bluuebunny/crossref_metadata_2025_split huggingface dataset:
Biliproteins: Some Relationships Among Aggregation States, Spectra, and Excitation‐Energy Transfer
Authors: Maccoll Robert , Berns Donald S. 
Published: November 2013

AbstractThe key aggregation steps for C‐phycocyanin consist of denatured subunits (α and β polypeptide chains), monomers, trimers, hexamers and stacks. For allophycocyanin the steps are denatured subunits, monomers and trimers. Ultimately these assembly states lead to the formation of phycobilisomes in blue‐green and red algae. The spectroscopic changes that accompany each stage in the assembly can be monitored as a function of the ratio of visible to near‐UV absorption. For C‐phycocyanin, monomers and hexamers are the important levels in the modulation of tetrapyrrole conformation by protein‐chromophore interaction. The effect of protein aggregation on the protein‐chromophore interactions is the key to solar‐energy harvesting and excitation‐energy migration in photosynthesis.
Electronic Structure and Reactivity of 2,2‐ethano‐1‐methylene 1,2,3,4‐tetrahydronaphthalene and 1,1‐ethano‐2‐methylene 1,2,3,4‐tetrahydronaphthalene
Authors: Goldblum Amiram , Sarel Shalom 
Published: November 2013

AbstractThe electronic structures of the title compounds were calculated byab initioSTO‐3G minimal basis set following a MINDO/3 total geometry optimization of both. Their different reactivities towards electrophiles is suggested to result from different frontier (HOMO) orbital densities on the relevant atoms. Both MINDO/3 and STO‐3G calculations give similar results for the HOMO structures, where frontier density is comparable on exocyclic methylene and ethano bridge atoms in the 1,1‐ethano 2‐methylene analog, and much higher on the methylene than on the cyclopropyl atoms in the second molecule. Relative rates of reaction are also discussed.
Properties of the Triplet State of N,N,Nî,Nî‐Tetraethyloxonine
Authors: Kamat Prashant V. , Lichtin Norman N. 
Published: November 2013

AbstractThe triplet state of N,N,Nî,Nî‐tetraethyloxonine, TEO+generated by means of sensitization with 9,10‐dibromoanthracene, was investigated in ethanol solution by laser flash photolysis‐kinetic spectrometry employing the first harmonic of a Q‐switched ruby laser at 347.2 nm. Quenching of both the protonated triplet, TEOH2+(T1), τ = 47 μs, and the unprotonated triplet, TEO+(T1), τ = 285 μs, by TEO+(S0) proceeds with substantial net electron transfer. No net electron transfer is detectable in the quenching of TEOH2+(T1), and TEO+(T1) by O2or 1,3,5,7‐cyclooctatetraene, processes which apparently involve transfer of energy. Specific rates of quenching are reported and the implications of the quenching processes for stability of TEO+in devices for conversion or storage of solar energy are discussed.
A Novel Effect of a Polyelectrolyte on a Photochemically Induced Electron Transfer Process Involving a Zwitterion
Authors: Sassoon Richard E. , Rabani Joseph 
Published: November 2013

AbstractA photoelectron transfer system containing the photosensitizer tris (2,2′‐bipyridyne) ruthenium II chloride, and the quencher N, Nî, di(4‐sulfonato toluyl) 4,4′‐bipyridine is described. Following a laser pulse the exited state of the photosensitizer is quenched by BSV giving a net yield of electron transfer of about 0.1. Addition of the polyelectrolyte polyvinyl sulfate (PVS) markedly enhances the rate of quenching. The decay of the electron transfer products Ru(bpy)3+3 and BSV+ is considerably retarded in the same system upon addition of PVS.
Photoinduced Electron Transfer Reactions as Photosynthetic Models: Reduction of Anthraquinones in SiO2 and ZrO2 Colloids
Authors: Willner Itamar , Degani Yinon 
Published: November 2013

