bluuebunny/crossref_metadata_2025_split huggingface dataset:
Geometries and Energies of Dilithioethylene Isomers and of Vinyl Lithium. An Ab Initio Study
Authors: Apeloig Yitzhak , Clark Timothy , Kos Alexander J. , Jemmis Eluvathingal D. , Schleyer Paul V. R. 
Published: November 2013

AbstractThe structures (4‐31G optimization) and energies (6‐31G*) of vinyl lithium (5) and of the 1,1 (3 and 4), cis‐1,2 (6), trans‐1,2 (7) and doubly bridged (8) dilithioethylenes have been calculated using standard ab initio methods. Vinyl lithium (5), indicated to have a classical geometry, is about 11 kcal/mole more stable than ethyl lithium in an hydride exhange reaction (Eq. (2)). Singlet trans‐1,2‐dilithioethylene, the most stable C2H2Li2 isomer (see also addendum), has a highly distorted, partially bridged structure (7a) with < CCLi = 87°. Symmetrically bridged 8 and the cis‐isomer 6 are considerably less stable, 54 and 19 kcal/mole, respectively. The partial Li‐H bonding in 7a contributes significantly to this behavior. 1,1‐Dilithioethylene is found to prefer a perpendicular (3) over the usual planar (4) geometry both in singlet and in triplet states. Triplet symmetrically bridged 8 and triplet cis dilithioethylene (6) are indicated to be local minima, but are less stable than singlet trans 7a. Cis‐dilithioethylene (6) is indicated to be thermodynamically unstable towards dissociation into Li2 and acetylene. This factor, and the low lying triplet state, may contribute to the experimental difficulties in preparing 6. The nature of the carbon–lithium bonding in these molecules is discussed. We find that an electrostatic model cannot account for the structural details or for the relative energies of the C2H2Li2 isomers. The importance of the multicenter covalent nature of bonding to lithium is emphasized.
Thermal and Photochemical Transformation Modes of Cyclopentadienone‐Dimers: Structural and Stereochemical Aspects
Authors: Fuchs Benzion 
Published: November 2013

AbstractSchemes are presented describing all possible rearrangement modes of cyclopentadienone‐dimers, the basic assumption being intramolecularity and C6‐C7bond breaking in the (4π+ 2π) dimers. For theendo‐dimer (2) all theoretically possible transformations (Fig. 1) are backed up by experiment. In theexo‐dimer (9) series (Fig. 3), some still await detection. Symmetry and stereochemical features are emphasized, as they enter structural and mechanistic considerations.
Bridged Annulenes
Authors: Vogel Emanuel 
Published: November 2013

AbstractThe synthesis and aromaticity of 1,6‐methano[10]annulene and its analogues containing a heteroatom bridge suggested the possibility that the homologous series of bridged [4n + 2]annulenes 1, 2, 3, etc., formally derived from the acene series, could be developed. In the pursuit of this concept a set of systematically bent bridged [14]annulenes which permitted a scrutinization of the steric prerequisite of the Hückel rule — the requirement of a planar ring skeleton — were built up. Moreover, a new synthetic strategy based on the synthon cycloheptatriene‐1,6‐dicarboxaldehyde has provided access to the two next higher homologues (syn‐series) of 1,6‐methano[10]annulene, i.e., syn‐1,6:8,13‐bismethano[14]annulene and syn, syn‐1,6:8,17:10,15‐trismethano[18]annulene, respectively.
Unusual Rearrangements in the C12H12‐Manifold 1,5,9‐Cyclododecatriyne, 1,2:3,4:5,6‐Tricyclobutabenzene, Hexaradialene
Authors: Barkovich A. J. , Strauss E. S. , Vollhardt K. P. C. 
Published: November 2013

