Showing papers in "The Journal of Physical Chemistry in 1991"

Charge equilibration for molecular dynamics simulations

Abstract: We report here an approach for predicting charge distributions in molecules for use in molecular dynamics simulations. The input data are experimental atomic ionization potentials, electron affinities, and atomic radii. An atomic chemical potential is constructed by using these quantities plus shielded electrostatic interactions between all charges. Requiring equal chemical potentials leads to equilibrium charges that depend upon geometry. This charge equilibration (QEq) approach leads to charges in excellent agreement with experimental dipole moments and with the atomic charges obtained from the electrostatic potentials of accurate ab initio calculations. QEq can be used to predict charges for any polymer, ceramic, semiconductor, or biological system, allowing extension of molecular dynamics studies to broad classes of new systems. The charges depend upon environment and change during molecular dynamics calculations. We indicate how this approach can also be used to predict infrared intensities, dielectric constants, and other charge-related properties.

Nanometer-sized semiconductor clusters: materials synthesis, quantum size effects, and photophysical properties

Abstract: Recent advances in the synthesis of semiconductor clusters open a doorway for the systematic study of size-dependent cluster properties in the condensed phase. This article focuses on the size effect on the optical and photophysical properties. The authors first introduce fundamental concepts and proceed to a discussion of recent progress toward the understanding of the quantum size effect and dielectric confinement effect. They then discuss the current status of materials synthesis and the prospect for making monodisperse clusters of well-defined surfaces.

Photophysical properties of C60

Abstract: A number of important photophysical properties of C{sub 60} have been determined, including its lowest triplet-state energy (near 33 kcal/mol), lifetime, and triplet-triplet absorption spectrum. The triplet state is formed in near quantitative yield and produces a very high yield of singlet oxygen by energy transfer. C{sub 60} does not react with singlet molecular oxygen and quenches it only slowly by an unknown mechanism. These results are discussed in terms of the unusual geometry of this molecule.

Fullerenes with metals inside

Abstract: Fullerenes with a single lanthanum atom trapped on the inside of the carbon cage were produced by laser vaporization of a lanthanum oxide/graphite composite rod in a flow of argon gas at 1200 °C. When sublimed with C_(60) and C_(70), they formed an air-stable film containing principally LaC_(60), LaC_(70), LaC_(74), and LaC_(82). When dissolved in toluene and exposed to air, LaC_(82) was found to be uniquely stable. Evidence was also obtained for coalescence reactions between these fullerenes at high temperatures to form larger cages with as many as three lanthanum atoms inside. Indications have also been obtained for the successful production of KC_(60), C_(59)B, and KC_(59)B where the boron has substituted for a carbon in the soccerball cage. The use of the @ symbol is advocated for specifying such complex fullerenes as (K@C_(59)B).

Determination of water diffusion coefficients in perfluorosulfonate ionomeric membranes

Abstract: Pulsed field gradient spin-echo 1 H NMR measurements of 1 H intradiffusion coefficients at 30 o C in hydrated Nafion membranes are reported. The dependence of the 1 H self-diffusion coefficient on membrane water content was a central part of this investigation. 1 H diffusion coefficients ranged from 0.6×10 −6 to 5.8×10 −6 cm 2 /s for the range of membrane water content 2-14 water molecules per sulfonate

Direct visualization of monolayers at the air-water interface by Brewster angle microscopy

Abstract: No light is reflected from the air-water interface under Brewster angle incidence if p-polarized light is used. With constant angle of incidence, the formation of a monolayer on the water surface modifies the Brewster angle condition, and light reflection is observed. Reflectivity-area isotherms of arachidic acid monolayers on pure water and on aqueous solution of different pH have been measured. Areas of different brightness due to different molecular density and/or refractive index in the monolayer have been recorded with a video system as micrographic pictures. This Brewster angle microscopy enables us also to detect monolayer inhomogeneities and the reversible formation of solid-phase domains upon compression of monolayers of DMPE

The role of oxygen in photooxidation of organic molecules on semiconductor particles

