Showing papers in "The Journal of Physical Chemistry in 1989"

Coupled-cluster approach to molecular structure and spectra: a step toward predictive quantum chemistry

Abstract: Etude de l'une des limitations dans la solution de l'equation de Schrodinger a savoir, le probleme de la correlation electronique et d'une nouvelle approche de sa resolution, la theorie des amas couples

Phase transition of aqueous solutions of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide)

Abstract: Variation brusque des dimensions moleculaires du poly (N-isopropylacrylamide) en solution aqueuse vers 32 o C, suivie d'une agregation des chaines de polymeres en particules globulaires. Transition de phase independante de la masse moleculaire et de la concentration du polymere. La transition de phase du poly (N-isopropylmethacrylamide) presente un retard lors du refroidissement

Intermediates and barrier crossing in a random energy model (with applications to protein folding)

Abstract: Utilisation d'un hamiltonien stochastique pour etudier une reaction chimique complexe. Une simple generalisation de la theorie de l'etat de transition est developpee et est utilisee pour estimer le temps de repliement d'une proteine

Phase relations and vitrification in saccharide-water solutions and the trehalose anomaly

Abstract: Glass transition temperatures and phase relations for several saccharide-water systems are reported. The trehalose-water system is distinguished from the others by a significantly higher T g at all water contents with a particularly large advantage near the stoichiometry of one water molecule per glucose ring. We relate this to the observation that desert insects, which survive dehydration and can exist in suspended animation for decades, produce trehalose intracellularly during desiccation

Cubic lipid-water phases: structures and biomembrane aspects

Abstract: Etude des transitions de phase dans les membranes formees de couches bimoleculaires de phospholipide ou d'acides gras

Relation between charge transfer absorption and fluorescence spectra and the inverted region

Abstract: The theoretical expression for the plot of a charge transfer (CT) band intensity vs frequency is known to parallel that for the plot of electron-transfer rate constant k^(ET) vs –ΔG^0. We use this parallelism to explore a recent model in the literature on the possible role, if any, of partial dielectric saturation of the solvent on the “inverted region” of electron-transfer reactions. A simple approximate expression is obtained for the full width at half-maximum for the k^(ET) and spectral plots and is tested using recent numerical results for calculated k^(ET) vs –ΔG^0 curves. Studies of experimental widths of CT bands are used, thereby, to test ideas on the possible presence and effects of partial dielectric saturation and on the observation of the inverted region. Further uses of charge-transfer spectra, when both the absorption and fluorescence data are available, are also described, including the determination of the “0 → 0 transition energy” ΔE_( 0 → 0) for systems displaying little or no vibrational structure.

Reduction potentials of CO2- and the alcohol radicals

Abstract: Equilibrium constants were measured at 25{degree}C for the reactions CO{sub 2}{sup {sm bullet}{minus}} + Tl{sup +} {rightleftharpoons} CH{sub 3}CHO + Tl{sup 0} (K = 0.53), and (CH{sub 3}){sub 2}CO{sup {sm bullet}{minus}} + Tl{sup +} {rightleftharpoons} (CH{sub 3}){sub 2}CO + Tl{sup 0} (K = 520). These constants give reduction potentials for CO{sub 2}{sup {sm bullet}{minus}}, CH{sub 2}O{sup {sm bullet}{minus}}, CH{sub 3}CHO{sup {sm bullet}{minus}}, and (CH{sub 3}){sub 2}CO{sup {sm bullet}{minus}} of {minus}1.90, {minus}1.81, {minus}1.93, and {minus}2.10 V, and for {sup {sm bullet}}CH{sub 2}OH, CH{sub 3}CHOH, and (CH{sub 3)3}COH of {minus}1.18, {minus}1.25, and {minus}1.39 V. All potentials are based on E{degree} (Tl{sup +}/Tl{sub aq}{sup 0}) = {minus}1.94 V. The stability constant for the reaction Tl{sup 0} + Tl{sup +} {rightleftharpoons} Tl{sub 2}{sup +} was found to be 140 M{sup {minus}1}. Values of {Delta}G{sub f}{sup 0} for the radicals in solution are given and values of {Delta}G{sub f}{degree} and {Delta}H{sub f}{degree} for the radicals in the gas phase are estimated based on the assumption that the free energy of solution of the neutral radicals is the same as for the corresponding alcohol or formic acid.

