Ab initio studies of the structure and vibrational spectra of methyl isocyanide-borane and acetonitrile-borane

The SCF/DZP and MP2/DZP methods of ab initio quantum chemistry have been utilized to study the structure, vibrational spectra, binding energy, and barrier to internal rotation of methyl isocyanide-borane and acetonitrile-borane adducts. The eclipsed conformation of the complexes was predicted to be a minimum, and the staggered form is a transition state with a barrier height of about 10 cal/mol. The vibrational analyses of CH3NC-BH3 and CH3CN-BH3 and several of their isotopomers have been carried out by the GF matrix method. Computations have also been carried out for free CH3NC and CH3CN in order to investigate the changes in CH3NC and CH3CN as a result of their complex formation with BH3. To obtain an acceptable set of force constants, a recently proposed procedure ''RECOVES'' has been utilized. The increase in the N=C/C=N stretching force constant of CH3NC/CH3CN on adduct formation is interpreted with the help of Parr and Borkman's model. The binding energies for the two adducts have been determined taking basis set superposition error (BSSE) into consideration. The effect of the BSSE on structure, dipole moment, and vibrational frequencies of CH3CN and CH3NC is also evaluated. The predicted infrared band intensities for the two complexes are in good agreement with the experimentally observed features, and they have been utilized in the assignment of vibrational frequencies.

Electrical transport studies in alkali borovanadate glasses

The ac conductivity and dielectric behaviors of sodium borovanadate glasses have been studied over wide ranges of composition and frequency. The de activation energies calculated from the complex impedance plots decrease linearly with the Na2O concentration, indicating that ionic conductivity dominates in these glasses. The possible origin of low-temperature departures of conductivity curves (from linearity) of vanadium-rich glasses in log sigma versus 1/T plots is discussed. The ac conductivities have been fitted to the Almond-West type power law expression with use of a single value of s. It is found that in most of the glasses s exhibits a temperature-dependent minimum. The dielectric data are converted into moduli (M*) and are analyzed using the Kohlrausch-William-Watts stretched exponential function, The activation barriers, W, calculated from the temperature-dependent dielectric loss peaks compare well with the activation barriers calculated from the de conductivity plots. The stretching exponent beta is found to be temperature independent and is not likely to be related as in the equation beta = 1 - s, An attempt is made to elucidate the origin of the stretching phenomena. It appears that either a model of the increased contribution of polarization energy (caused by the increased modifier concentration) and hence the increased monopole-induced dipole interactions or a model based on increased intercationic interactions can explain the slowing down of the primitive relaxation in ionically conducting glasses.

Electron donor-acceptor complexes of I-2 with diethyl ether and diethyl sulphide. An ab initio MO study

The charge-transfer complexes of I-2 with the n-donors diethyl ether and diethyl sulfide were studied at the Hartree-Fock and MP2 levels. The structures were fully optimized using the 3-21G((*)) basis set as well as with effective core potentials. The calculations consistently yield a C-2v structure for the ether-I-2 complex, but an unsymmetrical form for the sulfide-I-2 complex. A natural bond orbital analysis and the BSSE-corrected complexation energies reveal stronger interactions in the sulfide complex. The computed orbital energies of the monomers and complexes reproduce the trends in experimentally observed vertical ionization potentials.

Electron donor-acceptor complex of ICl with diethyl ether - He I photoelectron spectroscopy and ab initio molecular orbital study

The He I photoelectron spectrum of the diethyl ether-ICl complex has been obtained. The oxygen orbitals are shifted to higher binding energies and that of ICl to lower binding energies owing to complex formation. Ab initio molecular orbital (MO) calculations of the complex molecule showed that the bonding is between the sigma-type lone pair of oxygen and the I atom and that the complex has C-2v symmetry. The binding energy of the complex is computed to be 8.06 kcal mol(-1) at the MP2/3-21G* level. The orbital energies obtained from the photoelectron spectra of the complex are compared and assigned with orbital energies obtained by MO calculations. Natural bond orbital analysis (NBO) shows that charge transfer is from the sigma-type oxygen lone pair to the iodine atom and the magnitude of charge transfer is 0.0744 e.

