The effect of halogen atom on the molecular quadratic nonlinearity of half sandwich complexes in the presence of an acceptor

Half sandwich complexes of the type [CpM(CO)(n)X] {X=Cl, Br, I; If, M=Fe, Ru; n=2 and if M=Mo; n=3} and [CpNiPPh3X] {X=Cl, Br, I} have been synthesized and their second order molecular nonlinearity (beta) measured at 1064 nm in CHCl3 by the hyper-Rayleigh scattering technique. Iron complexes consistently display larger beta values than ruthenium complexes while nickel complexes have marginally larger beta values than iron complexes. In the presence of an acceptor ligand such as CO or PPh3, the role of the halogen atom is that of a pi donor. The better overlap of Cl orbitals with Fe and Ni metal centres make Cl a better pi donor than Br or I in the respective complexes. Consequently, M-pi interaction is stronger in Fe/Ni-Cl complexes. The value of beta decreases as one goes down the halogen group. For the complexes of 4d metal ions where the metal-ligand distance is larger, the influence of pi orbital overlap appears to be less important, resulting in moderate changes in beta as a function of halogen substitution. (C) 2006 Elsevier B.V. All rights reserved.

Microstructural evolution during laser resolidification of Fe-25 atom percent Ge alloy

The microstructural evolution of concentrated alloys is relatively less understood both in terms of experiments as well as theory. Laser resolidification represents a powerful technique to study the solidification behavior under controlled growth conditions. This technique has been utilized in the current study to probe experimentally microstructural selection during rapid solidification of concentrated Fe-25 atom pct Ge alloy. Under the equilibrium solidification condition, the alloy undergoes a peritectic reaction between ordered alpha(2) (B2) and its liquid, leading to the formation of ordered hexagonal intermetallic phase epsilon (DO19). In general, the as-cast microstructure consists of epsilon phase and e-p eutectic and alpha(2) that forms as a result of an incomplete peritectic reaction. With increasing laser scanning velocity, the solidification front undergoes a number of morphological transitions leading to the selection of the microstructure corresponding to metastable alpha(2)/beta eutectic to alpha(2) dendrite + alpha(2)/beta eutectic to alpha(2) dendrite. The transition velocities as obtained from the experiments are well characterized. The microstructural selection is discussed using competitive growth kinetics.

Metal Nanoparticles via the Atom-Economy Green Approach

Metal nanoparticles (NPs) of Cu(air-stable),Ag,and Au have been prepared using an atom-economy green approach Simple mechanical stirring of solid mixtures (no solvent) of a metal salt and ammonia borane at 60 degrees C resulted in the formation of metal NPs. In this reaction, ammonia borane is transformed into a BNHx polymer, which protects the NPs formed and halts their growth. This results in the formation of the BNHx polymer protected monodisperse NPs Thus, ammonia borane used in these reactions plays a dual role (reducing agent andprecursor for the stabilizing agent).

Thermal stresses in a finite hollow cylinder due to an axisymmetric temperature field at the end surface

A three-dimensional exact solution for determining the thermal stresses in a finite hollow cylinder subject to a steady state axisymmetric temperature field over one of its end surfaces has been given. Numerical results for a hollow cylinder, having lenght to outer diameter ratio equal to one and inner to outer diameter ratio equal to 0.75, subjected to a symmetric temperature variation over the end surfaces of the cylinder have been given.

Photodissociation of isomeric dichloroethylenes in the ultraviolet: Effect of the second chlorine atom substitution on the dynamics

We report the Cl-*(P-2(1/2)) production dynamics in the near-UV dissociation of three isomers (cis-, gem-, and trans-) of dichloroethylene using the conventional resonance enhanced multiphoton ionization technique. Substantial amounts of Cl-* are produced in the wavelength range 222-304 nm. The Cl-* quantum yield (phi(*)) i maximum at 304 nm for all the isomers and phi(*)(cis) is markedly higher than phi(*)(gem) and phi(*)(trans) except at 222 nm. Existence of both direct and indirect dissociation pathways at these wavelengths complicates the Cl* production dynamics. The higher value of phi(*)(cis) originates from a large contribution from direct dissociation via the (n, sigma(*)) state.

Asynchronous behaviour in some three-atom linear concerted radical-exchange reactions

A few simple three-atom thermoneutral radical exchange reactions (i.e. A + BC --> AB + C) are examined by ab initio SCF methods. Emphasis is laid on the detailed analysis of density matrices rather than on energetics. Results reveal that the sum of the bond orders of the breaking and forming bonds is not conserved to unity, due to development of free valence on the migrating atom 'B' in the transition state. Bond orders, free valence and spin densities on the atoms are calculated. The present analysis shows that the bond-cleavage process is always more advanced than the bond-formation process in the transition state. Further analysis shows a development of the negative spin density on the migrating atom 'B' in the transition state. The depletion of the alpha-spin density on the radical site "A" in the reactant during the reaction lags behind the growth of the alpha-spin density on the terminal atom "C" of the reactant bond, 'B-C' in the transition state. But all these processes are completed simultaneously at the end of the reaction. Hence, the reactions are asynchronous but kinetically concerted in most cases.