Structural studies of Aerosol OT reverse micellar aggregates by FT-IR spectroscopy

Abstract: La microstructure des microemulsions eau/isooctane/aerosol OT est determinee par spectrometrie IR a transformee de Fourier

Synthesis and electrochemistry of alkyl ring-substituted polyanilines

Abstract: Polymerisation electrochimique de l'aniline sur une electrode d'or ou de platine dans une solution d'acide chlorhydrique

Solid-state vanadium-51 NMR structural studies on supported vanadium(V) oxide catalysts: vanadium oxide surface layers on alumina and titania supports

Abstract: Solid-state wide-line, magic-angle spinning (MAS) and pulse excitation (nutation) NMR techniques are applied in a study of local environments in two-dimensional vanadium(V) oxide surface layers on titania and alumina supports. Two main surface vanadium oxide species with different bonding environments are detected, which are, on the basis of their anisotropic chemical shift and nuclear electric quadrupolar coupling properties, assigned to 4- and 6-coordinate V-O environments. In all materials, the relative amount of 6-coordinate surface species increases monotonically as a function of the vanadium oxide surface coverage. However, the results indicate a marked dependence of the surface vanadium oxide structure on the metal oxide support material. Vanadium(V) oxide on TiO{sub 2} (anatase) substrates displays the highest tendency to be 6-coordinated down to very low surface coverages. Hydrous species (OH and H{sub 2}O) participate in this environment, and upon dehydration in vacuo, a new tetrahedral species is formed. These results illustrate the suitability of {sup 51}V NMR as a unique quantitative spectroscopic tool in the structural analysis of vanadium(V) oxide catalytic materials.

Stability of lyotropic phases with curved interfaces

Abstract: The diverse structural forms of lipid-water phases are representative of those found in many lyotropic and amphiphilic systems. These structures consist of interfaces which divide the material into hydrophobic and hydrophilic volumes. The net result of a very complex set of intermolecular interactions is that the interfaces behave as surfaces endowed with a spontaneous curvature and whose separations are subject to constraints of molecular lengths and component densities. Phase transitions which result in an abrupt change of the curvature of the interfaces may be understood phenomenologically as a competition between the elastic energy of bending the interfaces and energies resulting from the constraints of interfacial separation. The application of this approach is reviewed for transitions between lamellar, hexagonally packed cylindrical, close-packed spherical, and bicontinuous cubic structures of diacyl biomembrane lipids. It is shown that the structural dimensions and phase transitions of the lipid mesomorphs may be largely understood in terms of the phenomenological model. The existence of a spontaneous curvature associated with the lipid monolayers of bilayers implies that biomembranes exist in a state of compositionally controlled elastic stress which may provide a chemically nonspecific rationale for the types of lipids found in cell membranes. A discussion is given ofmore » the evidence that membrane proteins are functionally sensitive to the elastic stress. The article concludes with a summary of related outstanding problems.« less

Forming benzene in flames by chemically activated isomerization

Abstract: Benzene is not formed in flames by high-pressure-limit addition reactions, as had been implied previously, but by chemically activated addition and isomerization reactions. First, mole-fraction and rate data for molecules and free radicals were measured in a lightly sooting, laminar, premixed flame of C{sub 2}H{sub 2}/O{sub 2}/Ar at 1000-1700 K and 2.67 kPa (20 Torr) by using molecular-beam mass spectrometry. Second, mechanisms were screened in this flame and a similar 1,3-butadiene flame by using high-pressure-limit rate constants. Third, pressure-dependent rate constants for all channels of successful mechanisms were analyzed by bimolecular quantum-RRK calculations. Finally, data tests with these more accurate rate constants showed that only additions of vinylic 1-C{sub 4}H{sub 5} and 1-C{sub 4}H{sub 3} radicals to C{sub 2}H{sub 2} were fast enough to account for the highest observed rates of benzene formation, forming benzene and phenyl directly by chemically activated channels. These reactants have been suggested before, but the pathways are crucially different from high-pressure-limit routes.

Calculation of bond energies in compounds of heavy elements by a quasi-relativistic approach

Abstract: A quasi-relativistic method, in which the valence density is optimized with respect to the first-order relativistic Hamiltonian, has been evaluated by calculations on systems containing heavy elements including third-row transition metals and actinides. The method adopts the statistical energy expression and employs in addition the frozen core approximation. The quasi-relativistic method has been applied in calculations on atomic orbital energies for the valence shells of heavy elements. It is concluded from these calculations that the quasi-relativistic scheme affords results in better accord with the fully relativistic Dirac-Slater method than the first-order relativistic method based on perturbation theory. Calculations on the M-X bond energies in MX{sub 4} (M = Th, U; X = F, Cl, Br, I) as well as the M-R bond energies in Cl{sub 3}MR (M = Th, U; R = H, CH{sub 3}) revealed in addition that bond energies based on the quasi-relativistic method (QR) were in better agreement with experimental data than bond energies based on the first-order perturbation theory (FO). The absolute mean derivations with respect to experimental values were 6.9 and 16.5 kcal mol{sup {minus}1} for QR and FO, respectively, the case of the MX{sub 4} systems. It is concluded that the quasi-relativistic method, inmore » which changes in the electron density induced by relativity ({Delta}{rho}{sup R}) are approximately taken into account in the energy expression, should be used for compounds containing actinides. Both QR and FO (in which contributions from {Delta}{rho}{sup R} to the total energy are absent, even though they are present in the orbital energies) are appropriate for elements up to Z = 80, although QR represents a slight improvement for the elements in the third transition series.« less