Metal-insulator transitions in metal clusters: a high-energy spectroscopy study of palladium and silver clusters

Bremsstrahlung isochromat spectroscopy (BIS) along with ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) has been employed to investigate the electron states of Pd and Ag deposited on amorphous graphite at different coverages. The metal core level binding energies increase with decreasing cluster size while the UPS valence bands show a decrease in the 4d states at E(F) accompanied by a shift in the intensity maximum to higher binding energies. BIS measurements show the emergence of new states closer to E(F) with increase in the cluster size. It is pointed out that the observed spectral shifts cannot be accounted for by final-state effects alone and that initial-state effects have a significant role. It therefore appears that a decrease in cluster size is accompanied by a metal-insulator transition.

New uniform solid catalyst for the low-temperature oxidation of carbon monoxide: A triple-layered rare earth perovskite containing Co and Cu ions

Oxygen reactivity and catalytic activity of the cobalt-containing layered defect perovskites, YBa2Cu2CoO7+delta and LaBa2Cu2CoO7+delta, in comparison with LaBa2Cu3O7-delta have been investigated employing temperature-programmed desorption (TPD) and temperature-programmed surface reactions (TPSR) in the stoichiometric and catalytic mode using carbon monoxide as a probe molecule. TPD studies showed evidence for the presence of two distinct labile oxygen species, one at (0 0 1/2) sites and the other at (0 1/2 0) sites in LaBa2Cu2CoO7+delta against a single labile species at (0 1/2 0) in the case of two other oxides. The activation energies for the catalytic oxidation of carbon monoxide by oxygen over LaBa2Cu3O7-delta, YBa2Cu2CoO7+delta, and LaBa2Cu2CoO7+delta have been estimated to be 24.2, 15.9, and 13.6 kcal/mol, respectively. The reactivity and catalytic activity of the oxide systems have been interpreted in terms of the structural changes brought about by substituents, guided by a directing effect of the larger rare earth cation. TPSR profiles, structural analysis, and infrared spectroscopic investigations suggest that the oxygen present at (0 0 1/2) sites in the case of LaBa2Cu2CoO7+delta is accessible to catalytic oxidation of CO through a Mars-Van Krevelen pathway. Catalytic conversion of CO to CO2 over LaBa2Cu2CoO7+delta occurs at 200 degrees C. The enhanced reactivity is explained in terms of changes brought about in the coordination polyhedra around transition metals, enhanced basal plane oxygen diffusivity, and redox potentials of the different transition metal cations.

Dielectric function and optical conductivity of TiOx (0.8

Reflection electron energy-loss spectra are reported for the family of compounds TiOx over the entire homogeneity range (0.8 < a: < 1.3). The spectra exhibit a plasmon feature on the low-energy side, while several interband transitions are prominent at higher energies. The real and imaginary parts of dielectric functions and optical conductivity for these compounds are determined using the Kramers-Kronig analysis. The results exhibit systematic behavior with varying oxygen stoichiometry.

Electrical characterization of surface defects in GaSb created by hydrogen plasma

Using steady state and transient capacitance measurements, the electrical characteristics of a defect layer on the surface of bulk GaSb created during the hydrogen plasma treatment is presented. The trap density, activation energies, and the thickness of the defect layer have been calculated. The trap densities are comparable in magnitude to the carrier concentration. The defects introduce multiple energy levels in the band gap. Typical defect layer thicknesses range from a few angstroms to a fraction of a micron. © 1995 American Institute of Physics.

Swing amplification of nonaxisymmetric perturbations in stars and gas in a sheared galactic disk