Abstract1,5,9‐Cyclododecatriyne has been synthesized by a bromination‐dehydrobromination sequence starting from all‐cis‐1,5,9‐cyclododecatriene. Its physical properties point to substantial interaction of the acetylenic groups through space and through bond although this effect does not result in significant homoaromatic stabilization. Attempts to convert the triyne to isolable 1,2:3,4:S,6‐tricyclobutabenzene failed, but condensed phase thermolysis in the presence of dimethylmaleate led to the isolation of a formal triadduct. Flash pyrolysis resulted in the formation of hexaradialene. The latter is an extremely air sensitive, polymerizable molecule, devoid of detectable aromatic stabilization, in accord with theoretical treatments. Thermochemical estimates are presented which indicate the exothermicity of the 1,5,9‐cyclododecatriyne → hexaradialene interconversion. The most likely intermediate in this transformation is 1,2:3,4:5,6‐tricyclobutabenzene, also in accord with group equivalent calculations, although other pathways cannot be.ruled out rigorously.
The Perimeter Model and Magnetic Circular Dichroism of Aceheptylene
Authors: Wallace Susan L. , Michl Josef 
Published: November 2013

AbstractThe use of the perimeter model for the interpretation of magnetic circular dichroism of aromatic molecules is briefly reviewed. The model is applied to a rather difficult case, aceheptylene, whose electronic states are only distantly related to those of its perimeter. Correct signs are still predicted for the effects of magnetic L1–L2 and B1‐B2 mixing.
Topological Resonance Energies of Conjugated Structures
Authors: Ilić P. , Sinković B. , Trinajstić N. 
Published: November 2013

AbstractThe graph‐theoretical backbone of the topological resonance energy (TRE) concept is outlined and the connection with the Dewar resonance energy (DRE) approach is described. It appears that the TRE is not a new aromaticity index, but an optimal DRE‐like quantity of great practical value for understanding and predicting the aromatic stability of conjugated structures. In the TRE concept the most important result is the introduction of the acyclic polynomial which closely approximates the acyclic‐polyene reference structure. The rules for the construction of the acyclic (reference) polynomial are presented. Application of TRE to monocyclic systems, to polycyclic molecules and their radicals and ions, to homoaromatic systems, and to excited states is discussed. In most cases the agreement between the theory and experimental findings is good. However, in some examples the predictions appear to be inconsistent with reported data. The reasons for the occasional anomalies in the TRE are investigated. In addition, the problem of normalization of the TRE is considered.
Synthesis and Ionization of 1,1‐Dihalogenonaphtho[b]cyclopropenes
Authors: Müller Paul , Thi Huong Can Nguyen 
Published: November 2013

AbstractThe synthesis of 1,1‐dichloro and 1,1‐difluoronaphtho[b]cyclopropenes 1 and 2 starting from 4,5‐dimethylenecyclohexene and tetrachloro‐ or 1,2‐dichloro‐3,3‐ difluorocyclopropene, respectively, is described. Both 1 and 2 ionize in cold fluorosulfonic acid to yield stable aromatic cations 3 and 4. Upon quenching with water, 1‐chloronaphtho[b]cyclopropenium ion 3 affords β‐naphthoic acid.
Synthesis of Polycyclic Ring Systems via Intramolecular [2 + 2]‐Cycloaddition Reactions of Cyclopropene Derivatives
Authors: Padwa Albert , Blacklock Thomas J. , Rieker William F. 
Published: November 2013

AbstractStrained bicyclic and polycyclic compounds continue to play an important role in the understanding of many aspects of organic chemistry. For this reason, synthetic efforts in this area have been extensive. Our research group has found that the thermolysis of a number of cyclopropene derivatives containing π‐substituents undergo a novel [2 + 2]‐cycloaddition. This reaction is unique in that the other reported examples of thermal olefin cycloadditions either occur in compounds in which the double bond is subjected to severe torsional strain or else involve reactants that bear substituents capable of stabilizing diradical or dipolar intermediates. In cyclopropene, the torsional angle is close to zero and p‐p overlap should not be significantly different from that of a normal olefin. Thus the propensity of the cyclopropene ring to undergo internal cycloaddition is primarily due to relief of angle bending rather than torsional strain. Details of the results obtained and the probable mechanism of the intramolecular [2 + 2]‐cycloaddition are presented in some detail.
Ring‐Expansion of Cyclopropyl Ketones to Furans
Authors: Donnelly John A. , Macken Patrick J. , O'Brien Simon 
Published: November 2013

