Showing papers in "The Journal of Physical Chemistry in 1988"

Time-dependent quantum-mechanical methods for molecular dynamics

Abstract: The basic framework of time-dependent quantum-mechanical methods for molecular dynamics calculations is described. The central problem addressed by computational methods is a discrete representation of phase space. In classical mechanics, phase space is represented by a set of points whereas in quantum mechanics it is represented by a discrete Hilbert space. The discretization described in this paper is based on collocation. Special cases of this method include the discrete variable representation (DVR) and the Fourier method. The Fourier method is able to represent a system in phase space with the efficiency of one sampling point per unit volume in phase space h, so that, with the proper choice of the initial wave function, exponential convergence is obtained in relation to the number of sampling points. The numerical efficiency of the Fourier method leads to the conclusion that computational effort scales semilinearly with the volume in the phase space occupied by the molecular system. Methods of time propagation are described for time-dependent and time-independent Hamiltonians. The time-independent approaches are based on a polynomial expansion of the evolution operator. Two of these approaches, the Chebychev propagation and the Lanczos recurrence, are also compared. Methods to obtain the Raman spectra directly by using the Chebychev propagation method are shown. For time-dependent problems unitary short-time propagators are described: the second-order differencing and the split operator. Consideration of all these methods has led to scaling laws of computation. The conclusion from such scaling laws is that, for simulations of complex molecular systems, approximation techniques have to be employed which reduce the dimensionality of the problem. The time-dependent self-consistent field (TDSCF) is suggested. Finally, a brief description is presented of current applications of the time-dependent method.

Preparation and characterization of quantum-size titanium dioxide

Abstract: The syntheses of transparent colloidal solutions of extremely small titanium dioxide particles (d < 3 nm) in water, ethanol, 2-propanol, and acetonitrile are presented. Quantum-size effects are observed during particle growth and at the final stages of synthesis. They are quantitatively interpreted by using a quantum mechanical model developed by Brus. The particles prepared in aqueous solution possess the anatase structure and consist of about 200 TiO_2 molecules at their final growth stage. The colloidal particles can be isolated from solution as white powders that are soluble in water and ethanol with no apparent change in their properties. In organic solvents the quantum-size TiO_2 particles appear to form with rutile structure. Excess negative charge on the particles resulting either from deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers causes a blue shift in the electronic absorption spectrum, which is explained by an electrostatic model. Electrons can be trapped in the solid as a Ti^(3+) species, which has a characteristic visible absorption spectrum. As much as 10% of the available Ti^(4+) ions can be reduced photochemically in the solid with a quantum yield of 3%. Molecular oxygen reoxidizes the Ti^(3+) centers, leading to detectable amounts of surface-bound peroxides. The pH of zero point of charge (pH_(zpc)) of the aqueous colloidal suspension has been determined to be 5.1 ± 0.2. The acid-catalyzed dissolution of the aqueous colloid yielding Ti(IV) oligomers has been studied, and an activation energy E_a = 58 ± 4 kJ/mol has been measured for this reaction. The photocatalytic activity of the small TiO_2 particles is demonstrated.

Molecular quasi-species.

Abstract: 1. The Sequence Space 2. The Kinetic Equations 3. How Realistic is the Kinetic Ansatz? 4. Solutions of the Rate Equations 5. Potential Functions, Optimization, and Guided Evolution 6 . Population Structures Error Threshold For Quasi-species Localization 1. Error Threshold and Selective Advantage 2. Localization Threshold for Statistically Distributed Replication Rates 3. Extreme-Value Theory for Effective Superiority 4. Relaxed Error Threshold and Gene Duplication 5. Analogies to Phase Transitions Examples of Fitness Landscapes and Stationary Populations 1. Error Threshold 2. Degenerate Quasi-species and Neutral Mutants 3. Conformation-dependent Value Functions and Fitness Landscapes 4. Asymmetry of Fitness Landscapes: Apparent Guidance of Evolution

Rheological properties of viscoelastic surfactant systems

Abstract: Les proprietes rheologiques ainsi que la viscosite de cisaillement des solutions de chlorure de cetylpyridinium et de salicylate de sodium sont mesurees en fonction de la concentration en agent de surface et en sel

Kinetics studies in heterogeneous photocatalysis. I. Photocatalytic degradation of chlorinated phenols in aerated aqueous solutions over titania supported on a glass matrix

Abstract: The photocatalytic degradation of phenol, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,5-trichlorophenol over TiO/sub 2/ (anatase) has been investigated by using three photochemical reactors. TiO/sub 2/ was used as a thin film, coating the internal surface of a glass coil (reactors I and II) or the external surface of glass beads (reactor III). The degradation of the four phenolic compounds, in a continuous recirculation mode in all three reactors, approximates first-order kinetics to near-complete degradation. The Langmuir-Hinshelwood kinetics have been modified slightly to rationalize the first-order behavior in solid-liquid reactions and to argue in favor of a surface reaction; the degradation reactions occur on the TiO/sub 2/ particle surface. In the multipass mode experiments, both reactors I and II exhibit higher degradation rates for phenol at the higher flow rates. By contrast, the greater degradation is associated with the lower flow rates in the single-pass mode experiments.

