Showing papers in "The Journal of Physical Chemistry in 1995"

Conductor-like Screening Model for Real Solvents: A New Approach to the Quantitative Calculation of Solvation Phenomena

Abstract: Starting from the question of why dielectric continuum models give a fairly good description of molecules in water and some other solvents, a totally new approach for the calculation of solvation phenomena is presented. It is based on the perfect, Le., conductor-like, screening of the solute molecule and a quantitative calculation of the deviations from ideality appearing in real solvents. Thus, a new point of view to solvation phenomena is presented, which provides an alternative access to many questions of scientific and technical importance. The whole theory is based on the results of molecular orbital continuum solvation models. A few representative solvents are considered, and the use of the theory is demonstrated by the calculation of vapor pressures, surface tensions, and octanol/water partition coefficients.

Characterization of c-h-o hydrogen-bonds on the basis of the charge-density

Abstract: It is shown that the total charge density is a valid source to confirm hydrogen bonding without invoking a reference charge density. A set of criteria are proposed based on the theory of “atoms in molecules” to establish hydrogen bonding, even for multiple interactions involving C-H-O hydrogen bonds. These criteria are applied to several van der Waals complexes. Finally a bifurcated intramolecular C-H-O hydrogen bond is predicted in the anti-AIDS drug AZT, which may highlight a crucial feature of the biological activity of a whole class of anti-AIDS drugs. Almost all the methods of physical chemistry, spectroscopy, and diffraction can be used to recognize and study hydrogen bonding.] Each technique focuses on specific properties in order to detect and characterize this phenomenon in its own way. This work is concerned with the manifestation of hydrogen bonding in the charge density obtained from ab initio calculations. Whereas crystallographers have concluded upon hydrogen bonding via purely geometrical criteria, recent deformation density2 studies allow one to observe hydrogen bonding beyond mere ge~metry.~ However, it is not necessary to subtract an arbitrary (promolecular) charge density from the total density to reveal hydrogen bonding, not even in the interpretation of X-ray experiment^.^ Boyd and Choi have shown in two important contribution^^^^ that the theory of “atoms in molecules’’ (AIM)7,8 can be used to characterize hydrogen bonding solely from the (total) charge density for a large set of acceptor molecules, involving HF and HC1 as donors. In a next stage Carroll and Bader performed a more extended analysis on a large set of BASE-HF comple~es.~ This theory has not only provided new insights in conventional intermolecular hydrogenI0.’ ] bonding but has also been successful in intramolecularI33l4 and x-type hydrogen bonds.I5 Drawing from earlier ob~ervations~~~~ ~.’~~~~ and the present work, we formulate eight concerted effects occurring in the charge density which are indicative of hydrogen bonding. All of these effects can be viewed as necessary criteria to conclude that hydrogen bonding is present. By observation one of these conditions has proven to be sufficient as well. This case study on C-H-O interactions shows that this less common type of hydrogen bonding obeys all of the proposed criteria. Moreover, the multiple interactions appearing in the present five examples do not impair the consistency of the global phenomenon of hydrogen bonding as it expresses itself in the charge density. In spite of an early affirmative infrared review,I6 the old controversy on whether C-H-O hydrogen bonds really exist continued for another decade,” but now the dust has settled’* (for an entertaining account of this controversy, see ref 19). The importance of these bonds has been recognized in crystal engineering’9,20 since C-H-O contacts have a determining influence on packing motifs.21

Subpicosecond Measurements of Polar Solvation Dynamics: Coumarin 153 Revisited

Abstract: Time-resolved emission measurements of the solute coumarin 153 (C153) are used to probe the time dependence of solvation in 24 common solvents at room temperature. Significant improvements in experimental time resolution ({approx}100 fs instrument response) as well as corresponding improvements in analysis methods provide confidence that all of the spectral evolution (including both the inertial and the diffusive parts of the response) are observed in these measurements. Extensive data concerning the steady-state solvatochromism of C153, coupled to an examination of the effects of vibrational relaxation, further demonstrate that the spectral dynamics being observed accurately monitor the dynamics of nonspecific solvation. Comparisons to theoretical predictions show that models based on the dielectric response of the pure solvent provide a semiquantitative understanding of the dynamics observed. 156 refs., 26 figs., 5 tabs.

Oxygen reduction on platinum low-index single-crystal surfaces in sulfuric acid solution. Rotating ring - Pt(hkl) disk studies

Abstract: The first results of a study of the oxygen reduction reaction on a rotating ring-disk electrode using single-crystal Pt disk electrodes are presented. The order of activity of Pt(hkl) in H{sub 2}SO{sub 4} increased in the sequence (111) (110), with 100% H{sub 2}O{sub 2} formation on Pt(111). 30 refs., 4 figs.