AbstractRigid cyclopropyl ketones were rearranged by phosphorus pentachloride to furans. This ring‐expansion was also possible using bromine and an acid. A β, γ‐unsaturated ketone, a supposed intermediate in the reaction, gave furan products when treated with bromine alone.
Chlorophyll‐Amino Acid Interactions in Synthetic Models
Authors: Boxer Steven G. , Bucks Rodney R. 
Published: November 2013

AbstractA number of amino acids and molecules of related functionality have been covalently connected to pyrochlorophyllide a (PChla) and bacteriopyrochlorophyllide a (BPChla) to determine their impact on the properties of the chromophore. PChla and BPChla esterified with side chains containing the thio‐ether functionality (methyl‐6‐hydroxyhexylsulfide, a Met analogue) were studied in detail by NMR. Intramolecular coordination at the central metal is observed for the Zn‐PChla and Zn‐BPChla thio‐ether derivatives, but not for the Mg‐PChla derivative. The NMR data for the Zn‐PChla derivative is interpreted as providing evidence for two, distinguishable five‐coordinate complexes, with the ligand on either side of the macrocycle. In contrast, the Zn‐BPChla derivative appears to be coordinated exclusively on one side. It is proposed that stereoselective coordination may provide a simple mechanism for altering the spectroscopic properties of BChla when it binds to a protein. PChla was covalently attached to the methyl esters of L‐Trp, L‐Tyr, L‐Phe and L‐Val. Very large, similar chemical shift changes in the NMR spectra in the absence of a coordinating ligand indicate that all of these compounds form a novel dimer, in which the carbonyl group of the carbomethoxy group on the amino acid coordinates to the central metal, bridging between macrocycles. An average structure is proposed for this dimer in which the macrocycle planes are parallel, overlap in ring IV and have their x‐axes antiparallel.
Interaction of the Quinone Analogue, DBMIB, with the Photosynthetic Rieske Iron‐Sulfur Center
Authors: Malkin Richard 
Published: November 2013

AbstractAddition of the plastoquinone analogue, DBMIB, to chloroplasts results in the appearance of an EPR signal at g = 1.94 which is observed at cryogenic temperatures. Studies with subchloroplast fragments which either contain or lack the Rieske iron‐sulfur center indicate the presence of this electron carrier is required for the DBMIB‐induced EPR signal. A correlation between the disappearance of the Rieske center signal at g = 1.89 and the appearance of the DBMIB‐induced signal at g = 1.94 suggests DBMIB alters the Rieske center to produce the new signal. Similar effects were observed in photosynthetic membrane fragments from a blue‐green alga and a photosynthetic bacterium. It has been possible to use the DBMIB‐induced signal to test for interaction of other inhibitors in the region of the Rieske center. These results are considered in terms of models in which quinone and the Rieske center function as a donor‐acceptor pair during physiological electron transport.
Near Infra‐Red Absorption Spectra of the Chlorophyll a Cations and Triplet State in vitro and in vivo
Authors: Mathis Paul , Setif Pierre 
Published: November 2013