Surface characterization of alumina-supported ceria

Abstract: CeO/sub 2//Al/sub 2/O/sub 3/ samples were characterized by XPS, Raman spectroscopy, and TPR. Three cerium species were detected, namely, a CeAlO/sub 3/ precursor in the dispersed phase, small CeO/sub 2/ crystallites, and large CeO/sub 2/ particles. The CeAlO/sub 3/ precursor is so named because it gives a Ce(III)-like XPS spectrum and a completely different Raman spectrum from that of CeO/sub 2/, while TPR reveals that this species is more easily reduced to CeAlO/sub 3/ than the CeO/sub 2/ particles on alumina. It was also found that ceria in the CeAlO/sub 3/ precursor and in a small crystallite form can be transformed to surface CeAlO/sub 3/ upon H/sub 2/ reduction at temperatures higher than 600/degrees/C, while even a partial conversion to CeAlO/sub 3/ thus formed shows good thermal stability in air in the temperature above 800/degrees/C. CeAlO/sub 3/ thus formed shows good thermal stability in air in the temperature range up to 600/degrees/C.

Linear solvation energy relationship. 46. An improved equation for correlation and prediction of octanol/water partition coefficients of organic nonelectrolytes (including strong hydrogen bond donor solutes)

Abstract: Les coefficients de partage octanol/eau de 245 composes aliphatiques et aromatiques sont bien correles par la relation d'energie de solvatation lineaire: logK ow =0,35+5,35V I /100-1,04(π*−0,35δ)-3,84 β m +0,10α m (r=0,9959, SD=0,131) ou V I est le volume molaire intrinseque, π*, β m et α m sont des parametres solvatochromiques

Photochemistry and radiation chemistry of colloidal semiconductors. 23. Electron storage on zinc oxide particles and size quantization

Abstract: Improved methods for the preparation of colloidal ZnO solutions of different particle size are described, and the relation between absorption threshold and particle size is reported. CH/sub 2/OH radicals, radiolytically generated, transfer electrons to ZnO particles. The electrons are long-lived and cause a substantial blue shift of the absorption spectrum of ZnO in a wavelength range of 60 nm below the threshold. The wavelength of maximum bleaching is shifted to shorter wavelengths with decreasing particle size (size quantization effect). Maximum bleaching occurs with a negative absorption coefficient of 1.1 x 10/sup 5/ M/sup -1/ cm/sup -1/. Electrons are also stored upon UV illumination of colloidal ZnO. The stored electrons react rather slowly with oxygen, the rate constant becoming lower with increasing particle size, and more rapidly with peroxy radicals.

Classical and modern methods in reaction rate theory

Abstract: Principes physiques courants permettant la determination des constantes de vitesses dans les gaz et les liquides. Methodes theoriques et techniques numeriques pour des systemes isoles ou dans des solvants

Photoelectrochemistry of cadmium sulfide. 1. Reanalysis of photocorrosion and flat-band potential

Abstract: The photoelectrochemical behavior of cadmium sulfide depends strongly on the pretreatment of the electrodes. Sulfur, the main photocorrosion product of CdS, changes the potential drop across the Helmholtz double layer and shifts the flat-band potential of CdS in the anodic direction. Removal of sulfur from the surface is possible by prepolarizing the electrode at about -1.1 V(SCE) in the presence of oxygen. The flat-band potential of the clean (001) Cd surface has been determined to about -1.8 V(SCE) in the dark. Illumination of the electrode leads to surface-state charging due to formation of S/sup .-/, the intermediate in the formation of both possible photooxidation products: S/sup 0/ in the absence of oxygen and SO/sub 4//sup 2 -/ in the presence of oxygen.

