Optical and magnetic resonance studies of the interaction of metallo tetraphenylporphyrins with nitrobenzofuroxan

Metallo tetraphenylporphyrins form I : I molecular complexes with 4,6-dinitrobenzofuroxan. The molecular association is described in terms of T-n. interaction with porphyrins functioning as donors. The association constants and thermodynamic parameters have been evaluated using optical absorption and 'H nmr spectral methods. Based on the binding constants, the donor ability of various metalloporphyrins can be arranged in the following order: Pd(I1) > Co(I1) > Cu(I1) > Ni(I1) - VO(1V) - 2H > Zn(l1). Electron paramagnetic resonance studies of the complexes reveal that the IT-complexation results in changes in the electronic structure of the central metal ions which are reflected in the changes in the M-N 5 bonding. The dipolar contribution to the acceptor proton chemical shifts in the CoTPP complex has been partitioned from ring current contributions using the shifts observed in the ZnTPP complex. The shifts, along with the line broadening ratios observed for the CoTPP complex, are used to arrive at the possible solution structures of the complexes.

Electron Spectroscopic Studies of Oxygen and Carbon Dioxide Adsorbed on Metal Surfaces

Adsorption of oxygen on Ni, Cu, Pd, Ag, and Au surfaces has been investigated by employing UV and X-ray photoelectron spectrscopy as well as electron energy loss spectroscopy (EELS). Molecularly chemisorbed (singlet) oxygen is found on Ni, Cu, Ag, and Au surfaces showing features such as stabilization of the rB* orbital, destabilization of the .nu orbital, higher O(1s) binding energy than the atomic species, and a band 2-3 eV below the Fermi level due to metal d-O(2p)u* interaction. 0-0 and metal-oxygen stretching frequencies have been observed in EELS. Physical adsorption of O2 is found to occur on Pd and Ni surfaces, only at high exposures in the latter case. Physical adsorption and multilayer condensation of CO, on metal surfaces are distinguished by characteristic relaxation shifts in UPS as well as O(1s) binding energies. Adsorption of CO on a Ni surface covered with presorbed atomic oxygen gives rise to C02.

Metal-insulator transition of naxwo3 studied by angle-resolved photoemission spectroscopy

The electronic structure of sodium tungsten bronzes NaxWO3 is investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The ARPES spectra measured in both insulating and metallic phases of NaxWO3 reveals the origin of metal-insulator transition (MIT) in sodium tungsten bronze system. It is found that in insulating NaxWO3 the states near the Fermi level (E-F) are localized due to the strong disorder caused by the random distribution of Na+ ions in WO3 lattice. Due to the presence of disorder and long-range Coulomb interaction of conduction electrons, a soft Coulomb gap arises, where the density of states vanishes exactly at E-F. In the metallic regime the states near E-F are populated and the Fermi level shifts upward rigidly with increasing electron doping (x). Volume of electron-like Fermi surface (FS) at the Gamma(X) point of the Brillouin zone gradually increases with increasing Na concentration due to W 5d t(2g) band filling. A rigid shift of the Fermi energy is found to give a qualitatively good description of the Fermi surface evolution. As we move from bulk-sensitive to more surface sensitive photon energy, we found the emergence of Fermi surfaces at X(M) and M(R) point similar to the one at the Gamma(X) point in the metallic regime, suggesting that the reconstruction of surface was due to rotation/deformation of WO6 octahedra.

The structure of turbulent boundary layers along mildly curved surfaces

This paper describes a detailed study of the structure of turbulence in boundary layers along mildly curved convex and concave surfaces. The surface curvature studied corresponds to δ/Rw = ± 0·01, δ being the boundary-layer thickness and Rw the radius of curvature of the wall, taken as positive for convex and negative for concave curvature. Measurements of turbulent energy balance, autocorrelations, auto- and cross-power spectra, amplitude probability distributions and conditional correlations are reported. It is observed that even mild curvature has very strong effects on the various aspects of the turbulent structure. For example, convex curvature suppresses the diffusion of turbulent energy away from the wall, reduces drastically the integral time scales and shifts the spectral distributions of turbulent energy and Reynolds shear stress towards high wavenumbers. Exactly opposite effects, though generally of a smaller magnitude, are produced by concave wall curvature. It is also found that curvature of either sign affects the v fluctuations more strongly than the u fluctuations and that curvature effects are more significant in the outer region of the boundary layer than in the region close to the wall. The data on the conditional correlations are used to study, in detail, the mechanism of turbulent transport in curved boundary layers. (Published Online April 12 2006)

Multilevel aperiodic Huffman sequences

Certain sequences that have zero aperiodic autocorrelation except for zero and the maximum shifts are described. They are useful in radar pulse compression.

