bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the Effect of Electron Relay Redox Potential on Electron Transfer Reactions in a Water Photoreduction Model System
Authors: Amouyal Edmond , Zidler Bertrand 
Published: November 2013

AbstractThe influence of electron relay redox potential (E1/2) on the electron‐transfer reactions occurring in the photo‐induced hydrogen generation from water has been investigated using the Ru(bipy)2+3/MV2+/EDTA/colloidal Pt model system. Quenching rate constants kq for electron transfer quenching of Ru(bipy)2+*3 by a series of 2,2′‐ and 4,4′‐bipyridinium ions and by a series of phenanthroline ions having different redox potential E1/2 have been determined by laser flash spectroscopy in deaerated aqueous solutions at pH 5. The rate constants kb of the back electron transfer following the quenching reaction have been obtained for five bipyridinium ions under the same experimental conditions. Our results show that: (1) for the same E1/2, the quencher molecular structure of the two homogeneous series does not affect kq significantly; (2) for the two series, kq increases with increasing E1/2 up to a value which is close to the diffusion‐controlled limit. The same correlation between kq and E1/2 is obtained for the two series. Such a correlation is satisfactorily interpreted in the frame of existing theories describing electron‐transfer reactions, particularly with the Rehm‐Weller treatment, provided that an intrinsic barrier ΔG*(0) = 5.2 kcal. M−1 is used; (3) kb is close to the diffusion‐controlled limit for E1/2 comprised between ‐ 0.33 and ‐ 0.65 V (vs NHE); (4) a correlation between H2 production rates and E1/2 is suggested, and a maximum H2 production is observed for E1/2 ≃ −0.45 V (vs. NHE) corresponding to compounds 1 and 14.
Solar Photoelectrochemical Cells Based on Photoinduced Hydrogen Atom Abstraction Reactions in Solution
Authors: Chandrasekaran K. , Whitten David G. 
Published: November 2013

AbstractSolar irradiation of a two‐component photoelectrochemical cell containing aqueous alcohol (2‐propanol, ethanol or methanol) solutions of a “mediator,” such as methyl viologen or diquat, and benzophenone in the photoanode compartment and aqueous acid in the cathode leads to the generation of moderately stable photocurrents in the 400–1500 μA range. Similar results are obtained, but with generally lower currents, when benzophenone‐free solutions are irradiated. The reactions in the photoanode compartment all originate with an excited state hydrogen atom abstraction from the alcohol to generate two reducing free radicals; the ketyl radicals formed are rapidly oxidized by the mediator to give an electrochemically active but relatively stable semi‐reduced mediator species which is oxidized at the electrode surface. The basic photochemical and subsequent dark reactions are reasonably efficient and well‐understood; the overall peak efficiencies of the present cell system are relatively low (1–2%) reflecting the low fraction of the solar spectrum absorbed by the photochemically reactive reagents. The net chemical change occurring is endothermic and the products are in principle readily recyclable. The solutions are photochromic due to transient generation of the semi‐reduced mediator. A primary advantage of this system is that the use of a mediator to avert radical combination or other energy‐wasting processes provides the system with a directionality analogous to that obtained via band bending in semiconductors.
Modified P‐Type Si Photocathodes for Photochemical Hydrogen Generation: Surface Texturing, Molecular Derivatizing Reagents, and Noble Metal Catalysts for Multi‐Electron Processes
Authors: Bruce James A. , Wrighton Mark S. 
Published: November 2013

AbstractModification of single‐crystal (100), p‐type Si photocathode material to improve photoelectrochemical H2 generation from H2O is reported. The (100) surface has been first modified by chemical etching to reveal pyramids (1–5 μM in height) having (111) sides to give a “textured” surface having lower specular reflection losses than polished (100) or (111) single‐crystal Si. Second, the textured surface has been derivatized with {N,Nî‐bis‐[3‐trimethoxysilyl)propyl]‐4,4′‐bipyridinium}dibromide, I, to yield large coverages (0.5–5 × 10−8 mol/cm2) of redox centers [PQ2+/+]surf in a polysiloxane network. The surface redox polymer is a fast electron acceptor and can be reduced photoelectrochemically, [PQ2+]surf → [PQ+]surf, by ≥ Eg = 1.1 eV light at ∼0.0 to ‐ 0.1 V vs. SCE in aqueous electrolyte solution, or ∼ 500 mV more positive than on a reversible electrode such as Pt. Third, the surface polymer is then further modified by depositing Pd(O) or Pt(O) onto the outermost surface to catalyze the reaction 2[PQ+]surf + 2H+ → 2[PQ2+]surf + H2. Energy conversion efficiency from the modified photocathodes for 632.8 nm (∼ 10 mW/cm2) light‐driven H2 evolution is ∼ 5% at pH = 4, representing significant improvement compared to polished, (100) or (111) p‐type Si‐based photoelectrochemical devices where the surface of the p‐type Si is not modified.
A Study of Lineshapes in Selective Adsorption in Atom—Surface Scattering
Authors: Celli V. , Evans D. 
Published: November 2013