AbstractAbsorption spectra in the near infra‐red have been obtained for transient species obtained from chlorophyll a, by flash absorption spectroscopy. The triplet state of Chl a presents an absorption band around 760 nm (ϵ = 7,500 M−1 cm−1) and a very broad band (ϵ = 1,700 M−1 cm−1) around 1100 nm. At low temperature, evidence was obtained for the formation of the triplet state of aggregated Chl a. Its t1/2 of decay is around 0.5 ms instead of 1 ms for the monomeric species. The radical‐cation Chl a+ was formed by reaction of 3Chl a with a quinone; it has an absorption band at 840 nm (in cyclohexanol) with a shoulder around 740 nm and no absorption between 1000 and 1650 nm. The cation has also been formed in vivo, as the oxidized state of the photosynthetic primary electron donors, P680 and P700. The spectrum of P+680 resembles that of Chl a+; its absorption maximum is at 820 nm. The spectrum of P+680, maximum at 810 nm, is significantly broader. P+700 has no absorption between 1000 and 1650 nm. These spectra are in favor of recent suggestions on a monomeric nature of P680 and of a dimeric nature for P700.
A Reexamination of the Walsh‐ and Förster—Coulson—Moffitt‐ (F.C.M.) Orbital Concept, and its Relevance for the Interpretation of PE Spectra
Authors: Honegger Evi , Heilbronner Edgar , Schmelzer Andreas , Jian‐Qi Wang 
Published: November 2013

AbstractContrary to what is often believed, the description of the bonding orbitals of cyclopropane in terms of the so‐called Walsh‐orbitals or F.C.M.‐ (bent‐bond) ‐orbitals is not equivalent. These two sets of orbitals transform into each other only if their antibonding counterparts are taken into account. The bent‐bond orbitals can be obtained directly from the set of occupied canonical SCF orbitals of cyclopropane via a unitary transformation into localized orbitals. In contrast, the bonding Walsh‐orbitals are not contained in the occupied manifold of the canonical orbitals, because of their non‐equivalence to the F.C.M.‐ orbitals. This is the reason why the anti‐bonding Walsh‐orbitals have to be taken into consideration if one wants to draw valid conclusions, e.g., about the photoelectron spectra of compounds containing the cyclopropane moiety.
The Effect of Fluoride on the Aqueous Iron–Thionine Photogalvanic System
Authors: Archer M. D. , Wilford J. H. , Ferreira M. I. C. 
Published: November 2013

AbstractThe influence of fluoride ions on the photocurrent and photostationary state composition of the acid, aqueous iron‐thionine system at bare and thionine‐coated platinum electrodes was investigated. Observed photocurrents are in some circumstances increased by the addition of fluoride, in accordance with the theory of characteristic lengths of Albery et al. However, the Gibbs energy of the forward reaction, and hence the open‐circuit voltage available from a differential photogalvanic cell, are sharply decreased.
Improved Hydrogen Production with Tetramethyl‐and Hexamethylviologen as Electron‐Transfer Agents in the System H2O‐Ru(bpy)2+3‐edta‐Pt
Authors: Launikonis Anton , Loder John W. , Mau Albert W.‐H. , Sasse Wolfgang H. F. , Wells Darrell 
Published: November 2013

AbstractThe 1,1′,2,2′‐tetramethyl‐4,4′‐bipyridinium and the 1,1′,2,2′,6,6′‐hexamethyl‐4,4′‐bipyridinium dications have been compared with methylviologen as electron‐transfer agents in the photoreduction of water by the title system at pH 5.0 and 7.0. Rates of formation and yields of hydrogen, turnover numbers of viologens, and rates of decay of viologen radicals have been measured at different concentrations of platinum and in the presence of glutathione. The results suggest that improved performance by tetramethyl‐ and hexamethylviologen in the production of hydrogen is related to higher rates in the hydrogen‐producing step and to increased stability towards platinum‐induced side reactions, such as catalytic hydrogenation. These side reactions can be largely suppressed by addition of glutathione to give improved yields and rates of formation of hydrogen and increased turnover numbers for the viologens.
Water Photolysis by UV Irradiation of Rhodium Loaded Strontium Titanate Catalysts. Relation between Catalytic Activity and Nature of the Deposit from Combined Photolysis and ESCA Studies
Authors: Lehn J. M. , Sauvage J. P. , Zlessel R. , Hilaire L. 
Published: November 2013