AbstractThe photoinduced reduction of anthraquinone‐2‐sulfonate, AQS−, and anthraquinone‐2,6‐disulfonate AQS2‐2, in a negatively charged SiO2 colloid has been examined. Ru(bipy)2+3 was used as sensitizer and triethanolamine, TEOA, as an ultimate electron donor. The reduction of the quinones to the corresponding semiquinone radicals, AQS2‐ and AQS3‐2 proceeds only with propylviologen sulfonate, PVS0, (1), as a mediating electron acceptor. The primary electron acceptor, PVS0, functions as an efficient quencher of the excited species. The formation of AQS3‐2 is 4‐fold enhanced as compared to that of AQS2‐. This enhancement is attributed to the additional stabilization of AQS3‐2 from recombination with the oxidized species, Ru(bipy)3+3, due to electrostatic repulsions from the colloidal interface. Consequently, the subsequent oxidation of TEOA is facilitated. Similarly, a positively charged ZrO2 colloid has been used in controlling photosensitized electron transfer reactions. Anthraquinone‐2,6‐disulfonate, AQS2‐2, is photoreduced in the ZrO2 colloid to the respective AQHS2‐2 with the neutral sensitizer, Ru(bipy)2(CN)02, and cysteine as electron donor (Φ = 3 × 10−2). No photoreduction of the quinone is observed in a homogeneous aqueous phase. The success in driving the photoinduced reaction in the ZrO2 colloid is again attributed to the stabilization of the intermediate photoproducts from recombination by means of electrostatic interactions with the charged interface.
X‐ray Diffraction as a Tool for Studying Stoichiometry and Kinetics of Solid State Thermal Decomposition Reactions. Application to the Thermal Decomposition of Bischofite MgCl2 · 6H2O
Authors: Herbstein F. H. , Kapon M. , Weissman A. 
Published: November 2013

AbstractHigh temperature in situ X‐ray diffractometry of polycrystalline samples is shown to be a very convenient method both for identification of the various stages of a complicated thermal decomposition reaction and for determination of the kinetics of each stage of the reaction. This method when used together with thermogravimetric and chemical analyses has enabled us to determine the stoichiometry and kinetics of the various stages in the thermal decomposition of bischofite (MgCl2 · 6H2O) to periclase (MgO) in atmospheres of dry nitrogen, moist air and dry steam. The overall reaction rate determined in the laboratory for the conversion of MgCl2 · 6H2O to MgO is similar to that found in industry.
Oxidation of Polycrystalline Niobium Nitride
Authors: Gallagher P. K. , Sinclair W. R. 
Published: November 2013

AbstractIsothermal and dynamic TG were used to follow the oxidation of a sieved fraction, 177–250 μm, of niobium nitride powder in a flow of dry 0.1% O2 in Ar. The material underwent enormous expansion during the oxidation. The isothermal weight gain was measured in the temperature range from 817 to 897 K. It followed a pseudo‐first order reaction rate law best with an Arrhenius activation energy of 50 kcal mole−1. Dynamic studies at 1 K min−1 the pseudo‐first order rate law yielded similar values of 48 kcal mole−1 in 0.1% O2 in Ar and 55 kcal mole−1 in air. Because of its highly exothermic nature, the reaction will lead to a thermal runaway situation at heating rates approaching a temperature of 600 K at 20 K min−1, in air or at even lower rates of heating in pure O2, for material in this particle size range. Examination of the weight gain and X‐ray diffraction patterns of partially oxidized samples did not give any indication of intermediate species, such as oxynitrides or oxides of niobium, other than Nb2O5. The final product was a low temperature, pseudohexagonal polymorph of Nb2O5 but monoclinic γ'‐Nb2O5 appeared as an intermediate.
Thermal Evaluation of Bituminous Rocks and Pyroproducts
Authors: Aizenshtat Zeev 
Published: November 2013

AbstractThe stepwise pyrolytic method is compared with the “bulk flow” method on a CDS‐820 ZA automated pyro‐analyser. Bituminous rock samples as well as isolated kerogen were studied and evaluated as potential energy sources. The two approaches are correlated to the Fischer—Assay retorting method. Some samples of trapped pyro‐hydrocarbons are studied in detail. Applications of these methods are proposed and discussed.
Thermoanalytical Characterization of Flame Retardant Additive Action in Solids
Authors: Simon J. , Androsits B. 
Published: November 2013

AbstractThermal analysis, in particular the derivatograph and its improved variants, were used to characterize flame retardation processes in solids. The techniques presented in this paper are applicable both to solid—liquid and to gas—liquid transitions. Naturally‐occurring and synthetic polymers and flame retardants of practical importance were chosen as model substances. The results confirm data obtained using other methods. The advantages and the limits, in the application of thermal analysis to the field of flame retardation, are discussed.
Energy Dependence of Diffraction Intensities: 4He + LiF (001)
Authors: Drolshagen Gerhard , Kaufhold Achim , Toennies J. Peter 
Published: November 2013