AbstractCalculations are presented involving the scattering of low energy4 He atoms from LiF(001), and compared with experiment. Resonant scattering theory is briefly reviewed, and a prescription for determining the shapes and signatures of resonant features is presented. Inelastic effects are included by means of a Debye—Waller scaling.
Quantum‐Mechanical Treatments of Rotationally Inelastic Molecule‐Surface Scattering
Authors: Kouri D. J. , Gerber R. B. 
Published: November 2013

AbstractQuantum‐mechanical approximation methods and calculations for rotational transitions in molecule‐surface collisions are reviewed. The methods are analyzed with regard to predictions of several observable effects: (1) Large ΔJ (rotational quantum number) transitions, and rotational rainbow scattering. (2) The relation between rotational and diffractive transitions. (3) Scaling properties of the rotationally‐inelastic scattering amplitudes. (4) Trapping and resonances induced by rotational‐translational energy transfer. The methods examined with regard to some of these effects include: coupled‐channel calculations; the Sudden approximation with regard to both the rotational and the diffractive transitions, and the hard corrugated wall model. It is concluded that whilst available methods provide a qualitative description of the effects mentioned, quantitative treatment of real systems remains an open problem. The main difficulties in formulating a satisfactory quantitative model are examined. Finally, the article presents new results on molecular reorientation processes (ΔMJ transitions) in molecule‐surface collisions. It is shown, using the Sudden approximation, that molecular reorientation probabilities should reflect sensitively on surface structural corrugation.
The Auger Parameter of Adsorbed Xenon as a Probe of Surface Polarizability
Authors: Stair P. C. 
Published: November 2013

AbstractThe change in Auger parameter, α (α = Auger kinetic energy minus photoelectron kinetic energy from core levels) between gas phase and adsorbed phase xenon provides an unambiguous measure of final state electronic relaxation in the surface toward the xenon ion. For many surfaces this relaxation is purely electronic polarization in the surface so that the Auger parameter provides a measure of the surface polarizability. Moreover, the Auger parameter for adsorbed xenon can be related to the relaxation energy in core level photoemission with high accuracy. Measurement of the xenon Auger parameter and its sensitivity to surface dielectric properties is illustrated by experiments on a series of oxygen treated Mo(100) surfaces.
A Theoretical Discrimination of Active Regions of Molecules in Chemical Reactions
Authors: Fujimoto Hiroshi , Koga Nobuaki , Endo Masayuki , Hataue Itaru , Fukui Kenichi 
Published: November 2013

AbstractPairwise orbital transformations in reagents and in reactants indicate which parts of molecules participate actively in chemical reactions. Similar orbitals are obtained for different molecules, demonstrating common features of the relevant reaction, and different orbitals are obtained for the same molecule, reflecting multiple types of reactivities of organic compounds. The possibility of analyzing the roles of orbital interactions along a reaction pathway is demonstrated.
Application of Molecular Orbital‐Valence Bond Theory to Geometric Isomerism
Authors: Epiotis Nicolaos D. , Larson J. R. 
Published: November 2013

AbstractThe Molecular Orbital‐Valence Bond (MOVB) method is briefly outlined and it is subsequently applied to the problem of geometric isomerism of molecules having eight and ten sigma valence electrons such as bent HC≡CH and HN=≡NH. It is shown that orbital symmetry dictates a trans bending tendency for HCCH and in the hypothetical absence of nonbonded repulsion, a cis geometry for XNNY and XOOY.
Strain Energies of Small Ring Alkenes
Authors: Wiberg Kenneth B. , Bonneville George , Dempsey Richard 
Published: November 2013