Kinetics of CO oxidation on single-crystal Pd, Pt, and Ir

Abstract: The activity of Pt(100), Pd(110), Ir(111), and Ir(110) single-crystal catalysts for CO oxidation has been studied as a function of temperature and partial pressure of O/sub 2/ and CO in a high-pressure reactor-ultra-high-vacuum surface analysis apparatus over the temperature range 425-725 K and pressure range 0.1-600 Torr. The specific rates and the partial pressure dependencies determined for the single crystals are in excellent agreement with results obtained previously for high surface area supported catalysts, demonstrating the structure insensitivity of this reaction. The single-crystal catalysts exhibit simple Arrhenius behavior over most of the temperature range studied, and the observed activation energies lie between 22 and 33 kcal/mol, close to the desorption energy of CO from these surfaces. These results are consistent with the generally accepted model in which the surface is primarily covered with CO and the reaction rate is controlled by the desorption of CO. Deviation from Arrhenius behavior below 500 K for Pt is interpreted as a change in the reaction mechanism. Under highly oxidizing conditions surfaces of both Pd and Ir show negative-order dependence on O/sub 2/ partial pressure, indicating the presence of a strongly bound oxygen species. The oxygen species was similar to surface oxide formed bymore » deliberate oxidation and could be detected as CO/sub 2/ desorbing at high temperatures in postreaction temperature-programmed desorption. Oxide formed by oxidation of the Pd and Ir samples prior to high-pressure reaction was only stable at 475 K on Pd(110) in an 11:1 O/sub 2/:CO mixture and to 500 K on Ir(111) in an 80:1 O/sub 2/CO:CO mixture. Deliberate oxidation resulted in a rate decrease but did not affect the activation energy significantly, indicating that the oxide served merely as a simple site blocker.« less

Adsorption of ordered zirconium phosphonate multilayer films on silicon and gold surfaces

Abstract: Multilayer films of zirconium 1,10-decanediylbis(phosphonate) have been prepared on silicon and gold substrates and characterized by ellipsometry, XPS, and electrochemical measurements. The deposition technique requires first covalent attachment or adsorption of a phosphonic acid anchoring agent; HO(CH/sub 3/)/sub 2/Si(CH/sub 2/)/sub 3/PO/sub 3/H/sub 2/ (I) and (-S(CH/sub 2/)/sub 4/PO/sub 3/H/sub 2/)/sub 2/ (II) were used with Si and Au, respectively. The functionalized substrates are exposed alternately to aqueous ZrOCl/sub 2/ and 1,10-decanediylbis(phosphonic acid) solutions to yield multilayer films. Ellipsometry shows an increase in film thickness, on Si, of 17 A/layer, which corresponds to the layer spacing in bulk Zr(O/sub 3/PC/sub 10/H/sub 20/PO/sub 3/). Variable take-off angle X-ray photoelectron spectra from four-layer films have attenuated Si peaks but strong Zr and P peaks when the detector is 70/sup 0/ off the surface normal, implying that the films on Si are continuous. Electrochemical comparison of bare and functionalized Au shows facile electron transfer between Au or Au-II electrodes and 1 mM aqueous Fe(CN)/sub 6//sup 3 -/ but nearly complete blocking of electron transfer at Au-II electrodes immersed once each in ZrOCl/sub 2/ and 1,10-decanediylbis(phosphonic acid) solutions.

Water in Silicate Glasses: Quantitation and Structural Studies by ^(1)H Solid Echo and MAS-NMR Methods

Abstract: ^(1)H wideline and magic angle spinning (MAS) NMR results are reported for water in a series of synthetic and naturally occurring silicate glasses containing from 0.04 to 9.4 wt % H_(2)0. For glasses free of paramagnetic metal ions, the absolute water contents can be accurately determined by a solid echo ^(1)H NMR technique with pyrophyllite, A1_(2)Si,0_(10)(OH)_2, as an intensity reference. The MAS-NMR spectra can be interpreted as superpositions of the individual spectra of OH and anisotropically constrained H_(2)0 groups, the latter giving rise to spinning sidebands extending over ca. 100 kHz. Two methods are described to obtain percentages of OH and H_(2)0 groups from the relative intensities of the centerband and the spinning sidebands in these glasses. The MAS-NMR results are consistent with previous IR analyses indicating that low levels of water (<2-4 wt %) are mainly present as OH groups whereas at higher concentrations molecular H_(2)0 species dominate. Simulations of the MAS-NMR spectra based on the individual spectra of compounds in which the hydrogen-bearing species are structurally isolated (OH in tremolite and H_(2)0 in analcite) accurately reproduce the experimental spectra, indicating that the OH or H_(2)0 groups in the glasses are not preferentially clustered. The MAS-NMR centerband line shapes are dominated by a distribution of isotropic chemical shifts. The well-established linear dependence of ^(1)H chemical shifts on the O-H•••O distance (a measure of the hydrogen bonding strength) leads to average distances of 290 ± 1.5 pm in all synthetic glasses except silica, 293 ± 1.5 pm in the volcanic rhyolite glasses, and 298 pm in silica glass. This value does not depend on the total water contents, indicating that the hydrogen-bonding characteristics of OH and H_(2)0 species in the glasses are similar. The ^(1)H wideline NMR procedure above yields underestimates of the total water content for synthetic and volcanic glasses containing ca. 1 wt 5% iron, presumably due to extreme signal broadening by the strong dipolar fields from the electron spins of the paramagnetic ions. These dipolar couplings also affect the line shape of the observable portion of the hydrogen resonance and produce intense spinning sidebands in the MAS-NMR spectra which invalidate determinations of OH/H_(2)0 ratios in these cases.

