Showing papers in "The Journal of Physical Chemistry in 1987"

The missing term in effective pair potentials

Abstract: on carbon. These particles can interact with the active sites of the carbon to form a species a t the metal-carbon interface which is stable against high-temperature reduction. This interaction may occur via oxygen atoms bonding with both the carbon surface and iron atoms contacting the carbon. This oxygen could be provided by dissociation of the C O ligands during decomposition. The remainder of the surface Fe can exist in either a reduced or carburized state, but the small size of these particles results in either case in a superparamagnetically collapsed Mossbauer spectrum. For carbons with fewer active sites, more of the particles are unbound, and these particles sinter and carburize in a manner more similar to that of Fe particles supported on graphite.

Molecular dynamics study of melting and freezing of small Lennard-Jones clusters

Abstract: The properties of small clusters of Lennard-Jones atoms were studied by using molecular dynamics simulations. Clusters containing from 13 to 147 atoms were found to undergo a transition, analogous to a melting transition for bulk materials, from a low-energy solidlike structure at low temperatures to a set of higher energy liquidlike structures at high temperatures. The transition creates noticeable features in plots of the equilibrium energy of clusters as a function of temperature, but the shapes of the curves depend on the size of the cluster and on whether a canonical ensemble or microcanonical ensemble is used for calculating the properties. The nonequilibrium energy as a function of temperature on heating or cooling at a finite rate can differ significantly from the equilibrium results. Coexistence among the minimum-energy structure, high-energy liquidlike structures, and typically one or more intermediate structures was observed over a range of temperatures. Results from potential energy minimization of fee and icosahedral structures of various sizes indicate that a Lennard-Jones cluster must contain at least 5000 atoms before the fee structure becomes more stable than the structure of the Mackay icosahedra.

Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study

Abstract: We report the synthesis of transparent colloidal suspensions of small zinc oxide particles in water, 2-propano1, and acetonitrile. Quantum (Q)-size effects are observed during particle growth and qualitatively interpreted by using a simple molecular orbital (MO) picture. The particles at the final stage of growth are approximately spherical in shape and consist of 2000-3000 ZnO molecules. They exhibit many of the photophysical properties of bulk zinc oxide. However, pronounced shifts in the absorption spectrum during the illumination of anoxic suspensions of ZnO reveal a distinctively different behavior of these small particles. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band into lower lying traps. Two fluorescence maxima are observed at the final growth stage of the ZnO particles. The bandgap fluorescence at 365 nm has an extremely short lifetime (τ < 100 ps), while the visible luminescence at 520 nm exhibits a slower biexponential decay (i.e., τ = 14 and 190 ns). The latter fluorescence is attributed to photogenerated electrons tunneling to preexisting, trapped holes. The low overall fluorescence quantum yield of Ф = 0.03 measured in these zinc oxide suspensions is indicative of radiationless transitions accompanying the emissions. A pronounced pH dependence of the Stern-Volmer constants obtained with various ionic substances, that effectively quench the 520-nm emission, is explained by specific adsorption to the charged particle surface. The zero point of charge (pH_(zpc)) of the aqueous colloidal suspension was determined to be 9.3 ± 0.2 by several independent methods.

An accurate electromagnetic theory study of surface enhancement factors for silver, gold, copper, lithium, sodium, aluminum, gallium, indium, zinc, and cadmium

Abstract: Utilisation d'une methode simple de determination du champ electromagnetique au-voisinage des surfaces des petites particules metalliques spheroidales, pour determiner les augmentations de la diffusion Raman et du champ dans le cas des 10 metaux du titre

Direct electrical communication between chemically modified enzymes and metal electrodes. I. Electron transfer from glucose oxidase to metal electrodes via electron relays, bound covalently to the enzyme