We present a study of the growth of local, nonaxisymmetric perturbations in gravitationally coupled stars and gas in a differentially rotating galactic disk. The stars and gas are treated as two isothermal fluids of different velocity dispersions, with the stellar velocity dispersion being greater than that for the gas. We examine the physical effects of inclusion of a low-velocity dispersion component (gas) on the growth of non-axisymmetric perturbations in both stars and gas, as done for the axisymmetric case by Jog & Solomon. The amplified perturbations in stars and gas constitute trailing, material, spiral features which may be identified with the local spiral features seen in all spiral galaxies. The formulation of the two-fluid equations closely follows the one-fluid treatment by Goldreich & Lynden-Bell. The local, linearized perturbation equations in the sheared frame are solved to obtain the results for a temporary growth via swing amplification. The problem is formulated in terms of five dimensionless parameters-namely, the Q-factors for stars and gas, respectively; the gas mass fraction; the shearing rate in the galactic disk; and the length scale of perturbation. By using the observed values of these parameters, we obtain the amplifications and the pitch angles for features in stars and gas for dynamically distinct cases, as applicable for different regions of spiral galaxies. A real galaxy consisting of stars and gas may display growth of nonaxisymmetric perturbations even when it is stable against axisymmetric perturbations and/or when either fluid by itself is stable against non-axisymmetric perturbations. Due to its lower velocity dispersion, the gas exhibits a higher amplification than do the stars, and the amplified gas features are slightly more tightly wound than the stellar features. When the gas contribution is high, the stellar amplification and the range of pitch angles over which it can occur are both increased, due to the gravitational coupling between the two fluids. Thus, the two-fluid scheme can explain the origin of the broad spiral arms in the underlying old stellar populations of galaxies, as observed by Schweizer and Elmegreen & Elmegreen. The arms are predicted to be broader in gas-rich galaxies, as is indeed seen for example in M33. In the linear regime studied here, the arm contrast is shown to increase with radius in the inner Galaxy, in agreement with observations of external galaxies by Schweizer. These results follow directly due to the inclusion of gas in the problem.

Synchrotron Study of the Dynamical Effects in the LVV Auger Transitions in the First-Row Transition Elements

The intensity ratio between L2-M45M45 and L3-M45M45 spectral features for both Fe and Co indicates significant tranfer of intensity from L2- to L3-M45M45 region due to Coster-Kronig L2-M45M45 transition. The L2-L3M45 transition can be suppressed by turning the photon energy between the L2 and L3 thresholds; however, the L3-M45M45 spectral shapes for Fe and Co do not change very significantly even at these photom energies unlike the cases of Ni, Cu and Zn, thus establishing that the M45-hole decays predominantly before the L3-hole Auger decay in the early transition elements in contrast to the late ones.

Ultraviolet photoelectron spectroscopy of the hydrogen bonded heterodimer H2Scdots, three dots, centeredHCl

The HeI photoelectron spectrum of the hydrogen bonded hetero-dimer H2Scdots, three dots, centeredHCl shows two vertical ionization energies at 10.91 and 12.16 eV. Ab initio MO calculations reveal that these features are due to the sulphur and chlorine lone pair ionizations respectively. Results show that while the ground ionic state is repulsive the first excited ionic state is strongly bound. The photoelectron spectrum of the diethyl sulphidecdots, three dots, centeredHCl complex is similar to that of H2Scdots, three dots, centeredHCl.

Tie lines and activities in the system NiO-MgO-SiO2 at 1373 K

The isothermal section of the phase diagram for the system NiO-MgO-SiO2 at 1373 K is established, The tie lines between (NiXMg1-X)O solid solution with rock salt structure and orthosilicate solid solution (NiYMg1-Y)Si0.5O2 and between orthosilicate and metasilicate (NiZMg1-Z)SiO3 crystalline solutions are determined using electron probe microanalysis (EPMA) and lattice parameter measurement on equilibrated samples, Although the monoxides and orthosilicates of Ni and Mg form a continuous range of solid solutions, the metasilicate phase exists only for 0 < Z < 0.096, The activity of NiO in the rock salt solid solution is determined as a function of composition and temperature in the range of 1023 to 1377 K using a solid state galvanic cell, The Gibbs energy of mixing of the monoxide solid solution can be expressed by a pseudo-subregular solution model: Delta G(ex) = X(1 - X)[(-2430 + 0.925T)X + (-5390 + 1.758T)(1 - X)] J/mol, The thermodynamic data for the rock salt phase are combined with information on interphase partitioning of Ni and Mg to generate the mixing properties for the orthosilicate and the metasilicate solid solutions, The regular solution model describes the orthosilicate and the metasilicate solid solutions at 1373 K within experimental uncertainties, The regular solution parameter Delta G(ex)/Y(1 - Y) is -820 (+/-70) J/mol for the orthosilicate solid solution, The corresponding value for the metasilicate solid solution is -220 (+/-150) J/mol, The derived activities for the orthosilicate solid solution are discussed in relation to the intracrystalline ion exchange equilibrium between M1 and M2 sites. The tie line information, in conjunction with the activity data for orthosilicate and metasilicate solid solutions, is used to calculate the Gibbs energy changes for the intercrystalline ion exchange reactions, Combining this with the known data for NiSi0.5O2, Gibbs energies of formation of MgSi0.5O2, MgSiO3, and metastable NiSiO3 are calculated, The Gibbs energy of formation of NiSiO3, from its component oxides, is equal to 7.67 (+/-0.6) kJ/mol at 1373 K.