Product quantum yields for the 305-nm photodecomposition of nitrate in aqueous solution

Abstract: The radical scavenging technique with product analysis was employed to determine quantum yields for the two main processes (1) NO/sub 3//sup -/ + hv = NO/sub 2//sup -/ + O and (2) NO/sub 3//sup -/ + H/sup +/ hv = NO/sub 2/ + OH in dilute, air-saturated solutions of sodium nitrate. Oxygen atoms were detected by their reaction with cyclopentene and the production of ethene. The quantum yield found was /phi/(O) = (1.1 /plus minus/ 0.1) x 10/sup -3/. The generation of OH radicals was confirmed by their reactions with benzene to give phenol and with 2-propanol to produce acetone. The second reaction was used to establish an OH quantum yield of /phi/(OH) = (9.2 /plus minus/ 0.4) x 10/sup -3/. Nitrite was formed with a similar yield in the pH region 5-12, whereas at lower pH the yield declined. Formate as a scavenger converts OH radicals toward O/sub 2//sup -//HO/sub 2/ radicals which produce H/sub 2/O/sub 2/. The yield of H/sub 2/O/sub 2/ at pH 5.6 was lower than expected whereas that of NO/sub 2//sup -/ was higher. Reactions are postulated to explain this observation as well as the pH dependence of nitrite formation.

The physicochemical properties of self-assembled surfactant aggregates as determined by some molecular spectroscopic probe techniques

Abstract: Determination de la viscosite, de la constante dielectrique et du potentiel de surface de solutions micellaires

Very long range attractive forces between uncharged hydrocarbon and fluorocarbon surfaces in water

Abstract: A new method has been used to carry out accurate measurements of the hydrophobic interaction between uncharged hydrocarbon and fluorocarbon monolayer-coated surfaces in a surface force apparatus. The measurable range of the attraction between these deposited monolayers extends to a separation of 80 nm in conductivity water and can be approximated by an exponential function with two decay lengths of 2-3 and 13-16 nm, respectively. There is only a slight difference in the strength of the interaction between the fluorocarbon and hydrocarbon surfaces, in contrast to the significant difference in hydrophobicity as measured by the advancing contact angle of water. The authors suggest that the much shorter range attractive forces measured between monolayer adsorbed from solution are due to the formation of partial bilayers. They speculate that the very long range hydrophobic interaction found here is related to the metastability of water films between very hydrophobic surfaces.

Activity of chromium-ion-doped titania for the dinitrogen photoreduction to ammonia and for the phenol photodegradation

Abstract: The photoreduction of dinitrogen to ammonia in a gas-solid regime and the photodegradation of phenol in an aqueous liquid-solid regime were investigated by using a series of chromium-ion-containing titania specimens. For the same photoreactions, two iron-ions-containing titania specimens were also studied for comparison. For the photoreduction reaction it was found that the presence of the dopants is essential for the occurrence for the photoreactivity, the iron-ions-containing specimens being more effective than those containing chromium ions. For the photooxidation reaction the presence of the dopants instead is not essential; the photoreactivity of titania is not affected or, in some cases, detrimentally affected. The two contrasting behaviors are explained by taking into account the differences in the gas-solid and liquid-solid interfaces.