Abstract: Glucose-reduced glucose oxidase does not directly transfer electrons to conventional electrodes because the distance between its redox centers and the electrode surface exceeds, even on closest approach, the distance across which electrons are transferred at sufficient rates. Therefore, electrical communication between the redox centers of this enzyme and electrodes required either the presence, and diffusion to and from the enzyme's redox center, of O/sub 2/ and H/sub 2/O/sub 2/, or the presence of members of a redox couple, or the use of special electrodes like TTF/TCNQ. They show here that direct electrical communication between the redox center of a large enzyme molecule and a simple metal electrode can be established through chemical modification of the enzyme. When a sufficient number of electron-relaying centers are attached through covalent bonding to the protein of glucose oxidase, electrons are transferred from the enzyme's redox centers to relays that are closer to the periphery of the enzyme. Because some of the relays are located sufficiently close to the enzyme's surface, electrons are transferred at practical rates to the electrode. As a result, a glucose-concentration-dependent current flows in an electrochemical cell made with conventional electrodes when the electrolytic solution contains the relay-modified enzyme. Such amore » current does not flow when the solution contains the natural enzyme. Specifically, electrical communication is established between the FAD/FADH/sub 2/ centers of glucose oxidase and gold, platinum, or carbon electrodes through the covalent bonding of an average of 12 molecules of ferrocenecarboxylic acid per glucose oxidase molecule.« less

Production of Cl2O2 from the self-reaction of the ClO radical

Abstract: The species Cl2O2 has been generated in a gaseous flow system at 220-240 K by reacting Cl atoms with one of three different ClO precursors: O3, Cl2O, or OClO The infrared spectra of the reactive mixture indicate that at least two different dimers are produced: a predominant form with bands centered at 1225 and 1057/cm attributed to ClOOCl, and a second form with a band at 650/cm attributed to ClOClO The UV spectrum of the predominant form shows a maximum absorption cross section of about 65 x 10 to the -18th sq cm/molecule around 270 nm, with a wing extending beyond 300 nm The implications of these results for the chemistry of the stratosphere are discussed

Photooxidation of organic impurities in water using thin films of titanium dioxide

Abstract: Results of the destruction of organic solutes in a simple, thin film TiO2 reactor are described. The reactor was illuminated with a 20-W blacklight UV fluorescent tube and the aqueous stream containing the organic solute flowed past the stationary photocatalyst. In the continuous recirculation mode, the destructive rate of each solute obeyed approximately first-order kinetics. The reaction rate constant decreased with increasing solute concentration. The times for 50% destruction of 500 cmT of 10 M solutions of each of the solutes salicylic acid, phenol, 2-chlorophenol, 4-chlorophenol, benzoic acid, 2-naphthol, naphthalene, and fluorescein were 7.1, 7.2, 8.2, 8.7, 6.9, 8.5, 4.3, and 6.4 min, respectively. It was found that the observed apparent first-order dependence and the change in rate constant with concentration could by explained in terms of the integrated form of the Langmuir adsorption isotherm. A marked dependence of the destruction rate on flow rate was observed and an expression developed which allows the calculation of the destruction curve with good precision at any solute concentration and flow rate. A corresponding curve was observed for the formation of carbon dioxide from salicylic acid solution. It was shown that hydroxylation of the aromatic ring to give salicylic acid is a minormore » reaction path in the destruction of benzoic acid. The maximum quantum yield for the destruction of salicyclic acid at 25C was found to be 0.022. The activation energy for the photooxidation of salicyclic acid was determined to be 11.0 +/- 0.8 kJ mol .« less

Molecular-dynamics simulation of liquid methanol

Abstract: Four recently proposed intermolecular-potential models have been used in molecular-dynamics simulations of liquid methanol over a temperature range of approximately 70 K. Results are reported for thermodynamic and structural properties, self-diffusion coefficients, and reorientational correlation times. Two of the models are shown to give results in fair agreement with a wide variety of experimental data. The pattern of hydrogen bonding and the distribution of hydrogen-bond lifetimes in the simulated liquids have been investigated. The structure in each case is found to be dominated by winding chains, in agreement with earlier work. For the more realistic models, the mean hydrogen-bond lifetime at room temperature is approximately 1 to 2 ps, which is several times larger than the corresponding time for liquid water.

Spin-polarized electron paramagnetic resonance spectra of radical pairs in micelles: observation of electron spin-spin interactions

Abstract: A model for polarized EPR spectra generated in radical pair reactions in micelles is being proposed. The model is based on electron spin-spin interactions which remain observable because of limited diffusion in micelles. The experimental observable is a doubling of hyperfine transitions, split by the magnitude of the interaction. The polarization is generated by the nonadiabatic generation of the radical pair with a triplet or singlet precursor. The time evolution of the wave function leads to a non-Boltzmann distribution of the populations among the four energy levels. The theory is tested by comparison with experiments, previously reported and repeated in this laboratory, obtained by laser flash photolysis of benzophenone in sodium dodecyl sulfate (SDS) micelles. Simulations of the shape of the spectra and their time dependence give excellent agreement with experiment. The model is further supported by experiments in micelles modified by different salt concentrations as well as different chain lengths of the micelle-forming molecules.