Abstract: The kinetics of O 2 reduction accompanying oxidation of organic compounds on photocatalytic semiconductor particles is analyzed. The rate-controlling processes are defined and the electron reactivities are derived for two cases: A, electrons moving freely in the semiconductor particle and reacting with O 2 anywhere on its surface and B, electrons trapped at or near the surface of the semiconductor particle and transferred from the traps only to nearby O 2 molecules

Removal of nitrogen monoxide through a novel catalytic process. 1. Decomposition on excessively copper-ion-exchanged ZSM-5 zeolites

Abstract: Repeated ion exchange of the ZSM-5 zeolite using aqueous copper(II) acetate solution was found to bring about excess loading of copper ions above an exchange level of 100%. The high activity of the resulting catalyst for NO decomposition was consistent for at least 30 h even at short contact time and low NO pressure. The number of copper ions that can adsorb NO molecules has been determined by a temperature-programmed desorption technique combined with IR measurement; 94% of Cu{sup 2+} ions in ZSM-5 were active for the adsorption. The activity of excessively copper ion exchanged ZSM-5 zeolite was slightly reduced by the oxygen in the feed gas while that of zeolite, of which the loading amount of copper was less than 100%, was greatly diminished under the same condition. SO{sub 2} completely poisons the activity at 673-923 K, but the activity can be regenerated at the higher temperature treatment.

Determination of vanadium-oxygen bond distances and bond orders by Raman spectroscopy

Abstract: trans to the lone pair, thus weakening the trans bond. Similar effects have been seen previously in m e t h ~ l a m i n e ~ ~ and trimethylamine?’a If the methyl C-H peak is split by the methylene group, then the splitting is too small to detect in this experiment. Thus, in terms of overtone spectroscopy, the nitrogen lone pair significantly influences the methylene hydrogens but not the methyl hydrogens. In addition, the anisotropic environment that the methylene group provides for the methyl group apparently does not affect the methyl group absorption. The two-point harmonic frequencies and anharmonicities calculated for these absorptions compare well among analogous bonds in these molecules and with similar modes in other molecules. There are two N-H stretching peaks in the overtone spectrum, one a t higher frequency corresponding to the N-H trans to a methylene hydrogen and one at lower frequency corresponding to the N-H trans to the N-C bond. This agrees with earlier results on diethylamine and di~nethylamine.~~ The large number of C-H bonds in triethylamine makes detailed analysis of its spectra difficult, especially without accompanying data on deuterated analogues. There are a t least two C-H peaks in the overtone spectra, the one at higher frequency being a methyl absorption, while the one a t lower frequency is a methylene absorption. Data for both of these absorptions were fit to a BirgeSponer plot and the resulting parameters compare well to the data of Fang et al. for dieth~lamine.’~

Adsorbed .omega.-hydroxy thiol monolayers on gold electrodes: evidence for electron tunneling to redox species in solution

Abstract: Reference LPI-ARTICLE-1991-027View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Absolute pKa calculations with continuum dielectric methods

Abstract: Solvation free energies and pK a values of models for ionizable side chains of amino acids are calculated by using continuum dielectric methods; integral equation techniques are also investigated. The dependence of the solvation free energies on the parameters is explored by comparing different sets that are being used in protein and liquid simulations. To obtain the experimental solvation free energies and pK a values of the model compounds with the continuum dielectric method, an adjusted parameter set is introduced; only very small changes from the standard parameter values are required

Carbon-13 NMR study of the C60 cluster in the solid state: molecular motion and carbon chemical shift anisotropy

Abstract: {sup 13}C NMR spectra of solid buckminsterfullerene, the soccerball-like cluster of 60 carbon atoms, have been obtained at temperatures down to 77 K. The ambient spectrum shows rapid isotropic rotational motion. The motion is sufficiently slow at 77 K that a measurement of the chemical shift tensor of the carbon nucleus can be made. The tensor components (220, 186, 40 ppm) have values that are typical for an aromatic carbon. Spectra at intermediate temperatures suggest the possibility of either growth of a low-temperature phase in which C60 rotation is inhibited or a distribution of rotational correlation times.