AbstractThe energies of hydrogenation of a series of alkenes have been calculated using both the 4‐31G and 6‐31G* basis sets. The inclusion of polarization functions leads to only a small change in calculated energy for simple alkenes but a large change is found with the more highly strained compounds. The calculated enthalpies of hydrogenation have been compared with experimental values and show that the error in the calculation is essentially constant (6 ± 2 kcal/mol). This makes it possible to estimate the enthalpies of formation and strain energies of the small ring alkenes for which experimental data are not available. The calculated structures of the compounds also are discussed.
Ab Initio CI Calculations of the Energy Difference between Trimethylenemethane and Butadiene
Authors: Feller David , Davidson Ernest R. , Borden Weston Thatcher 
Published: November 2013

AbstractAb initio configuration interaction (CI) calculations have been carried out in order to determine the energy difference between 3A î2 trimethylenemethane (TMM) and 1Ag butadiene. The pi CI value of 38.3 kcal/mol is very similar to the CI values obtained when excitations of the sigma electrons are permitted. This finding suggests that correlation effects involving sigma orbitals make at most a small contribution to the energy differences between non‐Kekulé hydrocarbon diradicals and their closed‐shell isomers. Possible complications in the experimental measurement of this energy difference are discussed. It is pointed out that methylenecyclopropane might be formed in attempts to deprotonate the 2‐methallyl cation, without the involvement of TMM. From the experimental heats of formation of methylenecyclopropane and butadiene, an energy difference of 14–16 kcal/mol between 3A î2 TMM and methylenecyclopropane is estimated. This is in excellent agreement with a previous energy difference obtained by direct calculation.
Electrophilic Attack on Tetrahedral Metal Complexes
Authors: Silvestre Jerome , Albright Thomas A. 
Published: November 2013

AbstractA number of ML4Lî complexes exist where Lî contains an electropositive atom connected to the transition metal. The structures of these compounds are very distorted; they do not correspond to a trigonal bipyramid, square pyramid or somewhere between these two extremes. An electronic rationale for these alternative structures is presented with the aid of molecular orbital calculations of the extended Hückel type. It is shown that in these compounds the M‐Lî bond is quite ionic in the sense of Lî+ML−4. The deformation is then due, in part, to the tendency of ML−4 to adopt a tetrahedral arrangement. Furthermore, it is shown that an electrophile (Lî+) may attack a three‐fold face, two‐fold edge, or somewhere intermediate at the tetrahedral ML−4 group yielding an alternative structure for these five‐coordinate complexes.
Circular Dichroism of Phytochromobilin and Related Compounds
Authors: Thümmler F. , Rüdiger W. 
Published: November 2013

AbstractAbsorption and circular dichroism (CD) spectra are recorded for partially denatured phytochrome, Pfr chromopeptide as well as (ZZZ)‐ and (ZZE)‐configurated phycocyanin chromopeptides. Solvent induced circular dichroism (SICD) spectra are included for (ZZZ)‐ and (ZZE)‐octaethylbilindione. Irradiation of Pr in 4 M urea yields a bleached chromophore with weak absorption and Δϵ values. On acidification in the dark it shows the known absorption and CD spectra of the Pfr chromophore. Subsequent irradiation of the acidified solution leads to absorption and CD spectra of the Pr chromophore. The red shift of absorption and CD bands obtained during this Pfr ŕ Pr chromophore transformation is observed in both absorption and CD or SICD spectra of Pfr peptides, (E)‐configurated phycocyanin peptides and (E)‐configurated octaethylbilindione during irradiation in acid solution. The shift may be coupled with inversion of the sign of CD or SICD bands, e.g., in fraction 5, phycocyanin peptide and octaethylbilindione. Several fractions of phycocyanin peptides with almost identical absorption spectra show significant differences in their CD spectra. Differences are also found in the pH dependency of CD or SICD spectra of the various fractions of phycocyanin peptides and of octaethylbilindione.
On the Orientation Mechanism of Small Molecules in Liquid‐Crystalline Environments
Authors: Snijders J. G. , De Langea C. A. , Burnell E. E. 
Published: November 2013

AbstractA general molecular theory for the description of the orientation of small molecules in anisotropic environments is presented. Assuming a second‐order tensorial interaction between some solute property βij and the anisotropy in a liquid‐crystal “field” Fij rigid and non‐rigid contributions to the dipolar and quadrupolar couplings observable by NMR are evaluated. The behaviour of the solutes hydrogen, methane and their deuterated analogues in nematic phases can be described by special cases of this general theory and excellent agreement between observed and calculated anisotropic couplings is obtained. The present results suggest that the coupling between the solute molecular quadrupole moment and the anisotropy in the liquid‐crystal electric field gradient plays a significant role in the orienting process in the case of hydrogen.
The Orientational Ordering of Some Non‐cylindrically Symmetric Solutes in Nematic Solvents
Authors: Catalano D. , Forte C. , Veracini C. A. , Zannoni C. 
Published: November 2013