AbstractPhotochemical water splitting (H2O → H2 + 1/2O2) has been shown to occur on UV irradiation of aqueous suspensions of strontium titanate, modified by surface photodeposition of rhodium species.7 Further improvement of the catalytic materials has now been achieved, using thermal deposition techniques. The most active material, prepared by heating rhodium chloride impregnated SrTiO3 powder at 500°C in air, allowed one to produce appreciable quantities of (H2 + 1/2O2) gas and to reach high turnover numbers on the catalytic material. The photolysis efficiency presents a strong pH dependence with an optimum around pH ∼ 14 and increases markedly when the wavelength of the light decreases from 400 to 300 nm. The amount of rhodium deposited also has a marked effect with an optimum at about 0.2–0.3% Rh by weight. ESCA studies have provided information about the nature of the catalytic deposit and about its relation to the activity of the materials. Catalytic efficiency and mechanistic aspects are discussed. The oxidation state of the rhodium species has a drastic influence: whereas SrTiO3 loaded with reduced rhodium shows no catalytic activity, the efficiency increases on progressive oxidation of the deposited metal to surface rhodium(III) species, as detected by ESCA. The photoactive step may involve both band‐gap excitation of the semi‐conductor support and direct UV excitation of the deposited Rh(III) species.
Photo(Electro)Chemical Oxidation of Water by the Persulfate Ion Over Aqueous Suspensions of Strontium Titanate SrTiO3 Containing Lanthanum Chromite LaCrO3
Authors: Thewissen D. H. M. W. , Timmer K. , Eeuwhorst‐Reinten M. , Tinnemans A. H. A. , Mackor A. 
Published: November 2013

AbstractThe photodecomposition of the S2O2‐8 ion to the SO2‐4 ion with concomitant oxidation of water to molecular oxygen over n‐type SrTiO3 powders mixed with LaCrO3 (mole fraction x = 0–1), using UV light (λ > 300 nm), is considerably faster than the photochemical (UV) decomposition of S2O2‐8 in water or the thermal reaction with or without the powder under the same conditions. The rate of oxygen formation is dependent on the temperature (Eact = 61 kJ/mol), the persulfate concentration in first order and x. At 64°C and constant [S2O2‐8] a maximum rate is found at x = 0.25. The rate determining step, i.e. the transfer of an energy‐rich electron from the conduction band to the S2O2‐8 ion, is expected to proceed faster in the case of p‐type semiconductors. In view of the reported p‐type conductivity of LaCrO3, which has previously been doped with the charge‐compensating Sr11 ion, our results with the SrTiO3‐x LaCrO3 powders are in agreement with these expectations. The sensitivity of these materials towards visible irradiation is much lower than their UV response (about 1–2%) which has also been found by determining i/V and i/λ characteristics of SrTiO3‐x LaCrO3 single crystals under illumination in a photoelectrochemical cell.
Photoassisted Carbon Dioxide Reduction to Organic Compounds Using Rare Earth Doped Barium Titanate and Lithium Niobate as Photoactive Agents
Authors: Ulman M. , Aurian‐Blajeni B. , Halmann M. 
Published: November 2013

AbstractThe effect of rare earth dopants on the large bandgap semiconductors, barium titanate and lithium niobate, in the photoassisted reduction of carbon dioxide was studied in aqueous suspensions, by illumination with high pressure mercury lamps. The yields of the reduction products formic acid and formaldehyde was enhanced by the presence of low concentrations of the oxides of europium, neodymium, samarium and cerium in the above semiconductors. The optical properties of the materials were studied by diffuse reflectance spectroscopy.
The Photoelectrochemical Behavior of an n‐TiO2 Electrode Coated with a Thin Metal Film, as Revealed by Measurements of the Potential of the Metal Film
Authors: Nakato Yoshihiro , Tsubomura Hiroshi 
Published: November 2013