Optical properties of CdS and PbS clusters encapsulated in zeolites

Abstract: The authors report optical properties of semiconductor clusters, CdS and PbS, encapsulated in zeolites. At a low loading level of CdS in zeolite Y, isolated clusters with size <13 A are generated. They show an absorption peak around 280 nm and do not emit even at 4.2 K. At higher loading levels, the individual clusters aggregate into an extended structure, modulated by zeolite internal cavities. It shows an exciton shoulder near 350 nm and emits at low temperature. Neither heating nor increasing CdS concentration can further shift this exciton shoulder to the red. On the contrary, when CdS is outside of the zeolite crystallite interior a continuous red shift of the absorption threshold toward the bulk value is observed paralleling the behavior of CdS colloids. The transition from clusters to aggregates upon increasing CdS loading is not continuous but rather abrupt, as judged from optical absorption and emission spectra. This suggests that the aggregation of individual clusters inside the zeolite may be a percolative process.

Photochemistry of 3,4,9,10-perylenetetracarboxylic dianhydride dyes. 3. Singlet and triplet excited-state properties of the bis(2,5-di-tert-butylphenyl)imide derivative

Abstract: Fluorescence lifetime and triplet-state spectral, kinetic, and energetic properties are reported for the first time for a perylenebis(dicarboximide) dye, N,N'-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenebis(dicarboximide) (DBPI). The fluorescence lifetime (3.8 +/- 0.2 ns), quantum yield (greater than or equal to 0.95), and singlet-state energy (54 +/- 1 kcal mol/sup -1/) were determined for DBPI in a variety of organic solvents (24/sup 0/C). Triplet sensitization was used to characterize the triplet state of DBPI by laser flash photolysis and pulse radiolysis. The triplet-triplet absorption spectrum of DBPI had a maximum in the range 495-505 nm with an extinction coefficient of (6.3 +/- 0.7) x 10/sup 4/ M/sup -1/ cm/sup -1/, and the triplet-state energy and lifetime were 27.5 +/- 2 kcal mol /sup -1/ and approx. = 100 ..mu..s, respectively. Direct photoexcitation produced the DBPA triplet with very low quantum yields (<0.001). Excimerlike fluorescence spectral and lifetime data for DBPI in the solid state are also presented.

Studies of concentrated electrolyte solutions using the electrodynamic balance. 1. Water activities for single-electrolyte solutions

Abstract: An electrodynamic balance has been used to measure the water activity as a function of solute concentration at 20 °C for three aqueous electrolyte mixtures: NaCI-KCI, NaCl-KBr, and NaCl-(NH_4)_2SO_4. The measurements were performed by levitating single, charged particles of these electrolyte mixtures within the balance and measuring the mass of the particles as a function of the surrounding relative humidity. The deliquescence behavior of the mixed-electrolyte particles was also observed. For the alkali halide mixtures, the low-concentration data were consistent with earlier investigations. Data were obtained at higher concentrations than previously reported. The amount of water in the NaC1-(NH_4)_2SO_4 particles after drying was unknown. This, together with the lack of previously reported water activity data for this mixture, complicated the analysis of these experiments. Three models of mixed-electrolyte solutions-the Zdanovskii-Stokes-Robinson, Reilly-Wood-Robinson, and Pitzer methods-agreed well with the experimental data for the NaCl-KCl and NaCl-KBr systems over the range of concentrations that the models could be applied. The mixing rules' predictions were consistent with the experimental observations for the NaCl-(NH_4)_2SO_4 system assuming a small amount of water was retained in the particles after drying.

Structure of surface tungsten oxide species in the tungsten trioxide/alumina supported oxide system from x-ray absorption near-edge spectroscopy and Raman spectroscopy