Ab initio reaction paths and direct dynamics calculations

Abstract: A detailed study of methods for generating the minimum energy path of a chemical reaction using ab initio electronic structure calculations is presented; the convergence with respect to step size of the geometry and energy along this path is studied with several algorithms. The investigations are extended to the calculation of chemical reaction rate coefficients by interfacing the polyrate code for variational transition-state theory and semiclassical tunneling calculations with a locally modified Gaussian 82 electronic structure package that now contains reaction path following capabilities at both the Hartree-Fock and perturbation theory levels. This combined package is used to study the kinetics of the abstraction reaction CH{sub 3} + H{sub 2} {yields} CH{sub 4} + H, which is considered as a prototype organic reaction. They report calculations of reaction rates based on electronic structure theory and generalized transition-state theory, including a multidimensional tunneling correction, without performing an analytic fit to the potential surface. The calculation of dynamical processes directly from ab initio electronic structure input without the intermediary of a potential surface fit is called direct dynamics, and this paper demonstrates the feasibility of this approach for bimolecular reactions.

Theory of diffusion-influenced fluorescence quenching

Abstract: Afin de decrire la cinetique de la reaction A*+B→A+B, differentes approches sont utilisees: approches de Smoluchowski et du champ moyen.

Infrared intensities of liquids. 5. Optical and dielectric constants, integrated intensities, and dipole moment derivatives of water and water-d2 at 22.degree.C

Abstract: The authors have recorded multiple attenuated total reflection spectra of liquid H{sub 2}O and D{sub 2}O, using the Spectra-Tech CIRCLE cell, and calculated from them the infrared optical and dielectric constants and molar conductivities from 9000 to 1250 cm{sup {minus}1} for H{sub 2}O and from 8500 to 700 cm{sup {minus}1} for D{sub 2}O. Our results agree well with the literature for H{sub 2}O, while our results for D{sub 2}O are the most extensive reported to date. We have calculated the dipole moment derivatives with respect to stretching and bending internal coordinates from the areas under the bands in our molar conductivity spectra. For lack of information, we have used the assumptions of the simple bond-moment model, a diagonal force field, and neglect of stretch-bend interaction. We found {partial derivative}{mu}/{partial derivative}r for the stretching vibrations to be 3.02 D/{angstrom} {plus minus} 1% and 3.04 D/{angstrom} {plus minus} 0.5% for H{sub 2}O and D{sub 2}O, respectively, and {partial derivative}{mu}/{partial derivative}{theta} for the bending vibration to be 0.73 D {plus minus} 3% for H{sub 2}O and 0.63 D {plus minus} 5% for D{sub 2}O. The two values for the stretching vibrations are indistinguishable, but this is not true for the bend. The disagreementmore » for the bending vibration is probably due, at least in part, to our simulation of the absorption by three distinct bands of mixed Gauss-Lorentzian character, in order to try to separate the bending mode from the background absorption. It is probable that no such separation exists precisely. The bond moments for H{sub 2}O and D{sub 2}O agree with those calculated by the same approximations form literature data for HDO to about the extent allowed by the approximations.« less

Photocatalytic oxidation of sulfur dioxide in aqueous suspensions of .alpha.-iron oxide (Fe2O3)

Abstract: The kinetics and mechanism of the photoassisted autoxidation of S(IV) in aqueous colloidal suspensions of α-Fe_2O_3 have been studied over the pH range of 2-10.5. Similar kinetic behavior toward S(IV) was observed for colloidal suspensions of TiO_2. Quantum yields, Ф, 4, ranged from 0.08 to 0.3 with a maximum yield found at pH 5.7. Upon band-gap illumination conduction-band electrons and valence-band holes are separated; the trapped electrons are transferred either to surface bound dioxygen or to Fe(III) sites on or near the surface while the trapped holes accept electrons from adsorbed S(IV) to produce S(V). The formation of S(V) radicals indicates that the reaction proceeds via successive one-electron transfers. The relatively high quantum yields are attributed in part to the desorption of SO_3^- from the α-Fe_2O_3 surface and subsequent initiation of a homogeneous free radical chain autoxidation of S(IV) to S(VI). Kinetic and thermodynamic models for the photoassisted surface chemistry of S(IV) on α-Fe_2O_3 are presented.