AbstractPhotocurrent‐potential curves of an n‐TiO2 electrode coated with a thin gold or palladium film are studied in aqueous solutions, together with the change of the potential of the metal film with respect to the solution. It is found that, when the metal‐coated n‐TiO2 electrode is illuminated under anodic potentials (i.e., in the presence of band bending), the potential of the metal film shifts toward the positive until it reaches the oxygen evolution potential at the metal, with little change of the surface band energies of the n‐TiO2 electrode. This implies that the height of the Schottky barrier at the metal/ n‐TiO2 contact is considerably increased by illumination, and the photovoltage obtained is much larger than expected from the barrier height in the dark.
Thermal Stability of Apatites Containing Zinc Ions
Authors: Ben‐Dor Lina , Diab Hussam 
Published: November 2013

AbstractFluor‐carbonate‐apatites were prepared by a heterogeneous reaction from calcium carbonate, alkaline phosphate and fluoride and were doped with Zn2+ ions. The thermal stability of the doped apatites was studied by measuring the fluoride content before and after heat‐treatment using a selective electrode. It was shown that the doped apatites are thermally more stable than the pure material. The degree of stability is dependent on the amount of doping.
Iron‐Bearing Kaolinite in Venezuelan Laterites. II. DTA and Thermal Weight Losses of KCl and CsCl Mixtures of Laterites
Authors: Mendelovici E. , Villalba R. , Yariv S. 
Published: November 2013

AbstractKCl and CsCl mixtures of Venezuelan laterite samples were examined by DTA. These laterites contain iron‐bearing kaolinite. The first endothermic peak at ∼300°C is characteristic of goethite and gibbsite. A second endothermic peak at 470–512°C characterizes kaolinite. In KCl mixtures, the maximum of this peak decreases with increasing iron content of the kaolinite crystal. In CsCl mixtures, such a relationship is obtained only after the removal of the gibbsite from the lateritic sample. A third endothermic peak at 650–760°C is characteristic of the melting of the salt. The intensity of this peak may be used as a reference. KCl may thus serve as an internal standard for the determination of kaolinite in a series of laterite samples. CsCl, which forms a complex with kaolinite, cannot be used as an internal standard. The fourth endothermic peak results largely from the evaporation of the salt. The location of this peak in the CsCl mixtures depends on the iron content of the kaolinite. The laterite—alkali halide mixtures were subjected to weight losses at temperatures between 105 and 1000°C. In addition to the dehydroxylation processes, hydrolysis of the salts on the surfaces of kaolinite, and the evaporation of HCl take place in the CsCl mixtures and to a lesser extent in the KCl mixtures. In the CsCl mixtures, when the dehydroxylation stage of the kaolinite was studied in stages, it was found that kaolinites with a high iron content tend to lose weight mainly at a high temperature (550°C) whereas those with a low iron content tend to lose weight even at 450°C.
Interaction of NO2 with a Pt(111) Surface
Authors: Segner J. , Vielhaber W. , Ertl G. 
Published: November 2013

AbstractThe interaction of NO2 with a Pt(111) surface was studied by molecular beam and thermal desorption techniques. At 300 K NO2 adsorbs dissociatively into NOad + Oad with an initial sticking coefficient so = 0.97. so decreases slightly with increasing surface temperature; the variation of the sticking coefficient with coverage exhibits a marked “precursor” state behavior. The rate of formation of gaseous NO is limited by desorption of this molecule, i.e. there is no evidence for a direct process NO2 ŕ NOg + Oad. At ≈ 400 K NO desorption is complete and only Oad is building up on the surface. The maximum coverage Θo,max ≈ 0.77 is much higher than that so far reported for exposure to O2 (0.25), which is ascribed to kinetic effects, i.e. a much higher sticking coefficient in the case of NO2. Two additional states with desorption energies of about 32 and 38 kcal/mol characterize the high oxygen coverage situation. Desorption of O2 starts above 450 K.