Abstract: The X-ray absorption near-edge spectra (XANES) of the L1 tungsten edge in WO3/Al2O3 samples indicate that the symmetry of the tungsten environment depends on both the surface coverage and the presence of coordinated water. At coverages of less than 1/3 monolayer, in the absence of coordinated water, the XANES spectrum indicates a distorted tetrahedral structure for the surface tungsten oxide species. Samples exposed to air at room temperature have water molecules coordinated to the surface tungsten oxide species and produce an octahedral site symmetry, but the water is removed by heating to 500 "C. The Raman spectra of the W03/A1203 samples are consistent with a distorted tetrahedral tungsten oxide environment and, in addition, show features due to W=O and W-0-W bonds. These results suggest that the surface tungsten oxide is present as both isolated and dimeric tetrahedra. At coverages approaching a monolayer, in the absence of coordinated water, a significant fraction of the surface tungsten oxide sites appears to have a distorted octahedral environment in the XANES spectra. At this high coverage the effect of coordinated water molecules is much less evident than at low coverage. The Raman results, however, only provide information about the tetrahedral component because the Raman cross section of the tetrahedral tungsten oxide is much higher than the octahedral tungsten oxide. The Raman spectra show features of W4 and W-0-W bonds in the tetrahedral fraction of the surface tungsten oxide monolayer on alumina. These observations are consistent with a surface complex where the supported tungsten oxide has formed a polymeric structure on the alumina support composed of W04 and W06 units jointed in infinite chains.

Electrochemistry and spectroscopy of ortho-metalated complexes of iridium(III) and rhodium(III)

Abstract: The electrochemical and UV-visible spectroscopic properties of Rh(III) and Ir(III) complexes of the ortho-metalating (NC) ligands, 2-phenylpyridine (ppy) and benzo(h)quinone (bzq), have been studied. Cyclic voltammetric studies of several of the dimeric species, (M(NC)/sub 2/Cl)/sub 2/, indicate metal-centered oxidation occurs at moderate potentials. Cationic monomers of the type M(NC)/sub 2/(NN)/sup +/ where (NN) = 2,2'-bipyridine or 1,10-phenanthroline have been prepared by reaction of the chelating ligands with the parent dimers. Cyclic voltammetric studies of these monomers indicate that several reversible ligand-centered reductions are generally observed and that the chelating ligand is more easily reduced than is the ortho-metalating ligand. Spectroscopic studies of the mixed ligand monomers indicate that dual emissions from MLCT states associated with the ortho-metalating and chelating ligands occur in the Ir(III) complexes whereas a single emission from a ligand-localized excited state is observed in the Rh(III) complexes. These results are discussed in terms of electronic and nuclear coupling factors analogous to those encountered in descriptions of bimolecular energy and electron-transfer processes.

Effects of steric factors on the electrosynthesis and properties of conducting poly(3-alkylthiophenes)

Abstract: Two series of thiophene monomers 3-substituted with linear and branched alkyl chains have been synthesized, and their electropolymerization has been investigated together with the properties of the resulting polymers. An analysis of the effects of the electrosynthesis conditions by visible absorption spectroscopy shows that the mean conjugation length presents an increasing dependence on the applied electrical conditions as the length of the alkyl chain increases. In the case of linear alkyl substituents (R3 = CH/sub 3/ to C/sub 18/H/sub 37/), the length of the alkyl chain affects slightly the polymerization reaction but produces important modifications on the structure and on the properties of the polymers. Thus, the mean conjugation length and the electrochemical reversibility admit an optimum for the polymers containing 7-9 carbons in the alkyl chain while the polymer films remain highly conductive up to R3 = C/sub 10/H/sub 21/. In contrast, the steric hindrance to conjugation caused by branched alkyl substituents totally inhibits the polymerization (isopropyl) or leads to weakly conjugated polymers (isobutyl). It is shown that the distance between the thiophene ring and the branched alkyl chain is the determining parameter which governs the aptitude for electropolymerization and the properties of the resulting polymer.

Simple intramolecular model potentials for water

Abstract: An effective intramolecular potential is presented for use in conjunction with existing three-site models of water. Two commonly used internal geometries were fit to the same form yielding slightly different parametrizations. By including a Urey-Bradley-like term in an otherwise standard molecular mechanics form it was found that the experimental transition frequencies of water monomer can be reproduced accurately. Good qualitative agreements for spectral shifts were subsequently found for the models in condensed-phase applications. Harmonic analysis of clusters indicates good qualitative agreement with experimental environmental shifts in frequencies at low temperatures for these models. This model should be useful for a wide variety of applications including simulations of biopolymers and ionic solutions.

