K. Srivastava

X-ray absorption fine structure study of multinuclear copper(I) thiourea mixed ligand complexes

X-ray absorption fine structure spectra of five copper(I) thiourea complexes [Cu4(thu)6 (NO3)4 (H2O)4] (1), [Cu4(thu)9 (NO3)4 (H2O)4] (2), [Cu2(thu)6 (SO4) H2O] (3), [Cu2(thu)5 (SO4) (H2O)3] (4), and [Cu(thu)Cl 0.5H2O] (5) have been investigated. Complexes 1 and 3 are supposed to have one type of copper centers in trigonal planar and tetrahedral environment, respectively. Complexes 2 and 4 are supposed to have two types of copper centers, one center having trigonal planar geometry and another center having tetrahedral geometry. The aim of the present work is to show how extended X-ray absorption fine structure (EXAFS) spectra of these complexes, having different types of coordination environment, can be analyzed to yield the coordination geometry around one type of copper centers present in complexes 1 and 3, and two types of copper centers present in complexes 2 and 4. The crystal structure of complex 5 is unavailable due to inability of growing its single crystals, and hence the coordination geometry of this complex has been determined from EXAFS. The structural parameters determined from the EXAFS spectra have been reported and the coordination geometry has been depicted for the metal centers present in all the five complexes. Also, the chemical shifts have been used to determine the oxidation state of copper in these complexes. The X-ray absorption near edge spectra features have also been correlated with the coordination geometry. Also, the presence of both three and four coordinated Cu(I) centers in complexes 2 and 4 has been suggested from a comparison of the intensity of the feature at 8984 eV with those of 1 and 3. Further, in case of complex 5, the high intensity of peak A at 8986.5 eV is found to correspond to the presence of Cl coordinated to the copper center.

Coordination geometry around copper in a Schiff-base trinuclear copper complex using EXAFS spectroscopy

In the present investigation, we have studied extended X-ray absorption fine structure (EXAFS) spectra of a trinuclear Schiff-base copper complex tetraaqua-di-?3-(N-salicylidene-DL-glutamato)-tricopper(II)heptahydrate, [Cu3(C12H10NO5)2 (H2O)4]. 7H2O, in which three metal sites are present. One metal site is square-pyramidal (4+1) and other two similar metal sites are tetragonally distorted octahedral (4+2). EXAFS has been recorded at the K-edge of copper in the complex at the dispersive EXAFS beamline at 2 GeV Indus-2 synchrotron source at RRCAT, Indore, India. The analysis of EXAFS spectra of multinuclear metal complexes pose some problems due to the presence of many absorbing atoms, even when the absorbing atoms may be of the same element. Hence, using the available crystal structure of the complex, theoretical models have been generated for the different copper sites separately, which are then fitted to the experimental EXAFS data. The two coordination geometries around the copper sites have been determined. The contributions of the different copper sites to the experimental spectrum have been estimated. The structural parameters, which include bond-lengths, coordination numbers and thermal disorders, for the two types of copper sites have been reported. Further, copper has been found to be in +2 oxidation state at these metal sites.

XAFS study of copper(II) diethylenetriamine complexes having different coordination geometries

XAFS of three Cu(II) diethylenetriamine complexes (in crystalline form) having different coordination geometries have been investigated. First complex has distorted tetragonal pyramidal, second has distorted square planar and third has distorted square pyramidal geometry. The difference in coordination geometries has been inferred from the differences in pre-edge peak, rising part of edge and in shape of white line, which are seen clearly in the derivative XANES spectra. The distortion in geometry has been correlated with the intensity of peaks in derivative spectra. These inferences have been corroborated from EXAFS analysis where the different paths have been used in the theoretical fits in R space to show contributions of different scatterers at different distances.

Extended X- ray absorption fine structure study at the K-edge of copper in mixed ligand complexes having benzimidazole as one of the ligands

Extended X-ray absorption fine structure (EXAFS) spectra have been studied at the K-edge of copper in some of its biologically important complexes, viz., [Cu(BzImH)4X2] and [Cu(BzIm)2], where X= Cl, Br, 1/2SO4, ClO4, NO3, and BzIm = Benzimidazolato anion. The spectra have been recorded using a bent crystal 0.4 m Cauchois-type transmission spectrograph. The positions of EXAFS maxima and minima have been used to determine the bond lengths in the complexes with the help of three different methods, namely, Levys, Lytles and Lytle, Sayers and Sterns (L.S.S.) methods. The phase uncorrected bond lengths have also been determined from Fourier transforms of the experimental spectra. The results obtained from these methods have been discussed and it has been found that the results obtained by L.S.S. method are comparable with the results obtained by Fourier transformation method and that these two methods give phase uncorrected bond lengths.