Self-assembly of tubular fullerenes

Abstract: Carbon nanotubes in the form of multiwalled fullerenes are shown here to self-assemble under homogeneous gas-phase conditions of carbon condensation in an inert atmosphere heated to 1200$DGR@C-conditions previously thought to be optimal only for the annealing and growth of C{sub 60} and other spheroidal shells. Tubular fullerenes are known to be less stable than their spheroidal counterparts and have thus far been reported only in circumstances where some extrinsic factor (e.g., high electric fields, catalytic metal particles, hydrogen atoms, or a surface at low temperature) was available to help keep the fullerene structure open at its growing end. The experimental evidence reported here now indicates that multiwalled tube growth is inherent in the condensation of pure carbon vapors. Adatoms bonded between edge atoms of adjacent layers at the growing end are proposed to be the crucial intrinsic factor facilitating tube growth by stabilizing the open conformation against closure. This new view of the growing nanotube tip structure is likely to impact on nanotube growth mechanisms under other conditions, particularly the arc. 32 refs., 4 figs.

The Kinetics of Electron Transfer Through Ferrocene-Terminated Alkanethiol Monolayers on Gold

Abstract: The kinetics of electron transfer between a substrate gold electrode and a self-assembled monolayer formed from CH{sub 3}(CH{sub 2}){sub n-1}SH and ({eta}{sup 5} C{sub 5}H{sub 5})Fe ({eta}{sup 5}-C{sub 5}H{sub 4})CO{sub 2}(CH{sub 2}){sub n}SH were studied as a function of n, the number of methylenes in the alkyl chain tethering the ferrocene moiety to the electrode, using the indirect laser-induced temperature jump method (ILIT). For 5 {<=} n {<=} 9 the standard electron-transfer rate constants vary according to {kappa}{sub {tau}a,n=0} exp[-{beta}{sub n}n] where {kappa}{sub {tau}a,n=0} is the (extrapolated) rate constant for the electron transfer at n = 0. At {Tau} = 25{degree}C, {kappa}{sub {tau}a,n} 0 {approx_equal} 6 x 10{sup 8} s{sup -1} and {beta}{sub n} = 1.21 x 0.05. The ILIT method allows rates to be measured that are too fast to be measured by conventional chronoamperometry at a macroelectrode, which is limited to rate constants of {<=} 10{sup 4} s{sup -1}. Using a Marcus formalism, the reorganization energy, {lambda}, for the electron-transfer process at a given n was determined from the slope of an Arrhenius plot over the temperature range 15-55{degree}C. Values of {lambda} determined from Arrhenius slopes for n = 8 and 9 using ILIT are in reasonable agreementmore » with the value of {lambda} previously deduced from the potential dependence of the rate constant for n = 16. 39 refs., 13 figs., 3 tabs.« less

H2 and CO Electrooxidation on Well-Characterized Pt, Ru, and Pt-Ru. 1. Rotating Disk Electrode Studies of the Pure Gases Including Temperature Effects

Abstract: We discuss the electrooxidation of both CO and H{sub 2} on well characterized Pt, Ru, and Pt-Ru rotating disk electrodes in 0.5 M H{sub 2}SO{sub 4} at room temperature and at 62{degree}C. We focus on the mechanism and the kinetics of CO oxidation on Ru and Pt-Ru electrodes under well-defined mass transfer conditions, highlighting the expected differences from measurements on the oxidation of preadsorbed monolayers of CO in CO-free electrolyte. Analysis of the catalytic activity displayed by Pt, Ru, and Pt-Ru surfaces for the oxidation of the pure gases, H{sub 2} and CO is used as the basis for understanding the oxidation of H{sub 2}/CO mixtures, which will be presented in Part 2. Finally, our previously proposed mechanism for the oxidation of both methanol and formic acid on Pt-Ru electrodes is reassured on the basis of measurements with H{sub 2} and CO. 47 refs., 9 figs.

An Improved Photocatalyst of TiO2/SiO2 Prepared by a Sol-Gel Synthesis

Abstract: The effect of incorporation of Si02 on the behavior of a TiO2-based photocatalyst prepared by a sol-gel technique from organometallic precursors is described. Application of photocatalysts with different TiOd Si02 ratios to the photodecomposition of rhodamine-6G (R-6G) demonstrates that a ratio of 30/70 produces a catalyst about 3 times more active than Degussa P-25 TiO2. Larger amounts of Si02 decrease the activity. The adsorption of R-6G on the different materials and the photodecomposition of preadsorbed R-6G is also described. The studies suggest that photogenerated intermediates are sufficiently mobile to react with R-6G adsorbed on Si02 sites but that adsorption of R-6G on Ti02 is not a prerequisite for reaction.

Structure and Properties of Neat Liquids Using Nonadditive Molecular Dynamics: Water, Methanol, and N-Methylacetamide

Abstract: We present the first nonadditive molecular dynamics simulation of organic liquids, studying the structure and energetics of methanol and N-methylacetamide. Beginning with an additive potential that reproduces the structure and energetics of these liquids quite well, we have shown that one can simply reduce the atomic charges by a scale factor in the range of 0.88-0.90 and add isotropic atomic polarizabilities to create nonadditive models that also quite accurately reproduce the structures and energies of these liquids. Thus, we have a clear pathway for the general inclusion of nonadditive effects for organic and biological molecules.