Thermodynamic properties of LaFeO3-delta and LaFe12O19

The standard Gibbs energies of formation of lanthanum orthoferrite (LaFeO3-delta) and hexaferrite (LaFe12O19)were determined using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte and pure oxygen gas at ambient pressure as the reference electrode. From emf of the solid-state cell, the Gibbs energy of formation of nonstoichiometric orthoferrite (LaFeO3-delta) is obtained. To derive values for the stoichiometric phase, variation of the oxygen nonstoichiometric parameter with oxygen partial pressure was measured using thermogravimetry under controlled gas mixtures. The results obtained for LaFeO3 and LaFe12O19 can be summarized by the following equations, which represent the formation of ternary oxides from their component binary oxides: 1/2 La2O3 + 1/2 Fe2O3 -> LaFeO3: Delta G degrees (LaFeO3) (+/- 450) (J mol(-1)) = -62920 - 2.12T (K), and 1/2 La2O3 + 9/2Fe(2)O(3) + Fe3O4 -> LaFe12O19; Delta G degrees (LaFe12O19) (+/- 200) (J mol(-1)) = -103900 + 21.25T (K). These data are discussed critically in comparison with thermodynamic values reported in the literature from a variety of measurements. The values obtained in this study are consistent with calorimetric entropy and enthalpy of formation of the perovskite phase and with some of the Gibbs energy measurements reported in the literature. For the lanthanum hexaferrite (LaFe12O19) there are no prior thermodynamic measurements for comparison. (c) 2011 Elsevier B.V. All rights reserved.

Lopsidedness in WHISP galaxies II. Morphological lopsidedness

The distribution of stars and gas in many galaxies is asymmetric. This so-called lopsidedness is expected to significantly affect the dynamics and evolution of the disc, including the star formation activity. Here, we measure the degree of lopsidedness for the gas distribution in a selected sample of 70 galaxies from the Westerbork Hi Survey of Spiral and Irregular Galaxies. This complements our earlier work (Paper I) where the kinematic lopsidedness was derived for the same galaxies. The morphological lopsidedness is measured by performing a harmonic decomposition of the surface density maps. The amplitude of lopsidedness A(1), the fractional value of the first Fourier component, is typically quite high (about 0.1) within the optical disc and has a constant phase. Thus, lopsidedness is a common feature in galaxies and indicates a global mode. We measure A(1) out to typically one to four optical radii, sometimes even further. This is, on average, four times larger than the distance to which lopsidedness was measured in the past using near-IR as a tracer of the old stellar component, and therefore provides a new, more stringent constraint on the mechanism for the origin of lopsidedness. Interestingly, the value of A(1) saturates beyond the optical radius. Furthermore, the plot of A(1) versus radius shows fluctuations that we argue are due to local spiral features. We also try to explain the physical origin of this observed disc lopsidedness. No clear trend is found when the degree of lopsidedness is compared to a measure of the isolation or interaction probability of the sample galaxies. However, this does not rule out a tidal origin if the lopsidedness is long-lived. In addition, we find that the early-type galaxies tend to be more morphologically lopsided than the late-type galaxies. Both results together indicate that lopsidedness has a tidal origin.

Kinetics of copolyurethane network formation

Cure kinetics for the formation of copolyurethane networks of various compositions based on hydroxy-terminated polybutadiene(HTPB), poly(12-hydroxy stearic acid-co-TMP) ester polyol(PEP), and different isocyanates has been studied through viscosity build up during the cure reaction. The viscosity (N)-time (t) plots conform to the equation N = ae(bt), where a and b are empirical constants, dependent on the composition and the nature of the polyols and the isocyanates. The rate constants (b) for viscosity build up, evaluated from the slopes of dN/dt versus N plots at different temperatures, were found to vary significantly from 0.0073 to 0.25 min(-1); and the activation energies for gelation were found to be in the range 20 to 40 kJ mol(-1). The results have been interpreted in terms of the dependence of the rate constants on structural characteristics of the prepolymers. (C) 1997 John Wiley & Sons, Inc.