EXAFS study of mixed ligand copper complexes having nicotinic acid as one of the ligands

Copper K-edge EXAFS spectra of the copper (II) complexes [Cu3(NA)4(dca)2(H2O)8].2H2O (1) and [Cu2(2,2-bipy)(NA)2](ClO4)2.H2O (2) (where NA = Nicotinic acid; dca = dicyanamide anion; 2,2-bipy = 2-2 bipyridine) having different coordination environments and geometries have been investigated. The complex 1 is supposed to have six coordinated geometry and complex 2 five coordinated geometry. EXAFS has been recorded at the K-edge of copper in the complex at the dispersive EXAFS beamline at 2 GeV Indus-2 synchrotron source at RRCAT, Indore, India. Theoretical models have been generated for both the complexes separately using their reported crystal structures. These theoretical models have been fitted to the respective experimental EXAFS data and the structural parameters, which include bond lengths, coordination numbers and thermal disorders, have been determined. The results obtained have been found to be comparable with those reported using crystallography. The coordination geometry about the copper (II) ion has been depicted for both the complexes. Further, copper has been found to be in +2 oxidation state in both the complexes. The study shows that the dispersive EXAFS beamline can be used to obtain information about the coordination geometry of a sample, though the beamline provides limited range of EXAFS data.

X-ray absorption studies of copper (II) mixed ligand complexes with ethylenediamine (en) as one of the ligands

X-ray K-absorption spectra of six biologically important copper (II) complexes having ethylenediamine (en) as primary ligand and nitrate, thio-cynate, perchlorate, bromide, chloride and sulphate as secondary ligands have been studied using a bent crystal 0.4 m Cauchois-type transmission spectrograph. Various parameters, e.g., chemical shift, energy position of the principal absorption maximum, effective nuclear charge and edge-width have been determined. The K-absorption edges have been found to split into two components, i.e., K1 and K2 in all of these complexes. All the complexes have been found to have copper in oxidation state +2. The relative ionic character of the complexes has also been determined and correlated with spectrochemical series.

The structure and morphology of α-quartz

Some features of the quartz morphology such as the well-developed nature of the faces of the primary rhombo-hedron, their position to the left of electric axes in left quartz, and the plane and the nature of the cleavage in quartz have been explained on the basis of the crystal structure.

Variation of the piezo-electric constants of α-quartz with temperature

The method of finding the piezo-electric constants with the help of the variations of bond distances and bond angles on strain has been utilised in finding the variations of the piezo-electric constants ε11 and ε41 of α-quartz with temperature. It is found that the variations of ε11 with temperature can be explained on the basis of the change of co-ordinates with temperature. At 558° C. the silicon atoms are found to occupy the same positions as they do in β-quartz. As the transition temperature is reached, the longitudinal coefficient ε11 drops to zero, while the transverse coefficient ε41 decreases by only 15%. The piezo-electric constant of β-quartz has been similarly determined and its value comes out to be 1·05×104 for a non-ionic crystal (k=·724) and 1·45×104 for an ionic crystal (k=1).

A new method of calculating the piezo-electric constants of α-quartz

A method of calculating the piezo-electric constants of α-quartz is described which consists in determining the electric moment developed by changes of bond-lengths and bond-angles on strain. Considering three atoms P, Q, R, of which P and R are silicon atoms and Q an oxygen atom orvice-versa, the electric moment developed on strain may be resolved into moments in the plane PQR and moments perpendicular to this plane. Resolving these moments along the axes of the crystal and summing up for all the forty-two planes in the unit cell, we get the piezoelectric equations and constants of the crystal. The electric moment in the plane is equivalent to the moment produced by the change of bondlength along the direction of the bonds before strain, while for the moment normal to the plane we take the average value,i.e., half the value of the moment normal to the plane when the displacements of the atoms are wholly normal to the plane. The method gives good agreement with the observed values.