crystal data is as follows: orthorhombic, P21212, a = 17.860 A, b= 20.274 A, c= 8.704 A, Z= 2. 63Cu-enriched compound of 1 has also been synthesized for single-crystal ESR spectroscopy. ESR measurements of non-magnetically diluted single crystals of 1 have shown that Cu(II) ions are in the tetrahedral ligand field with great tetragonal distortion, being in accord with the molecular structural data on 1 by the X ray analysis.

Electronic and Molecular Structures of 1,3,5-Tris(Phenylmethylene)Benzene (S = 3) as Models for Discotic High-Spin Liquid Crystals: Single-Crystal ESR and X-Ray Diffraction Studies

Abstract The electronic and molecular structures of the two-dimensional septet-state molecules(S=3), l,3,5-tris(p-mcthoxyphenylmcthylcne)benzcnc A and 1,3,5-tris(phenylmethylene)benzene B, were investigated by single-crystal ESR spectroscopy. A was designed for a prototypical core model for discotic liquid-crystalline organic high-spin molecules. We determined the fine-structure constants of A by the ESR spectroscopy and analyzed the crystal and molecular structures of B by X-ray diffraction. The effect of the mcthoxy functional group on the fine-structure constants was found: the effect showed up in the significant reduction of the fine-structure constant D value for A, indicating the modification of the topologically controlled robust π spin polarization. The observed non-vanishing E value is in accord with the molecular symmetry and crystal structure of the host molecule, 1,3,5-tris(α-benzoyl)benzene.

Electronic and Molecular Structures of 2D Septet Tris(p-Methoxyphenylmethylene)Benzene as Studied by ESR: A Model for Discotic High-Spin Assemblages

Abstract The electronic and molecular structures of a two-dimensional septet-state molecule (S=3), 1,3,5-tris(p-methoxyphenylmethylene)benzene A, were investigated by single-crystal ESR spectroscopy. A was designed for a prototype core model for discotic liquid crystalline organic high-spin molecules. Tris(α-benzoyl)benzene was adopted as the host crystal and its crystallographic parameters were determined by X-ray diffraction: the space group Pi with Z=4, a=11.99 A, b=21.94 A, c=7.82 A, α=98.0°, β=90.9°, and γ=94.1°. The effect of the methoxy functional group on the fine-structure constants was found: the effect showed up in the significant reduction of the fine-structure constant D value, showing the modification of the topologically controlled robust π spin polarization. The observed finite E value is in accord with the molecular symmetry and crystal structure of the host molecule.

Molecular Design, Synthesis, and Electron Spin Resonance Detection of a Ground-State Tridecet (S=6) Hydrocarbon as a Model for 2D Organic Ferro- and Superpara-Magnets

The electronic and molecular structures of a high-spin carbene oligomer, hexakiscarbene B, having a two-dimensional spin structure were investigated by single-crystal ESR spectroscopy. B was design...

Role of High-Spin Molecules as Models for Organic Ferro- and Ferri-Magnets, and Pi-Toporegulated Magnetic Polymers

ABSTRACTA chemical modification exploiting functional groups such as ether and methylene as bridges between high-spin assemblies has been carried out as a usable method to increase dimensionality of spin structure. Exchange interaction via an ether or a methylene bridge between two high-spin assemblies has been studied by single-crystal ESR spectroscopy. Whether it is ferromagnetic or antiferromagnetic depends upon the substituted position of the bridge, demonstrating the important role of the topological nature in spin alignment. It turns out that superexchange interaction or hyperconjugation mechanism can dominate spin alignment between high-spin assemblies. Using the criteria obtained, model compounds for units of organic ferrimagnets have been synthesized. They possess antiferromagnetically-exchange coupled heterospins and the salient features of their spin structures are characterized in terms of the spin density distribution as determined by single-crystal 1H-ENDOR (Electron-Nuclear-DOuble Resonance) spectroscopy.