AbstractThe dependence of specular and first order diffraction probabilities on the atom beam energy has been calculated for the system He + LiF (001) in the energy range 18 to 58 meV. A realistic Lennard‐Jones—Devonshire atom surface potential with several values of the β‐parameter, determining the strength of the corrugated part, was used. A comparison with the predictions of the simple Rayleigh—Eikonal approximation shows agreement with respect to the overall trends, however the quantal calculations show some deviations and the additional effect of selective adsorption resonances. The results are compared with recent measurements for the (100) and (110) directions and good agreement is found for β10 = 0.086.
Rotationally Mediated Selective Adsorption as a Probe of Isotropic and Anisotropic Molecule—Surface Interaction Potentials: HD(J)/Ag(111)
Authors: Yu Chien‐Fan , Hogg Charles S. , Cowin James P. , Whaley K. Birgitta , Light John C. , Sibener Steven J. 
Published: November 2013

AbstractRotationally mediated selective adsorption scattering resonances are used to make an experimental and theoretical study of the laterally averaged interaction potential between HD and a weakly corrugated system, Ag(111). The experimentally observed resonances determine the vibrational levels of the HD/Ag(111) physisorption potential as a function of bound rotational state. These vibrational levels show J‐dependent shifts due to the orientational anisotropy of the potential. Exact quantum scattering calculations using a full laterally averaged potential of the form have been carried out to obtain rotationally inelastic transition probabilities. Experimental and theoretical resonance energies are compared for two forms of vo(z), a Morse and a variable exponent potential, as a function of β, and are found to be very close to the first‐order perturbed energies of a free rotor in bound states of vo(z). Both potential forms give equally good fits to the data, yielding an optimum value of the asymmetry parameter, β ≈ — 0.05. The determination of β is relatively insensitive to small changes in the vo(z) well depth.
Scattering of N2 from Ag(001)
Authors: Muhlhausen Carl W. , Serri John A. , Tully John C. , Becker Gordon E. , Cardillo Mark J. 
Published: November 2013

AbstractExperimental and theoretical studies of the scattering of nitrogen molecules from the (001) face of silver are presented. Angular and time‐of‐flight distributions of the scattered molecules have been measured for incident N2 energies between 7 and 96 kJ/mol, and for surface temperatures between 205 and 600 K. Stochastic trajectory simulations have been carried out for the same conditions. Good agreement between experiment and theory has been obtained. The angular distributions are shown to be very sensitive to the extent of corrugation of the gas—surface interaction potential, but are relatively insensitive to rotational energy transfer. Scattered velocity distributions also show only a small dependence on rotational energy transfer.
An Electrodynamic Theory of the Non‐adiabatic Effects in Rate Processes for Molecules Adsorbed on Metals
Authors: Metiu Horia 
Published: November 2013

AbstractWe present a theory which computes the role of the electronic excitations in a metal substrate in dynamic processes involving adsorbed molecules. We use a model in which the molecular skeleton has effective charges whose magnitude and position varies when the nuclei of the molecule move. These moving charges couple to the electrons in the metal and excite them; as a result the molecule loses energy. The rate of such energy loss affects the vibrational line‐width and the fluorescence lifetime of the molecule, the van der Waals interaction between adsorbed molecules and the motion of the molecule at the surface. We show how these effects can be discussed in a unified way in terms of an electromagnetic theory. We emphasize that for obtaining reasonable numerical results we must replace phenomenological Maxwell equations with a microscopic theory that takes into account the non‐locality of the electron response to moving charges and the continuous nature of the interface.
Non‐Markoffian Theory of Activated Rate Processes II. Thermal Desorption
Authors: Nitzan Abraham , Carmeli Benny 
Published: November 2013

AbstractA model for thermal desorption, described by a one dimensional classical generalized Langevin equation (GLE) for the motion of an adatom, is solved by reducing the GLE to a Fokker Planck equation in action space. The escape rate is obtained as the inverse mean first passage time for the particle to achieve a threshold energy (or action). A calculation using parameters corresponding to the desorption of Ar from W is compared with quantum mechanical results for the same model.
Ion Induced Surface Alterations Due to Electronic Charge Exchange and Chemical Reactions
Authors: Rabalais J. Wayne 
Published: November 2013

