Synchrotron radiation for structure analysis—EXAFS and XANES

In this paper, a newly developed technique for the structure analysis of materials by the use of the X-rays emitted from storage ring of synchrotron is introduced. Principle, applications and limitations of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) are discussed.     

Determination of the local structure and parameters of the one-electron potential from XANES data (inverse problem of XANES theory)

A method of obtaining complex information on the structure of polyatomic systems using experimental data for one-electron quasistationary state (shape resonance) is developed. The method involves the equation for S-matrix poles in the muffintin (MT) approximation. The parameters of the model (intemuclear distances, valence angles, and potential characteristics) are fitted in such a way that the S-matrix poles in a complex energy plane be maximally close to {E-iГ/2}, where E and Fare the energies and widths of the XANES maxima of one-electron origin. Testing the method on a number of objects shows that it determines internuclear distances with an accuracy of 1% and valence angles with an accuracy of 3%. Among the potentials of this type, the empirical potentials obtained are the best for describing shape resonances. The suggested scheme is applicable in determining the microstructure and in the cases where the diffraction methods of structural analysis do not work (unordered systems, molecular adsorption on the surface of solids, etc.). Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 1013–1017, November–December, 1998.     

Dual bimodal polyethylene prepared by intercalated silicate with nickel diimine complex

The ethylene polymerization was catalyzed by the intercalated montmorillonite with the nickel complex, [ArN?C(Me)? C(Me)?NAr]NiBr₂ (Ar = 2,6‐C₆H₃ (i‐Pr)₂). Polymer with low melting point and high molecular weight was produced at the early stage of polymerization followed by formation of polymer with high melting point and low molecular weight. It is proposed that the gallery of silicate lowers the propagation rate of polymerization and frequency of “chain walking” process of nickel complex anchored inside the gallery, which produces polymer with low molecular weight and low branching, whereas the nickel complex immobilized on the surface of silicate generates polymer with high molecular weight and high branching. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5506–5511, 2005     

Evaluation of diimine ligand exchange on Cu(I)

Four ligands have been prepared, 8,8-dimethyl-6,7,9-trihydropyrido[1,2-b]acridine and three 4,4',6,6'-tetrasubstituted derivatives of 2,2'-bipyrimidine where the substituents are methyl, phenyl, and p-tolyl. The corresponding [CuL(2)](+) salts of these ligands evidence nonequivalent NMR signals that allow an estimation of the ligand exchange barrier in both acetonitrile and chloroform solution. Lower barriers are found in the former solvent and attributed to solvent participation in the exchange process. Corresponding differences in the oxidation potentials of the complexes are explained in a similar manner. The electronic absorption properties of the complexes are also consistent with the steric and electronic properties of the ligands. [Cu(2c)(2)](PF(6)), where 2c = 4,4',6,6'-tetraphenyl-2,2'-bipyrimidine, was analyzed by X-ray diffraction and found to crystallize in the space group Pccn with a = 14.761(2) A, b = 15.007(2) A, c = 24.407(4) A, and Z = 4. The internal and external phenyl rings are disposed quite differently, with the internal rings interacting strongly with the orthogonal ligand.     

Geometrical and electronic structure of small copper nanoclusters: XANES and DFT analysis

The geometrical and electronic structure of small copper nanoclusters was studied by density functional theory (DFT) and analysis of X-ray absorption spectra. It was shown that the icosahedral geometry of small copper nanoclusters of 13 atoms was energetically more stable than cuboctahedral geometry. The binding energies in these structures were compared; the theoretical XANES spectra were compared with experiment. The paper gives the results of ab initio calculations of the electronic structure of copper clusters differing in size.     

Molecular structure determination of Ni(II) diimine complex and DMA analysis of Ni(II) diimine‐based polyethenes

Dynamic mechanical thermoanalysis showed that polyethene, prepared under suitable polymerization conditions with the Brookhart‐type catalyst dibromo‐N,N′‐1,2‐acenaphthylenediylidenebis[2,6‐bis(1‐methylethyl)benzeneamine]Ni(II)/methylaluminoxane (MAO), behaved like an elastomer, even though no comonomer was added. A structural characterization showed that the polymers contained methyl to hexyl branches and some longer branches. The effect of the polymerization conditions on branching was investigated through variations in the pressure and temperature of the polymerization. Depending on the degree and type of branching, polyethene was either quite amorphous or highly crystalline with a high melting temperature. The solid‐state structure of the catalyst dibromo‐N,N′‐1,2‐acenaphthylenediylidenebis[2,6‐bis(1‐methylethyl)benzeneamine]Ni(II) consisted of two centrosymmetrically related monomeric moieties, where Ni atoms were bridged by two bromide ligands. The Ni atom was five‐coordinated, with a square pyramidal coordination polyhedron. The sixth coordination site of the octahedral geometry was effectively blocked by the isopropyl groups of the 2,6‐C₆H₃(i‐Pr) substituents of the diimine ligand. In solution in the presence of MAO, the longer bridging Ni? Br bonds broke, and the complex dissociated to a monomeric species. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1426–1434, 2001     

Uracil on Cu(110): a quantitative structure determination by energy-scanned photoelectron diffraction