Complexes of lanthanide iodides with 4-methylpyridine-1-oxide and 2-methylpyridine-1-oxide

Complexes of lanthanide iodides with 4-methylpyridine-1-oxide and 2-methylpyridine-1-oxide of the formulae Ln(4-MePyO)8I3.xH2O (x=0 or 2) and Ln(2-MePyO)5I3.xH2O (x=0, 1 or 3) have been prepared and characterized by analyses, conductance, infrared and proton NMR data. Infrared spectra of the complexes indicate that the coordination of the ligand to the metal ion takes place through the oxygen of the N-O group of the ligand. Proton NMR data for the paramagnetic complexes indicate that both contact and pseudocontact interactions are responsible for the isotropic shifts. Proton NMR spectra of the 2-methylpyridine-1-oxide complexes indicate a restricted rotation of the ligand about the N-O group.

Intersoliton forces in weak-coupling quantum field theories

We offer a procedure for evaluating the forces exerted by solitons of weak-coupling field theories on one another. We illustrate the procedure for the kink and the antikink of the two-dimensional φ4 theory. To do this, we construct analytically a static solution of the theory which can be interpreted as a kink and an antikink held a distance R apart. This leads to a definition of the potential energy U(R) for the pair, which is seen to have all the expected features. A corresponding evaluation is also done for U(R) between a soliton and an antisoliton of the sine-Gordon theory. When this U(R) is inserted into a nonrelativistic two-body problem for the pair, it yields a set of bound states and phase shifts. These are found to agree with exact results known for the sine-Gordon field theory in those regions where U(R) is expected to be significant, i.e., when R is large compared to the soliton size. We take this agreement as support that our procedure for defining U(R) yields the correct description of the dynamics of well-separated soliton pairs. An important feature of U(R) is that it seems to give strong intersoliton forces when the coupling constant is small, as distinct from the forces between the ordinary quanta of the theory. We suggest that this is a general feature of a class of theories, and emphasize the possible relevance of this feature to real strongly interacting hadrons.

Orientational ordering in sheared inelastic dumbbells

Using even driven simulations, we show that homogeneously sheared inelastic dumbbells in two dimensions are randomly orientated in the limit of low density. As the packing fraction is increased, particles first tend to orient along the extensional axis, and then as the packing fraction is further increased, the alignment shifts closer to the flow axis. The orientational order parameter displays a continuous increase with packing fraction and does not appear to exhibit a universal scaling with elongation. Except at the highest packing fractions, the orientational distribution function can be reconstructed with only the first coefficient of the Fourier expansion.

Interaction of surfactants with DNA. Role of hydrophobicity and surface charge on intercalation and DNA melting

A probe, 9-(anthrylmethyl)trimethylammonium chloride, 1, was prepared. 1 binds to calf-thymus DNA or Escherichia coli genomic DNA with high affinity, as evidenced from the absorption titration. Strong hypochromism, spectral broadening and red-shifts in the absorption spectra were observed. Half-reciprocal plot constructed from this experiment gave binding constant of 5±0.5×104 M−1 in base molarity. We employed this anthryl probe-DNA complex for studying the effects of addition of various surfactant to DNA. Surfactants of different charge types and chain lengths were used in this study and the effects of surfactant addition to such probe-DNA complex were compared with that of small organic cations or salts. Addition of either salts or cationic surfactants led to structural changes in DNA and under these conditions, the probe from the DNA-bound complex appeared to get released. However, the cationic surfactants could induce such release of the probe from the probe-DNA complex at a much lower concentration than that of the small organic cations or salts. In contrast the anionic surfactants failed to promote any destabilization of such probe-DNA complexes. The effects of additives on the probe-DNA complexes were also examined by using a different technique (fluorescence spectroscopy) using a different probe ethidium bromide. The association complexes formed between the cationic surfactants and the plasmid DNA pTZ19R, were further examined under agarose gel electrophoresis and could not be visualized by ethidium bromide staining presumably due to cationic surfactant-induced condensation of DNA. Most of the DNA from such association complexes can be recovered by extraction of surfactants with phenol-chloroform. Inclusion of surfactants and other additives into the DNA generally enhanced the DNA melting temperatures by a few °C and at high [surfactant], the corresponding melting profiles got broadened.

Study of As2(Se,Te)3 glasses by X-ray absorption and photoelectron spectroscopy

Arsenic selenide-telluride glasses have been investigated by X-ray absorption and photoelectron spectroscopy. The core electron energy shifts and chemical shifts in K-absorption edge measurements associated with the glass-crystal transitions of pure As2Se3 and As2Te3 have been studied. The effect of composition on the core level energy and valence bands of As2(Se,Te)3 glasses, has been discussed. Mixed-composition glasses are found to be considerably ionic.

X-ray spectroscopic studies of amorphous materials

Measurement of the chemical shifts ΔE of the K-absorption edge in both crystalline and amorphous states of several solids shows that ΔE is generally smaller in the amorphous state. More covalent solids appear to be associated with small values of ΔE.