AbstractA general discussion of the phenomena that can occur during ion—surface encounters is presented and specific examples of charge exchange processes on LiF surfaces and chemical reactions of N+2 on metals are shown. During rare gas ion bombardment of LiF, ejection of F+ ions and excited Li atoms and the energy distributions of secondary F+ ions can be correlated with Auger primary ion—surface charge exchange transitions determined by the potential energy of the primary ion vacancy and electron promotions resulting from close atomic encounters. Nitridation of Al, Cu, Mo, and Ni to produce very thin nitride surface films is induced by 0–50 eV mass selected N+2 beams. The efficiency of nitridation, monitored by the KL2L2 nitrogen Auger electron signal, exhibits a large kinetic energy Ek dependence below ca. 30 eV. The threshold impact energy for nitridation varies for the different metals; it is nominally zero for Al and Ni, ∼ 4 eV for Mo, probably > 0 for Cu, and does not occur at any energy below 200 eV for Ag. That the metal electronic structure is a critical parameter is evidenced by the facts that different ion doses are required to produce similar nitride signal levels on various metals and that plots of nitride intensity versus Ek exhibit structure or at least different shapes in the low Ek region. A simple quantum mechanical model is used to simulate the nitridation reaction by delineation into four elementary steps: (1) neutralization of incoming N+2, (2) decomposition of N2 into 2N, (3) deexcitation and thermalization of N, and (4) chemical reaction proper.
Dynamics of Surface Atoms and Molecules Under Irradiation
Authors: Shapira Yoram 
Published: November 2013

AbstractVarious classes of interactions between surface species and irradiation are discussed. Electron induced decomposition of halides, photon‐ and electron‐stimulated desorption and photodesorption are emphasized. Experimental work, involving a wide range of techniques, has been carried out on different materials whose surfaces can be subject to those interactions, enhancing our insight into the corresponding processes. The data are described in terms of existing and new mechanisms, while the similarities and differences between the various interactions are pointed out.
Electronic Structures and Reactivities of Pyramidal Alkenes and Carbonyls
Authors: Houk K. N. , Rondan Nelson G. , Brown Frank K. 
Published: November 2013

AbstractTheoretical predictions that alkenes and carbonyls pyramidalize in asymmetric environments were made before any conclusive experimental evidence for this phenomenon had been noted. In the last two years, several such pyramidalizations have been discovered. This review summarizes the predictions, experimental discoveries, and theoretical explanations of pyramidalizations of alkene and carbonyl carbons and describes the evidence indicating that both pyramidalization and stereoselectivity in reactions of such molecules arise from torsional (closed‐shell repulsion) effects.
Structure‐Resonance Valence Bond Theory for Ferrocenes and Ferrocenylalkyl Cations
Authors: Herndon William C. , Agranat Israel 
Published: November 2013

AbstractStructure‐resonance theory can be used to derive charge distributions and bond orders in ferrocenes, open ferrocenes and α‐ferrocenylalkyl cations. These species are of interest because they are unusually accessible and stable and understanding the electronic origin of this stabilization would aid in developing bonding models for organometallic systems. The methods for carrying out the calculations will be outlined. They involve shortcuts and algorithms for counting structures. The choice of structure basis, limited to conventional principal resonance structures, will be discussed. Charges and bond orders will be compared with NMR chemical shift and X‐ray data. The unusual stabilities of the ions and the role of the metal in stabilizing positive charge will be understandable within the conventional ideas of structural organic chemistry and resonance theory. These metal‐organic systems are highly delocalized and consequently resonance stabilized to a significant degree. They can be characterized as three‐dimensional aromatic compounds.
Structures, Energetics and Vibrational Frequencies of Cyclopropyne
Authors: Fitzgerald George , Schaefer Iii Henry F. , 
Published: November 2013

AbstractThe electronic structure of the lowest singlet and triplet states of cyclopropyne have been investigated using ab initio molecular quantum mechanics. Both double zeta (DZ) and double zeta plus carbon d function (DZ + d) basis sets have been used in conjunction with self‐consistent‐field (SCF), multiconfiguration (MC) SCF, and configuration interaction (CI) methods. Singlet cyclopropyne is predicted to be a transition state for the degenerate rearrangement of propadienylidene (H2C=C=C:). However, triplet cyclopropyne is a minimum on the C3H2 potential energy hypersurface and vibrational frequencies are predicted at two levels of theory. Structures and vibrational frequencies are compared with those of the relatively stable cyclopropene molecule.
The Origin of Alkyl Substituent Effects on the Reactivity and Thermodynamic Stability of Alkenes. A Theoretical Study on Alkene Radical Cations
Authors: Bach Robert D. , Wolber Gregory J. , Pross Addy 
Published: November 2013