The local adsorption site of the nucleobase uracil on Cu(110) has been determined quantitatively by energy-scanned photoelectron diffraction (PhD). Qualitative inspection of the O 1s and N 1s soft x-ray photoelectron spectra, PhD modulation spectra, and O K-edge near-edge x-ray adsorption fine structure indicate that uracil bonds to the surface through its nitrogen and oxygen constituent atoms, each in near atop sites, with the molecular plane essentially perpendicular to surface and aligned along the close packed [110] azimuth. Multiple scattering simulations of the PhD spectra confirm and refine this geometry. The Cu-N bondlength is 1.96 ± 0.04 ?, while the Cu-O bondlengths of the two inequivalent O atoms are 1.93 ± 0.04 ? and 1.96 ± 0.04 ?, respectively. The molecule is twisted out of the [110]direction by 11 ± 5°.     

MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations

The molecular structure and dynamics of the photoexcited metal-to-ligand-charge-transfer (MLCT) state of [Cu(I)(dmp)(2)](+), where dmp is 2,9-dimethyl-1,10-phenanthroline, in acetonitrile have been investigated by time-domain pump-probe X-ray absorption spectroscopy, femtosecond optical transient spectroscopy, and density functional theory (DFT). The time resolution for the excited state structural determination was 100 ps, provided by single X-ray pulses from a third generation synchrotron source. The copper ion in the thermally equilibrated MLCT state has the same oxidation state as the corresponding copper(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distance 0.04 A shorter than that of the ground state [Cu(I)(dmp)(2)](+). The results confirm the previously proposed "exciplex" structure of the MLCT state in Lewis basic solvents. The evolution from the photoexcited Franck-Condon MLCT state to the thermally equilibrated MLCT state was followed by femtosecond optical transient spectroscopy, revealing three time constants of 500-700 fs, 10-20 ps, and 1.6-1.7 ns, likely related to the kinetics for the formation of the triplet MLCT state, structural relaxation, and the MLCT excited-state decay to the ground state, respectively. DFT calculations are used to interpret the spectral shift on structural relaxation and to predict the geometries of the ground state, the tetracoordinate excited state, and the exciplex. The DFT calculations also indicate that the amount of charge transferred from copper to the dmp ligand upon photoexcitation is similar to the charge difference at the copper center between the ground-state copper(I) and copper(II) complexes.     

Effect of the alkylaluminum cocatalyst on ethylene polymerization by a nickel–diimine complex

Different chlorine-free alkylaluminum compounds were active cocatalysts for ethylene polymerization in the presence of 1,4-bis(2,6-diisopropylphenyl)-acenaphthenediimine-dichloronickel (II) (1). The combination of 1 with trimethylaluminum or triisobutylaluminum produced catalytically active species that polymerized ethylene with productivities up to 469 kg_polymer/(mol_Ni · h). The activity of the catalytic system and the properties of the polymeric materials were influenced strongly by the reaction temperature. The polymers had a high molecular weight (up to 642 × 10³ g · mol⁻¹), and the molecular weight increased with the reaction time. The polyethylenes were branched, and the branching could be modulated by the proper choice of reaction parameters. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4656–4663, 1999     

The role of trace metallic elements in neurodegenerative disorders: quantitative analysis using XRF and XANES spectroscopy.

The present paper focuses on the analysis of trace metallic elements and their role in neurodegenerative disorders. The use of synchrotron radiation microbeams allows investigation of pathological tissues from Alzheimer's disease, Parkinson's disease and Amyotrophic lateral sclerosis cases in a nondestructive manner and at cellular level. By employing X-ray absorption near edge structure (XANES) technique, the chemical state of the investigated elements can be determined, while energy-selective X-ray fluorescence spectroscopy provides the spatial distribution of each element in each oxidative state selectively. The investigated tissues (derived from human, monkey and mouse specimens) show distinct imbalances of metallic elements such as Zn and Cu as well as Fe(2+)/Fe(3+) redox pair, which point to oxidative stress as a crucial factor in the development or progress of these neurodegenerative diseases.     

Reactivity of an aluminum hydride complex with a redox-active diimine ligand

Russian Chemical Bulletin - The reaction of the hydride [(dpp-bian)Al(H)Cl] (1) containing the 1,2-bis[(2,6-diisopropylphenyl) imino]acenaphthene (dpp-bian) radical anion with MeLi affords the...     

MXAN analysis of the XANES energy region of a mononuclear copper complex: applications to bioinorganic systems

The near edge XAS spectra of the mononuclear copper complex [Cu(TMPA)(OH(2))](ClO(4))(2) (1) have been simulated using the multiple scattering edge simulation package MXAN (or Minuit XANes). These simulations, which employ the muffin-tin (MT) approximation, have been compared to simulations generated using the finite-difference method (FDM) to evaluate the effect of MT corrections. The sensitivity of the MXAN method was tested using structural models that included several different variations on the bond angles and bond distances for the first-shell atoms of 1. The sensitivity to small structural changes was also evaluated by comparing MXAN simulations of 1 and of structurally modified [Cu(TMPA)(L)](n)(+) complexes [where L = -O-(F(8)TPP)Fe(III), -F, -OPO(2)(O-p-nitrophenyl)Zn(II)(TMPA), and -NCMe] to the experimental data. The accuracy of the bond distances obtained from the MXAN simulations was then examined by comparison to the metrics of the crystal structures. The results show that MXAN can successfully extract geometric information from the edge structure of an XAS spectrum. The systematic application of MXAN to 1 indicates that this approach is sensitive to small structural changes in the molecule that are manifested in the XAS edge spectrum. These results represent the first step toward the application of this methodology to bioinorganic and biological systems.     