Aggregation behavior of hydrotropic compounds in aqueous solution

Abstract: Etude de l'agregation des particules de toluenesulfonate de sodium et de cumenesulfonate de sodium, de xylenesulfonate de sodium

Photocatalytic activities of microcrystalline titania incorporated in sheet silicates of clay

Abstract: Photochemical and photocatalytic properties of microcrystalline TiO{sub 2} incorporated in the interlayer space of montmorillonite were investigated in reference to those of TiO{sub 2} powder particles which were prepared by agglomeration of a titania sol that was used in the preparation of the titania-pillared clay. The pillared TiO{sub 2} microcrystallites having ca. 15-{angstrom} pillar height showed a ca. 0.58-eV blue shift in its absorption and fluorescence spectra compared with those of the TiO{sub 2} particles, exhibiting the quantum size effect. The excited electronic states of the pillared TiO{sub 2} were determined to be 0.36 V more negative than that of the TiO{sub 2} powder particles. Photocatalytic activities of the pillared TiO{sub 2} were greater than those of the TiO{sub 2} powder particles for decomposition of 2-propanol to give acetone and hydrogen and of n-carboxylic acids with up to eight carbons (from acetic acid to caprylic acid) to give the corresponding alkanes and carbon dioxide, though the pillared TiO{sub 2} exhibited lower activities for decomposition of capric acid, having 10 carbons in a molecule.

The temperature dependence of mass accommodation of sulfur dioxide and hydrogen peroxide on aqueous surfaces

Abstract: The authors report the laboratory determination of the temperature-dependent mass accommodation coefficients of SO{sub 2} and H{sub 2}O{sub 2} on aqueous surfaces over the range 260-292 K. Mass accommodation kinetics involve fundamental chemical interactions of liquid surfaces about which little is known. Uptake of SO{sub 2} and H{sub 2}O{sub 2} by cloud droplets is believed to be critical to S(IV) oxidation in the troposphere. Our experimental method combines a monodisperse train of droplets (200 {mu}m in diameter) and a low-pressure flow reactor. Uptake rates of trace gases are determined by measuring changes in trace gas number density as a function of exposed liquid surface area. Experiments with systematic variation of water vapor and rare gas partial pressures permit deconvolution of gas diffusion and temperature-dependent mass accommodation. In the case of SO{sub 2}, variation of droplet-gas interactions on millisecond time scales resolved pH-dependent saturation effects at the aqueous surface. Results for SO{sub 2} show {gamma}{sub 273} = 0.11 {plus minus} 0.02 with no significant temperature variation. H{sub 2}O{sub 2} shows a strong temperature dependence consistent with an attractive well depth of at least 26 kJ mol{sup {minus}1} with {gamma}{sub 273} = 0.18 {plus minus} 0.02. These results are discussed in termsmore » of the different aqueous solubilities of SO{sub 2} and H{sub 2}O{sub 2}.« less

Singlet−singlet and triplet−triplet intramolecular transfer processes in a covalently linked porphyrin−phthalocyanine heterodimer

Abstract: Etude des processus de transfert intramoleculaire dans un heterodimere porphyrine-phtalocyanine en fonction de la polarite du solvant. Les differences de comportement sont expliquees en termes de changements conformationnels dus a la nature de l'interaction avec le solvant

Attenuation Lengths of Photoelectrons in Hydrocarbon Films

Abstract: : Quantitative analysis of data obtained by X-ray photoelectron spectroscopy (XPS) requires a knowledge of the escape depths of electrons from the surface of a sample. In order to derive the composition of a homogeneous material from the intensities of the photoelectrons originating from different elements, one needs to know not only the relative atomic cross-sections, but also the variation of the attenuation length, gamma, with the energy of the photoelectrons. The ability to derive an elemental depth profile of a layered material from the variation in the photoelectron intensity with the angle of emission requires a knowledge of the absolute value of gamma. The recent growth of interest in thin organic films has generated an immediate need for accurate, reliable values of gamma in organic materials in general, and in thin, densely packed hydrocarbon films in particular. In this paper we have determined the attenuation length of electrons with energies in the range 940-1400 eV in self- assembled monolayers of n-alkanethiols adsorbed on gold. (JS)