Nature of hydrogen atom trapped inside palladium lattice

Colloid of palladium nanoparticles has been prepared by the Solvated Metal Atom Dispersion (SMAD) method. Reaction of Pd(0) nanopowder obtained upon precipitation from the colloid, with ammonia borane (H3N center dot BH3, AB) in aqueous solutions at room temperature results in the generation of active hydrogen atoms. The active hydrogen atoms either combine with one another resulting in H-2 evolution or diffuse into the Pd lattice to afford PdHx. Diffusion of hydrogen atoms leads to an expansion of the Pd lattice. The diffused hydrogen atoms are distributed uniformly over the entire particle. These features were established using powder XRD and electron microscopy studies. The H-1 NMR spectral studies of PdHx before and after desorption of H-2 revealed that the hydrogen atoms trapped inside Pd lattice are hydridic in nature. Desorption of hydrogen from PdHx did not result in complete reversibility suggesting that some hydrogen atoms are strongly trapped inside the Pd lattice. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

The intramolecular external heavy atom effect in bromo-, benzo-, and naphthonorbornenes

A theoretical analysis of the external heavy atom effect of a halogen atom on the radiative rate constant of phosphorescence is examined as a function of position of a bromine atom or atoms relative to a naphthalene or a benzene chromophore for a series of mono- and dibromo-, naphtho-, and benzonorbornenes. The theoretical results are then compared to experimentaldata and lead to the conclusion that the enhancement of the phosphorescence process takes place through the second-ordermixing of the triplet states of the chromophore with the singlet charge transfer states arising primarily from an electron transferfrom the orbitals of the heavy atom perturber to the unfilled x* orbitals of the chromophore.

Hollow hemisphere and microcapsules of nonionic copolymer

Hemispherical colloidal nanowells or microwells with hollow interiors are becoming increasingly important for the encapsulation of functional materials. There has been rapid progress to develop new methods to obtain such structures. In this work, we present emulsification approach to generate hemisphere and microcapsules of biocompatible organic polymer. The precise control over the size is exhibited by applying variable vortex effect. The hemispheres and microcapsules of a copolymer (BPVA-PVA) were characterized by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). These structures were used for loading of hydrophilic molecules and submicron colloidal particles to demonstrate their potential application. The introduction of hydrophobic groups on poly(vinyl alcohol) was crucial to obtain these structures.

Thermodynamic and structural consequences of changing a sulfur atom to a methylene group in the M13Nle mutation in ribonuclease-S

Two fragments of pancreatic ribonuclease A, a truncated version of S-peptide (residues 1-15) and S-protein (residues 21-124), combine to give a catalytically active complex. We have substituted the wild-type residue at position 13, methionine (Met), with norleucine (Nle), where the only covalent change is the replacement of the sulfur atom with a methylene group. The thermodynamic parameters associated with the binding of this variant to S-protein, determined by titration calorimetry in the temperature range 10-40 degrees C, are reported and compared to values previously reported [Varadarajan, R., Connelly, P. R., Sturtevant, J. M., & Richards, F. M. (1992) Biochemistry 31, 1421-1426] for other position 13 analogs. The differences in the free energy and enthalpy of binding between the Met and Nle peptides are 0.6 and 7.9 kcal/mol at 25 degrees C, respectively. These differences are slightly larger than, but comparable to, the differences in the values for the Met/Ile and Met/Leu pairs. The structure of the mutant complex was determined to 1.85 Angstrom resolution and refined to an R-factor of 17.4% The structures of mutant and wild-type complexes are practically identical although the Nle side chain has a significantly higher average B-factor than the corresponding Met side chain. In contrast, the B-factors of the atoms of the cage of residues surrounding position 13 are all somewhat lower in the Nle variant than in the Met wild-type. Thus, the large differences in the binding enthalpy appear to reside entirely in the difference in chemical properties or dynamic behavior of the -S- and -CH2- groups and not in differences in the geometry of the side chains or the internal cavity surface. In addition, a novel method of obtaining protein stability data by means of isothermal titration calorimetry is introduced.

Role of free valences and spin densities on the migrating atom in location of saddle points in radical-exchange reactions

The principle of the conservation of bond orders during radical-exchange reactions is examined using Mayer's definition of bond orders. This simple intuitive approximation is not valid in a quantitative sense. Ab initio results reveal that free valences (or spin densities) develop on the migrating atom during reactions. For several examples of hydrogen-transfer reactions, the sum of the reaction coordinate bond orders in the transition state was found to be 0.92 +/- 0.04 instead of the theoretical 1.00 because free valences (or spin densities) develop on the migrating atom during reactions. It is shown that free valence is almost equal to the square of the spin density on the migrating hydrogen atom and the maxima in the free valence (or spin density) profiles coincide (or nearly coincide) with the saddle points in the corresponding energy profiles.

Thermal Stability of Spherical Nanoporous Aggregates and Formation of Hollow Structures by Sintering-A Phase-Field Study

Nanoporous structures are widely used for many applications and hence it Is important to investigate their thermal stability. We study the stability of spherical nanoporous aggregates using phase-field simulations that explore systematically the effect of grain boundary diffusion, surface diffusion, and grain boundary mobility on the pathways for microstructural evolution. Our simulations for different combinations of surface and GB diffusivity and GB mobility show four distinct microstructural pathways en route to 100% density: multiple dosed pores, hollow shells, hollow shells with a core, and multiple interconnected pores. The microstructures from our simulations are consistent with experimental observations in several different systems. Our results have important implications for rational synthesis of hollow nanostructures or aggregates with open pores, and for controlling the stability of nanoporous aggregates that are widely used for many applications.

Metal-billed and hollow carbon nanotubes obtained by the decomposition of metal-containing free precursor molecules

Vapor-phase pyrolysis of Fe(CO)(5) in the presence of another carbon source such as CO or Ca He yields iron-filled or hollow nanotubes depending on the relative concentration of the carbon source. Essentially single-walled nanotubes are obtained when the C6H6/Fe(CO)(5) ratio is high. Pyrolysis of metallocenes yields metal-filled nanotubes and hollow nanotubes are obtained when metallocenes are pyrolyzed along with benzene. Metal-decorated nanotubes are also obtained by this method.