Microstructure of microemulsions by freeze fracture electron microscopy

Abstract: Refroidissement rapide de microemulsions dans le propane liquide. La fracture sous vide et la replique d'une face fracturee permet la discrimination des domaines riches en eau et huile par microscopie electronique a transmission

Photoluminescence and relaxation dynamics of cadmium sulfide superclusters in zeolites

Abstract: Photoluminescence properties of CdS superclusters in zeolites have been studied. Three different emission bands, all attributed to defects rather than carrier recombination, are observed. The yellow-green emission, which can be observed in zeolites X, Y, and A, is attribued to Cd atoms. The red emission is attributed to sulfur vacancies, following previous works on colloidal CdS. The blue emission is due to shallow donors, the nature of which is not yet clear but we suggest it is sulfur related. Both red and blue emissions can be observed only in zeolite A

Effect of solvent polarity on nonradiative processes in xanthene dyes: Rhodamine B in normal alcohols

Abstract: The fluorescence lifetime of rhodamine B in a series of normal alcohols (C/sub n/H/sub 2n-1/OH, n = 1-6) was measured as a function of temperature. The nonradiative rate constants were calculated from the fluorescence lifetimes and quantum yields. Activation energies were obtained from Arrhenius plots of the nonradiative rate constant. The variation of the nonradiative rate constant with solvent polarity and temperature was consistent with a photophysical mechanism that involves equilibrium between the planar and twisted configurations of the diethylamino groups on the xanthene ring of rhodamine B and internal conversion from the twisted configuration. The activation energy is equal to the free energy difference between the twisted and planar configurations. The solvent polarity dependence of the free energy difference and of the rate constant for internal conversion from the twisted configuration determines the variation of the nonradiative rate constant with solvent. When solvent polarity effects are taken into account by using the parameter E/sub T/(30), the nonradiative rate constant shows weak or no dependence on the solvent viscosity.

Moeller-Plesset perturbation theory with spin projection

Abstract: A method has been developed for calculating energies by full spin projection of unrestricted Mdler-Plesset perturbation theory wave functions. The spin projection technique has been tested on bond dissociation potentials of LiH and HF and on symmetrically stretched H20. In the region where the UHF wave function is more stable than the RHF wave function, spin projected UMPn energies of low order (n I 4) have smaller errors than the corresponding spin restricted MPn and unprojected UMPn energies, when compared to full configuration interaction calculations. For higher order perturbation theory, spin restricted MPn energies may be more accurate than spin projected UMPn, but only for limited region near the RHF/UHF instability. An approximate spin projected UMPn formalism developed earlier yields energies that are in good agreement with the-present full spin projected UMPn calculations. A formula for spin projected energies in the coupled clusters approach is also presented. It is shown that annihilation of any single spin contaminant leaves the CCSD energy una1 tered.

Surface geometry change in 2-naphthoic acid adsorbed on silver

Abstract: The surface geometry of quinoline, pyridine, and 2-naphthoic acid adsorbed on silver colloid surface has been determined by observing the CH stretching and CH bending bands in their SERS spectra. 2-Naphthoic acid has been observed to have two different surface geometries depending on the surface concentration. At high surface concentration, this molecule is «standing up», while at low surface concentration, it is «lying down» on the silver surface

Dihexadecyl phosphate, vesicle-stabilized and in situ generated mixed CdS and ZnS semiconductor particles. Preparation and utilization for photosensitized charge separation and hydrogen generation

Abstract: Semiconductor particles, compromised of either homogeneous mixed crystals of Zn/sub x/Cd/sub 1-x/S or crystals of CdS coated on the surface with ZnS, were in situ generated in and stabilized by dihexadecyl phosphate (DHP) vesicles. Introduction of H/sub 2/S into solutions which contained Cd/sup 2 +/- and Zn/sup 2 +/-coated DHP vesicles, at pH values higher than 9, resulted in the formation of colloidal mixed crystals of Zn/sub x/Cd/sub 1-x/S. Surfactant vesicles were found to regulate the composition and band gap of the Zn/sub x/Cd/sub 1-x/S particles produced. Addition of Zn/sup 2 +/ to a DHP-stabilized CdS solution followed by the introduction of H/sub 2/S led to the formation of ZnS-coated CdS particles. Microstructures of DHP-vesicle-incorporated semiconductor particles were investigated by absorption fluorescence spectroscopies and by powder X-ray diffraction. Transfer of conduction band electrons or holes to acceptors or donors have been observed by flash or continuous irradiation. DHP-vesicle-incorporated Zn/sub x/Cd/sub 1-x/S and ZnS-coated CdS particles efficiently sensitized water photoreduction even in the absence of noble metal catalysts. The hydrogen generation rate in the DHP vesicle system was found to be 5 times higher on using ZnS-coated CdS (ratio of Zn:Cd = 1:1) than pure CdS semiconductor particles as sensitizers.