AbstractFully optimized STO‐3G SCF MO calculations were conducted on a series of methyl substituted ethylene radical cations. The stabilizing effect of a methyl group is found to be 14 kcal/mol using ethylene as a reference. Analysis of the orbital make‐up of the radical cations and their calculated geometry suggests that hyperconjugative 1‐, 2‐, and 3‐electron interactions between the alkyl groups and the π‐system is largely responsible for the stabilization. Analysis of hyperconjugation in propene suggests that 2‐electron stabilization outweighs 4‐electron destabilization energetically but that effects of the 2‐electron stabilization are manifested in a low‐lying MO (Φ1, Fig. 6).
A MO Study of β‐Substituted Ethyl Radicals
Authors: Hoz Tova , Sprecher Milon , Basch Harold 
Published: November 2013

AbstractThe preferred ground state conformation of β‐substituted ethyl radicals, with the Cβ heteroatom substituent group being PH2, SH and CI1 have been determined by ab initio spin‐unrestricted self‐consistent field gradient optimization techniques. All the ground state geometries are found to have the Cβ‐heteroatom bond coplanar with the Cα‐centered radical orbital. No tendency towards heteroatom group bridging between Cα and Cβ is found in the systems studied here. The radical centers are calculated to be non‐planar, with the Cα hydrogen atoms tilting towards the Cβ substituent group. Calculated rotation energy profiles about the Cα‐Cβ and Cβ‐heteroatoms bonds, as well as the atomic orbital spin population distributions, are interpreted in terms of possible interactions between the Cα‐centered radical orbital and the Cβ‐heteroatom σ bond orbitals (hyperconjugation, type I) or the heteroatom 3d (type II) or lone pair (type III) atomic orbitals. Type I is the only significant stabilizing interaction found in these systems, whereas the type III (two orbital, three electron) interaction is actually found to be destabilizing.
The Insertion of Singlet Silylene into Ethylene
Authors: Anwari Freidun , Gordon Mark S. 
Published: November 2013

AbstractAb initio calculations at the minimal, split valence and extended basis set levels, both with and without electron correlation corrections, were used to study the insertion of singlet silylene into ethylene to form silacyclopropane. The least motion C2v path is symmetry forbidden and an artificial barrier of about 36 kcal/mol is found within C2v symmetry. Relaxation of symmetry to C, eliminates the symmetry forbiddenness of the reaction. The 3‐21G and 6‐31G* SCF calculations still predict a small barrier along this symmetry allowed path; however, the addition of second order correlation corrections results in the prediction of an insertion process with no barrier. The overall ΔE for the reaction is predicted by MP2/6‐31G*//3‐21G to be 47.9 kcal/mol.
NMR Spectroscopy of Bilirubin and its Derivatives
Authors: Kaplan Doron , Navon Gil 
Published: November 2013

AbstractLiterature NMR data of bilirubin and related compounds and their interpretation in terms of structure, solution conformation and dynamic properties have been reviewed. 1H‐spectrum assignments as well as solvent and temperature dependencies of the chemical shifts are described. Proton exchange rates, longitudinal relaxation times and nuclear Overhauser enhancements are surveyed. NMR is shown to provide knowledge on the basic structure, conformation, tautomerism, hydrogen bonding, acidity constants, segmental motion of side‐chains, interconversion between enantiomers and aggregation—disaggregation of these compounds in solution.
A Kinetic Study of the Interaction Between Bilirubin and Thermally Produced Singlet Oxygen
Authors: Galliani G. , Manitto P. , Monti D. 
Published: November 2013

AbstractThermal decomposition of 3‐(4‐methyl‐1‐naphthyl)‐propionic acid‐1,4‐endoperoxide at 34,7°C has been used to produce singlet oxygen in non‐photochemical conditions. Thus the reaction of bilirubin with singlet oxygen has been studied. A kinetic scheme is proposed taking into account physical and chemical quenching of singlet oxygen by bilirubin and quenching by the endoperoxide. From the lifetime, τ = 100 μs, of 102 in methylene chloride the following rate constants have been determined: for bilirubin: kphys quenching = 5.0 × 109 M−1s−1,kchem quenching = 5.0 × 108 M−1s−1 and for the endoperoxide: k = 8.5 × 107 M−1s−1. These same constants in the same order as above but in water (τ = 3μs) are: 4.9 × 109 M−1s−1, 1.0 × 109 M−1 s−1 and 9.0 × 108 M−1's−1 Moreover it has been shown that bilirubin is scarcely protected by albumin from singlet oxygenation. The protection in photochemical systems seems to be due to the lower ability of bilirubin to behave as a photosensitizer.