Micelle and Gel Formation in a Poly (ethylene oxide)/ Poly (propylene oxide)/ Poly (ethylene oxide) Triblock Copolymer in Water Solution. Dynamic and Static Light Scattering and Oscillatory Shear Measurements

Abstract: The properties of aqueous solutions of low molecular weight triblock copolymers of PEO/PPO/PEO have been investigated, mainly by dynamic and static light scattering. At low concentrations (C<10%) and temperature (<25°C) the relaxation time distributions from dynamic light scattering show the coexistence of the monomer (R H =18 A), micelles (R H =80 A), and micellar aggregates in relative proportions which depend critically on temperature and concentration

Predicting molecular structures of surface metal oxide species on oxide supports under ambient conditions

Abstract: The molecular structures of the two-dimensional vanadium oxide overlayers on different oxide supports (MgO, Al 2 O 3 , ZrO 2 , TiO 2 and SiO 2 ) were determined with Raman spectroscopy under ambient conditions. The surface vanadium oxide molecular structures were found to depend on the net pH at which the surface possesses zero surface charge (point of zero charge, pzc). The net surface pH at pzc is determined by the specific oxide support and the surface coverage of the acidic vanadium oxide overlayer

Ultrasonic irradiation of p-nitrophenol in aqueous solution

Abstract: The kinetics and mechanism of the sonochemical reactions of p-nitrophenol have been investigated in oxygenated aqueous solutions. In the presence of ultrasound (20 kHz, 84 W) p-nitrophenol was degraded primarily by denitration to yield NO_2^-, NO_3^-, benzoquinone, hydroquinone, 4-nitrocatechol, formate, and oxalate. These reaction products and the kinetic observations are consistent with a model involving high-temperature reactions of p-nitrophenol in the interfacial region of cavitation bubbles. The main reaction pathway appears to be carbon-nitrogen bond cleavage. Reaction with hydroxyl radical provides a secondary reaction channel. The average effective temperature of the interfacial region surrounding the cavitation bubbles was estimated to be T ≃ 800 K.

Attenuation of photoelectrons in monolayers of n-alkanethiols adsorbed on copper, silver, and gold

Abstract: and 22 A at 554 eV. Estimates of ), obtained by using SAMs as scattering layers are more consistent than those obtained by using thin organic films fabricated by other methods. The relationship between tr and the kinetic energy (KE) of photoelectrons, over the energy range studied by using SAMs, can be fit equally well by the expressions \ = 9.0 * 0.022K8 or tr o KEp where p = 0.67 + 0.11. Bethe's equation for electron scattering in matter also relates A to KE.

Contact and solvent-separated geminate radical ion pairs in electron-transfer photochemistry

Abstract: The two primary intermediates that play a major role in determining the efficiencies of bimolecular photoinduced electron-transfer reactions are the contact (A{sup {sm bullet}{minus}}D{sup {sm bullet}+}) and the solvent-separated (A{sup {sm bullet}{minus}}(S)D{sup {sm bullet}+}) radical ion pairs, CRIP and SSRIP, respectively. These two species are distinguished by differences in electronic coupling, which is much smaller for the SSRIP compared to the CRIP, and solvation, which is much larger for the SSRIP compared to the CRIP. The present work addresses the quantitative aspects of these and other factors that influence the rates of energy-wasting return electron transfer within the ion-pair intermediates. The electron acceptor tetracyanoanthracene (TCA) forms ground-state charge-transfer complexes with alkyl-substituted benzene donors. By a change of the excitation wavelength and/or donor concentration, either the free TCA or the CT complex can be excited. Quenching of free {sup 1}TCA{sup *} by the alkylbenzene donors that have low oxidation potentials, such as pentamethylbenzene and hexamethylbenzene, in acetonitrile solution leads to the direct formation of geminate SSRIP. Excitation of the corresponding charge-transfer complexes leads to the formation of geminate CRIP. Rates of return electron transfer within the two types of ion pair are determined from quantum yields for formation of free radicalmore » ions pairs depend upon the reaction exothermicity in a manner consistent with the Marcus inverted region.« less