Effect of Preparation Conditions of Pt Alloys on Their Electronic, Structural, and Electrocatalytic Activities for Oxygen Reduction-XRD, XAS, and Electrochemical Studies

Abstract: The effect of different alloying conditions (alloying temperature, annealing period) on the electrocatalytic activities for the oxygen reduction reaction (ORR) by three carbon-supported Pt alloy electrocatalysts (Pt/Cr, Pt/Co, Pt/Ni) was investigated and correlated with electronic and structural parameters determined by in-situ XAS. The results indicate that all the Pt alloys show enhanced ORR activities relative to a Pt/C electrocatalyst. The observations indicate that the dependence of electrocatalytic activities and activation energy for Pt/Co and Pt/Ni alloys on the thermal history and the absence of such an effect in the Pt/Cr alloy could be related to the differences in the Pt 5d-orbital vacancies. 52 refs., 12 figs., 5 tabs.

Thermodynamics of Micelle Formation as a Function of Temperature - A High-Sensitivity Titration Calorimetry Study

Abstract: Summary and Conclusions Titration calorimetry can be used as a routine method for the determination of all thermodynamic quantities for the micellization of surfactant^.'.^^^^-'^ Particularly, the availability of power compensated microtitration calorimeters with high sensitivity has improved the speed and precision of the method.I6 The cmc and the micellization enthalpy can be determined from a single experiment. The temperature dependence of these quantities is easily accessible in the temperature range between 0 and 80 “C. From the temperature dependence of the demicellization enthalpy, udemic, the heat capacity change, ACp,demic, can be directly determined and thus information on the change in hydrophobic contacts upon demicellization can be estimated. The temperature at which the cmc minimum occurs can be determined with great precision from the temperature where AHdemic is zero. The shape of the titration curves also contains information on the aggregation number. A simulation of the titration curves for deoxycholate was attempted using a simple mass action model with a low aggregation number and the association constant and three enthalpic terms as adjustable parameters. The calculations support previous findings that cholates and deoxycholates form micelles with low aggregation numbers.

Properties of Hydrogen-Bonded Complexes Obtained from the B3LYP Functional with 6-31G(d,p) and 6-31+G(d,p) Basis Sets: Comparison with MP2/6-31+G(d,p) Results and Experimental Data

Abstract: Author Institution: Youngatown State University, Youngatown, Ohio 44555; University of Florida, Gainesville, Florida 32611

Surface Structures of Supported Molybdenum Oxide Catalysts: Characterization by Raman and Mo L3-Edge XANES

Abstract: Supported molybdenum oxide catalysts on TiO{sub 2}, Al{sub 2}O{sub 3}, ZrO{sub 2}, SiO{sub 2}, and Nb{sub 2}0s were prepared by the incipient-wetness impregnation method employing aqueous solutions of ammonium heptamolybdate ((NH{sub 4}){sub 6}Mo{sub 7}O{sub 24}{center_dot}H{sub 2}O). The molecular structures of the surface molybdenum oxide species were investigated. Under ambient conditions, the structures of the hydrated surface molybdenum oxide species are controlled by the net surface pH at the point of zero charge (PZC) and are the same as observed in aqueous solutions: MoO{sub 4}{sup 2-}, Mo{sub 7}O{sub 24}{sup 6-}, and Mo{sub 8}O{sub 26}{sup 4-}. Under dehydrated conditions, the structures of the surface molybdenum oxide species depend on both the specific oxide support and surface coverage. At low surface coverages of MoO{sub 3} on Al{sub 2}O{sub 3} and TiO{sub 2} the primary species is isolated and tetrahedral coordinated. At high surface coverages of MoO{sub 3}, for TiO{sub 2} the primary species is polymerized and octahedral coordinated, but for Al{sub 2}O{sub 3} there is a mixture of tetrahedral and octahedral coordinated species. The surface molybdenum oxide species on SiO{sub 2} is isolated and appears to possess a coordination that is in between tetrahedral and octahedral. Monolayer coverage was achieved at the samemore » surface density of molybdenum oxide on the different oxide supports with the exception of SiO{sub 2}. 90 refs., 18 figs., 5 tabs.« less

Colloidal metal films as a substrate for surface-enhanced spectroscopy

Abstract: Colloidal films of gold and silver were prepared on glass or quartz slides. The slides were derivatized with (3-mercaptopropyl)trimethoxysilane and subsequently reacted with aqueous metal colloids for variable time periods. The formation of the sulfur-metal bond provides a stable colloidal film on the surface. Because of the electrostatic interaction between individual particles, a semiregular structure is produced, as can be seen from electron micrographs. The unique property of the colloidal film is that they possess the optical properties of colloidal metals and the convenience of solid substrates. The effect of the dielectric constant of solvents on the optical frequencies, as well as the specific interaction of the solvent molecules with the metal on the plasmon resonances, was examined in detail. The colloidal films exhibit strong enhancement of Raman scattering and fluorescence emission from molecules adsorbed on the surface. Enhancement of fluorescence was observed for fluorescein-labeled molecules spaced 0-200 A away from the surface. These substrates can be used in a number of analytical applications, such as surface-enhanced spectroscopies as well as for fundamental studies of plasmon resonances in small metal particles. 40 refs., 6 figs., 1 tab.