AbstractThe problem of obtaining information on the molecular properties of non‐cylindrically symmetric solute molcules in liquid crystals is examined. Nitrobenzene, tetrafluoro‐p‐benzoquinone and pyridine have been investigated in various nematic solvents at a series of temperatures using proton, fluorine and deuterium NMR where relevant. The ordering matrices for these essentially rigid molecules have been determined as a function of temperature. The predictions of a mean field theory for solutes deviating from cylindrical symmetry are considered. The relative importance of polarizability, steric forces and specific interactions in determining orientational ordering is discussed.
Deuterium NMR Studies of Discotic Liquid Crystals
Authors: Goldfarb D. , Luz Z. , Zimmermann H. 
Published: November 2013

AbstractDeuterium Nuclear Magnetic Resonance studies in discotic mesophases are reviewed. The studies include NMR measurements on neat compounds as well as deuterated probe molecules dissolved in the mesophase. The discotic compounds discussed include derivatives of benzene, triphenylene and truxene which exhibit a variety of mesophases. Particular emphasis is placed on comparing axially symmetric and biaxial phases, and on analyzing the various orientational order parameters required to characterize their NMR spectra. The conformation of side chains as derived from the deuterium resonance of the methylene and methyl deuterons is also reviewed. Finally, the properties of discotic mesophases as ordering solvents for solute probe molecules are discussed.
The Study of Bilayers in the Micelles of Lyotropic Nematic Mesophases
Authors: Reeves Leonard W. 
Published: November 2013

AbstractNematic micellar liquid crystals based on rod and disk shaped bilayers can be prepared with both signs of the magnetic susceptibility anisotropy giving 4 subclassifications, II DM, II CM, I DM and ICM. These bilayer systems, because they spontaneously align in magnetic fields, are especially useful as media for the study of the bilayer chemistry of the constituent micelles. The deuterium magnetic resonance of specific and perdeuterated hydrocarbon chains yield degree of order profiles, which contain information about whole chain movements and segmental motion within the chains. The degree of order ratios within a given chain for different segments only contain information about the segmental trans / gauche rotations in the carbon–carbon bonds. The order profile, in ratio form, can be used as a discriminating tool for detecting physical and chemical changes in the bilayers. Solubilisation of solutes in the bilayers can be studied either from the effect on motions in the host surfactants or directly in the solutes themselves. The disk‐micelle bilayers do not differ detectably from those of multi‐lamellar phases in thermodynamic equilibrium. Changes in trans / gauche equilibria in the chains can explain the accommodation of single chain amphiphiles to a host bilayer thickness in terms of changes in the probability of isolated gauche rotations near the non‐polar tail. Acid/base equilibria of guest amphiphiles may be investigated. In the particular case of an optically active guest, the non‐equivalence of the αCD2 group may be titrated because the non‐equivalence in quadrupole splitting is a function of the pH of the water used to prepare the mesophase. The use of α,ω amphiphiles as guests in the mesophase bilayer is instructive in revealing the presence of U‐shaped and spanning conformations of the intervening hyrocarbon chain. For short chain α,ω amphiphiles, the constraint angle effect on segmental motions is a function of the chemical nature of the bilayer matrix and the pH of the water.
Concerning the Radical Character of Superoxide. The H—O Bond Energy of HO−2
Authors: Liebman Joel F. , Valentine Joan S. 
Published: November 2013

AbstractThe strength of the H‐O bond in H−O2 should be a good indicator of the chemical reactivity of superoxide, O−2, as a hydrogen atom abstractor. This quantity can be calculated if the electron affinity of HO2 is known. A recent experimental determination of the electron affinity of HO2 [V. M. Bierbaum, R. J. Schmitt and C. H. DePuy, J. Am. Chem. Soc., 103, 6262–6263 (1981)] gave that value as 27.4 ± 0.2 kcal mol−1. The H‐O bond energy of HO−−2 can then be calculated from that value to be approximately 66 kcal mol−1. Various theoretical methods may also be used to approximate the value of the electron affinity of HO2. These methods generally give values in accord with the experimental result. Based on the low H—O bond energy of 66 kcal mol−1 for HO−2, superoxide is expected to be relatively unreactive, comparable to iodine atoms but less reactive than bromine atoms or HO2 and much less reactive than chlorine atoms or hydroxyl radicals. Based on our analysis, we predict that superoxide can only react as a hydrogen atom abstractor with substrates that contain relatively weak bonds to hydrogen such as hydroxylamine or hydroquinones.
H2O2‐Diffusion Through Liposomes
Authors: Frimer Aryeh A. , Forman Arthur , Borg Donald C. 
Published: November 2013