Adsorption of carbon monoxide on ZSM-5 zeolites: infrared spectroscopic study and quantum-chemical calculations

Abstract: Low temperature adsorption of CO was studied on H-ZSM-5 zeolites modified by dehydroxylation, ionic exchange with Al/sup 3 +/, and impregnation with Al/sub 2/O/sub 3/ and on Na-ZSM-5 and CaH-ZSM-5 zeolites. It was found that interaction of CO with framework OH groups results in the formation of a hydrogen-bonded CO complex whose OH bond frequency is decreased by 310-320 cm/sup -1/ compared with that of free hydroxyls. For the less acidic framework hydroxyls in large cavities of H/sub 70/Na/sub 30/-Y zeolite the observed shift is 275 cm/sup -1/. With ZSM-5 zeolites, at least six types of electron-accepting sites are observed originating from nonframework Al species (band of CO in the interaction complex: 2132, 2222, 2202, 2195, and 2198 cm/sup -1/) and the Al/sub 2/O/sub 3/ microcrystalline phase (CO band at 2153 cm/sup -1/). The CO bond orders calculated by the CNDO/2 method for the CO interaction complexes with models of surface sites increase in the following order: > O-CO OH-CO approx. ..-->.. Al-CO approx. = Na-CO .. Si-CO < Al(cationic)-CO. A correlation between the calculated bond orders of CO and the observed vibrational frequencies of CO-forming interaction complexes is drawn.

The mechanism of solute retention in reversed-phase liquid chromatography

Abstract: Developpement d'une theorie decrivant la retention de petites particules par des phases a chaines greffees en chromatographie phase inverse

Reduction potential of the carbon dioxide/carbon dioxide radical anion: a comparison with other C1 radicals

Abstract: The formate radical CO/sub 2//sup -/ is commonly used in aqueous radiation chemistry as a reducing agent for metalloproteins or as an intermediate in the formation of the superoxide anion. Although it is known to be a strong one-electron reductant, exact determination of the reduction potential of the couple CO/sub 2//CO/sub 2//sup -/ is difficult because of the irreversibility of the electron-transfer process CO/sub 2/ + e/sup -/ ..-->.. CO/sub 2//sup -/. The irreversibility might be related to the change in structure upon reduction from linear to bent. Since the electron affinity of carbon dioxide is known, the only missing parameter is the Gibbs energy of solution (..delta../sub s/G/sup 0/) of the formate radical, CO/sub 2//sup -/. The approach in the present study is based on the observed similarity of Gibbs energies of solution of bent triatomic anions which are structurally related to CO/sub 2//sup -/. The reduction potentials calculated allow a complete oxidation-state diagram of C/sub 1/ compounds containing hydrogen and oxygen to be constructed. It shows that the one-electron oxidation of formate is energetically comparable to the oxidation of methanol and formaldehyde, while oxidation of methane is much more unfavorable.

Infrared and x-ray photoelectron spectroscopy study of carbon monoxide and carbon dioxide on platinum/ceria

Abstract: The adsorptions of CO and CO/sub 2/ on Pt/CeO/sub 2/ were studied by infrared and X-ray photoelectron spectroscopy. A heterogeneous distribution of linearly adsorbed CO on Pt at terrace, step, and corner sites was proposed based on changes in the absorption of the CO infrared stretching frequencies from 2084 to 2060 cm/sup -1/ upon heating under vacuum from 300 to 473 K. Pretreatment of the sample with O/sub 2/ above 373 K lead to a blue-shifted peak at 2091 cm/sup -1/, interpreted as arising from CO adsorbed on Pt with an atom of oxygen at a neighboring site, and a new peak at 2131 cm/sup -1/, assigned to CO coadsorbed with an oxygen atom on the same Pt atom. Adsorption of CO/sub 2/ at room temperature resulted in the appearance of an IR band at 2065 cm/sup -1/, assigned to CO adsorbed on a Pt atom on a step or corner. The intensity of the CO band from CO/sub 2/ adsorption was sensitive to pretreatment of the sample, being completely suppressed by preoxidation but enhanced by prereduction. Lattice oxygen vacancies in the support, CeO/sub 2/, in the vicinity of the Pt particles are responsible for the formation of CO frommore » CO/sub 2/. XPS spectra show that Ce/sup 3 +/ is formed by the prereduction treatment, supporting the proposal that lattice oxygen vacancies play an important role in the CO/sub 2/ decomposition. Preoxidation is shown to lead to the formation of Pt/sup 2 +/ XPS (4f) spectra, which is removed by flash heating. Disproportionation of CO to carbon and CO/sub 2/ was shown not to occur by pretreatment of the surface with CO at 573 K.« less