Solution NMR studies of acetohydroxy acid synthase I: Identification of the sites of inter-subunit interactions using multidimensional NMR methods

The novel multidomain organization in the multimeric Escherichia coli AHAS I (ilvBN) enzyme has been dissected to generate polypeptide fragments. These fragments when cloned, expressed and purified reassemble in the presence of cofactors to yield a catalytically competent enzyme. Structural characterization of AHAS has been impeded due to the fact that the holoenzyme is prone to dissociation leading to heterogeneity in samples. Our approach has enabled the structural characterization using high-resolution nuclear magnetic resonance methods. Near complete sequence specific NMR assignments for backbone H-N, N-15, C-13 alpha and C-13(beta) atoms of the FAD binding domain of ilvB have been obtained on samples isotopically enriched in H-2, C-13 and N-15. The secondary structure determined on the basis of observed C-13(alpha) secondary chemical shifts and sequential NOEs indicates that the secondary structure of the FAD binding domain of E. coli AHAS large Subunit (ilvB) is similar to the structure of this domain in the catalytic subunit of yeast AHAS. Protein-protein interactions involving the regulatory subunit (ilvN) and the domains of the catalytic subunit (ilvB) were studied using circular dichroic and isotope edited solution nuclear magnetic resonance spectroscopic methods. Observed changes in circular dichroic spectra indicate that the regulatory subunit (ilvN) interacts with ilvB alpha and ilvB beta domains of the catalytic subunit and not with the ilvB gamma domain. NMR chemical shift mapping methods show that ilvN binds close to the FAD binding site in ilvB beta and proximal to the intrasubunit ilvB alpha/ilvB beta domain interface. The implication of this interaction on the role of the regulatory subunit oil the activity of the holoenzyme is discussed. NMR studies of the regulatory domains show that these domains are structured in solution. Preliminary evidence for the interaction of ilvN with the metabolic end product of the pathway, viz., valine is also presented.

Mechanisms of molecular doping of graphene: A first-principles study

Doping graphene with electron donating or accepting molecules is an interesting approach to introduce carriers into it, analogous to electrochemical doping accomplished in graphene when used in a field-effect transistor. Here, we use first-principles density-functional theory to determine changes in the electronic-structure and vibrational properties of graphene that arise from the adsorption of aromatic molecules such as aniline and nitrobenzene. Identifying the roles of various mechanisms of chemical interaction between graphene and a molecule, we bring out the contrast between electrochemical and molecular doping of graphene. Our estimates of various contributions to shifts in the Raman-active modes of graphene with molecular doping are fundamental to the possible use of Raman spectroscopy in (a) characterization of the nature and concentration of carriers in graphene with molecular doping, and (b) graphene-based chemical sensors.

Are Metallocene-Acetylene (M = Ti, Zr, Hf) Complexes Aromatic Metallacyclopropenes?

The bonding nature of metallocene acetylene complexes Cp2M(eta(2)-H3SiC2SiH3) 1M and CP2M (eta(2)- HC2H) 1M' (M = Ti, Zr, Hf) wits studied by density functional theory method. It is found that this acetylene complex has indeed it metallacyclopropene moiety with two in-plane M-C sigma-bonds and one out-of-plane pi-bond interacting with the metal center, resulting in the formation of it delocalized three-center and two-electron (3c-2e) system. Along with its delocalized out-of-plane bonding, this complex has been characterized its aromatic on the basis of the computed stabilizing energy and negative nucleus-independent chemical shifts (NICS). The aromatic stabilization increases from Ti to Zr and Hf, and this is because of the increased charge separation between the CP2M fragment and the H3SiC2SiH3 (also HC2H) unit. The decrease of the M-C bond length from Zr to Hf is ttributed to the increased s character of both M and C hybridization of the M-C a-bonds.

Rotational isomerism about the Ca-CO bond in proline derivatives. 1H and 13C NMR studies of benzyloxycarbonyl-Pro-N-methylamide and pivaloyl-Pro-N-methylamide

The 270 MHz 1H n.m.r. spectrum of benzyloxycarbonyl-Pro-N-methylamide in CDCl3 is exchange broadened at 293° K. Spectral lines due to two species are frozen out at 253° K and a dynamically averaged spectrum is obtained at 323° K. A selective broadening of the Cβ and Cγ resonances in the 13C n.m.r. spectrum is observed at 253° K, with a splitting of the Cβ and Cγ resonances into a pair of lines of unequal intensity. A similar broadening of Cβ and Cγ peaks is also detected in pivaloyl-Pro-N-methylamide where cis-trans interconversion about the imide bond is precluded by the bulky t-butyl group. The rate process is thus attributed to rotation about the Cα-CO bond (ψ) and a barrier (ΔG#) of 14kcal mol-1 is estimated. 13C n.m.r. data for pivaloyl-Pro-N-methylamide in a number of solvents is presented and the differences in the Cβ and Cγ chemical shifts are interpreted in terms of rotational isomerism about the Cα-CO bond.