AbstractThe toxicity of H2O2 suggests that it readily diffuses through cell membranes. The present paper demonstrates via a chromogenic peroxidase reaction that H2O2 can cross the lipid bilayer membrane without affecting the cell wall's integrity. In this demonstration, the chromogen ABTS was encapsulated with horseradish peroxidase into ovolecithin multilamellar liposomes. Upon the addition of dilute H2O2, the color was generated exclusively within the intact liposomes. Related results were observed with the AAP/DHBS chromogenic couple; however, this system is complicated by the small holes created by the phenol DHBS in the liposomal wall.
Reactions of Alkoxy Radicals in Aqueous Solutions
Authors: Bors Wolf , Tait David , Michel Christa , Saran Manfred , Erben‐Russ Michael 
Published: November 2013

AbstractThe kinetic and mechanistic properties of alkoxy radicals in organic solvents are briefly reviewed. Owing to the scarcity of such data in aqueous solutions and since reactions at the membrane/water interface may be also biologically important, we have studied the reactivity of these radicals in water and the results of our investigations are reported. Alkoxy radicals were generated by photolytic or radiolytic cleavage of peroxide precursors (tert‐butyl hydroperoxide and di‐tert‐butyl peroxide as well as hydroperoxides of polyunsaturated fatty acids). A quantitative correlation between the structure of various substances, in particular phenolic antioxidants, and their activity in inhibiting the alkoxy radical‐induced bleaching of the water‐soluble carotenoid crocin will be discussed. Rate constants for intermolecular reactions of t‐BuO radicals were determined by pulse radiolysis. The diffusion‐controlled reaction with the catechol antioxidant nordihydroguaiaretic acid demonstrates an effective competition with the very rapid intra molecular β‐fragmentation in water. The results support the view that a considerable amount of alkoxy radicals interact with substrates before they can rearrange intramolecularly.
Trimetalation of a Methallyl‐Thioether and Amine
Authors: Klein Joseph , Weiler Larry 
Published: November 2013

AbstractMethallylphenyl sulfide 2 undergoes trimetalation on treatment with n‐butyllithium and TMEDA. The trilithiated product gives with chlorotrimethylsilane the trisilyl derivative 3. A similar reaction of the tertiary amine 4 leads to a mixture of the trisilyl 7 and two disilyl derivatives 5 and 6.
Highly Chemoselective Allylic Reductions with Silicon Hydrides and Palladium Catalyst
Authors: Keinan Ehud , Greenspoon Noam 
Published: November 2013

AbstractNovel reducing systems, comprising various silicon hydrides and a soluble palladium(0) catalyst, allow very mild and chemoselective reductive cleavage of allylic heterosubstituents. Other easily‐reducible functionalities such as ketones, aldehydes and Michael‐acceptors are not affected under these conditions. The relative reactivities of twelve commercially available silanes towards allylic reduction were determined using 1H NMR techniques. Similarly, the role of various reaction parameters such as temperature, solvent and concentration of reactants, and the effect of added water and zinc chloride on the reduction rate have been studied.
Molybdenum Catalyzed Reactions Selectivity in Oxidations with Hydrogen Peroxide and Ammonium Molybdate
Authors: Trost Barry M. , Masuyama Yoshiro 
Published: November 2013

AbstractHydrogen peroxide in conjunction with ammonium molybdate effects the epoxidation of olefins, the oxidation of secondary alcohols to ketones, and the oxidation of aldehydes to acids. By controlling pH olefin epoxidation can be suppressed in favor of alcohol oxidation. Secondary alcohols are cleanly oxidized in the presence of primary alcohols. Sterically more hindered alcohols are oxidized more rapidly than less hindered alcohols. The latter rate preference permitted selective conversion of a 17‐hydroxyl group of several steroids into a.ketone in the presence of a 3‐hydroxyl group. The chemoselective oxidation of aldehydes to carboxylic acids is another use of this inexpensive oxidation system. A rate acceleration by addition of the lanthanide salt cerium chloride is noted and attributed to enhanced rates of hydration of the aldehyde. Potential mechanisms are discussed.