Preparation and adsorption of refined polyelectrolyte complex nanoparticles

We report on bulk and surface properties of centrifuged nonstoichiometric polyelectrolyte complex (PEC) dispersions. PECs were prepared by mixing poly(diallyldimethylammonium chloride) (PDADMAC) and sodium poly(maleic acid-co-alpha-methylstyrene) (PMA-MS) at the monomolar mixing ratio of 0.6 and polymer concentration >/=1 mmol/l. Centrifugation of initial PEC dispersions revealed three phases: supernatant (SUP), coacervate (COAC), and an insoluble precipitate. Mass, turbidity, particle hydrodynamic radii (R(h)), and the titratable charge amount were determined for those phases. The turbid COAC phase consisted of 200-nm nanoparticles and carried 60% of the polymer mass and 20% of the titratable charge amount of the initial PEC dispersion. The SUP phase showed no turbidity and no such nanoparticles, but carried 80% of the initial titratable charge amount, presumably caused by excess polycations. Furthermore, linear dependences of turbidity and R(h) on COAC concentration was observed. COAC adsorption was studied at polyelectrolyte multilayer (PEM) modified silicon surfaces in dependence on both adsorption time and concentration using attenuated total-reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. The adsorption data were fitted by the simple Langmuir model. Comparison of COAC particles and polystyrene latices revealed similar adsorption features. SEM and AFM measurements resulted in hemispherically shaped adsorbed COAC particles with coverages >/=25%, whose calculated volumes correlated well with those in dispersion obtained by PCS.     

Crystal structure and solid state IR-LD analysis of a mononuclear Cu(II) complex of 4-aminopyridine

A new mononuclear Cu(II) complex of 4-aminopyridine (4AP), [Cu(4AP)₄]Cl₂·2CH₃OH (1), was obtained and structurally characterized by single-crystal X-ray diffraction methods. The complex is square-planar involving four monodentate 4AP molecules coordinated through the N_py atoms. A solid-state linear-dichroic spectroscopic analysis (IR-LD) as a suspension in a nematic liquid crystal was carried out over the 4000–400?cm^?1 range and determined stereochemical parameters compared crystallographic data. Detailed IR characteristics are reported.     

Local structure analysis of Cu(II)-diazepam complexes by ESR spectroscopy

CuL₂X₂ (L = 7-chloro-1,3-dihydro-1-methyl-5-phenyl-3H-1,4-benzodiazepin-2-one, also known as diazepam, X = Cl, Br) complexes have been prepared and investigated by ESR spectroscopy. Powder ESR spectra of these complexes suggest a planar-rhombic distorted local symmetry. The CH₃Cl solutions spectra show the presence of pseudo-tetrahedral species with a 3d_xy+4p_z mixture ground state for the paramagnetic electron. The anisotropic spectra obtained for the Cu(II)-diazepam solution adsorbed on NaY zeolite confirm the existence of a CuN₂X₂ chromophore.     

Local structure analysis of Cu(II)-diazepam complexes by ESR spectroscopy

CuL(2)X(2) (L = 7-chloro-1,3-dihydro-1-methyl-5-phenyl-3H-1,4-benzodiazepin-2-one, also known as diazepam, X = Cl, Br) complexes have been prepared and investigated by ESR spectroscopy. Powder ESR spectra of these complexes suggest a planar-rhombic distorted local symmetry. The CH(3)Cl solutions spectra show the presence of pseudo-tetrahedral species with a 3d(xy)+4p(z) mixture ground state for the paramagnetic electron. The anisotropic spectra obtained for the Cu(II)-diazepam solution adsorbed on NaY zeolite confirm the existence of a CuN(2)X(2) chromophore.     

Oxidatively induced reactions of a diimine platinum(IV) tetramethyl complex

The oxidation of the Pt(IV) tetramethyl complex [ArNCHCHNAr]PtMe₄ (Ar = 2,6-Me₂C₆H₃) has been investigated in acetonitrile and dichloromethane. Cyclic voltammetry demonstrates that the irreversible oxidation of [ArNCHCHNAr]PtMe₄ occurs at a slightly less positive oxidation potential than the irreversible oxidation of the analogous Pt(II) species [ArNCHCHNAr]PtMe₂. The product distribution arising from the oxidation depends strongly on the reaction conditions and includes cationic Pt(IV) species (acetonitrile, dichloromethane solvents) and Pt(II) species (dichloromethane only). Evidence is presented that suggests that homolytic cleavage of a weakened PtC bond in is involved in the oxidatively induced reactions.