bluuebunny/crossref_metadata_2025 huggingface dataset:
Alkyl and Acyl Dicarbonylarenemanganese Complexes: The Reduction of Metal‐Bound Carbonyl Groups
Authors: Munro George A. M. , Pauson Peter L.
Published: November 2013

Hydride reduction of tricarbonylarenemanganese cations gives, in addition to the known cyclohexadienyl compounds, appreciable yields of dicarbonylarenemethylmanganese by direct reduction of a metal carbonyl group. Alkyllithiums also attack the cations at carbonyl groups as well as at the ring carbons and, in the case of polyalkylbenzene complexes, fair yields of acyl derivatives result. These may be decarbonylated photochemically and may also be converted to cationic carbene complexes by alkylation with triethyloxonium tetrafluoroborate.
Some Aspects of the Aqueous Chemistry of Molybdenum
Authors: Dori Zvi
Published: November 2013

Some aspects of the aqueous chemistry of molybdenum in the oxidation states +3, +4, +5 and +6 are reviewed. Mention is made only of well characterized species and complexes.
Cembranolids from Marine Origin (Red Sea), Survey, and Isolation of a New Sinulariolide Derivative
Authors: Kashman Y. , Bodner M. , Loya Y. , Benayahu Y.
Published: November 2013

The hitherto isolated and identified cembrane derivatives from alcyonarians of the Gulf of Elat (Red Sea) are briefly reviewed, including the thus far unreported isolation of nephtenol and 16‐deoxysarcophine from several soft‐corals. The structure of a new sinulariolide derivative, 11‐episinulariolide acetate, isolated together with 11‐dehydrosinulariolide, from two (out of seven tested) sinularia species is illustrated.
A New Flavolignan of Biogenetic Interest from Aegilops ovata L.—Part I
Authors: Cooper Raymond , Gottlieb Hugo E. , Lavie David
Published: November 2013

Two compounds, tricin and a related new flavolignan have been isolated from Aegilops ovata L. dispersal units. Evidence leading to assignment of structure of this new compound is presented together with 1H and 13C‐NMR chemical shifts of related model compounds. These phenolics found together in the same plant species with a previously discovered lignan possessing activity as a germination inhibitor, raise the possibility of a new biogenetic link.
Biogenetic Aspects of Sesquiterpene Lactone Chemistry
Authors: Herz Werner
Published: November 2013

Current notions concerning the biogenesis of this large group of naturally occurring substances are discussed with emphasis on stereochemical aspects.
Diterpenoid Irritants and Cocarcinogens in Euphorbiaceae and Thymelaeaceae: Structural Relationships in View of their Biogenesis
Authors: Adolf W. , Hecker E.
Published: November 2013

Diterpene esters exhibiting toxic, irritant, cocarcinogenic as well as antileukemic activities have been isolated from many Euphorbiaceae and Thymelaeaceae species. They contain the structurally closely related polycyclic skeletons of tigliane, daphnane and ingenane. They are being used in all facets of cancer research as both tumor promoters and inhibitors. They are also considered as second‐order carcinogenic risk factors in the etiology of human cancer. The authors review possible biosynthetic pathways for the plant diterpenes and discuss structural relationships in view of their biogenesis.
IX. Experimental Problems
Authors: Feil Dirk
Published: November 2013

Experimental problems encountered in accurate electron density studies are discussed: choice of crystal, temperature effects, scanning mode, multiple scattering, accuracy. The effects of inelastic scattering (TDS) are indicated.
X. Data Processing and Structure‐Factor Determination
Authors: Rees Bernard
Published: November 2013

The different steps in the determination of precise X‐ray or neutron structure factors are reviewed. Items discussed are: absorption, extinction, X‐ray polarization, scale factor, Λ/2 contamination, long‐range intensity variations, anomalous dispersion, averaging of equivalent reflections, magnitude and phase of the structure factor.
XI. Overcoming the Free‐Atom Bias with Modified Least‐Squares Formalisms
Authors: Coppens Philip
Published: November 2013

Modified least‐squares formalisms ranging from high‐order refinement and spherical‐atom refinement with a variable radial dependence of the valence shell to refinement with atom‐centered multipole expansions are discussed.
Azulene Revisited: Picosecond Decay of the S1 State in the Gas Phase and in Solution
Authors: Huppert Dan , Jortner Joshua , Rentzepis P. M.
Published: November 2013

In this paper we report the results of an experimental study of picosecond time‐resolved relaxation dynamics of the S1 state of the azulene molecule in solution and in the low pressure gas phase, utilizing consecutive two‐photon excitation S0 → S1(v) → S2 (or S3), with the exciting and the monitoring picosecond pulses chosen at the same wavelength. The nonradiative electronic relaxation times for azulene in solution were τnr = 4 ± 3 ps for S(v = 5), excited at 530 nm, and τnr = 3 ± 2 ps for S(v = 2), excited at 625 nm, while for the “isolated” molecule case τnr = 4 ± 3 ps for S(v = 2), excited at 625 nm. The electronic relaxation rate of the S1 manifold exhibits only a weak dependence on the excess vibrational energy Ev up to Ev = 4800 cm−1. The electronic relaxation rate is practically unaffected by solvent perturbations, as appropriate for the statistical limit of intramolecular radiationless transitions.
Gas and Solution Phase Reactivities of Triplet Aromatic Carbonyls
Authors: Berger Michael , Camp R. Nicholas , Demetrescu Ioanna , Giering Linda , Steel Colin
Published: November 2013

Absolute rate studies pertinent to the gas and solution phase photochemistry of triplet benzaldehyde, acetophenone and benzophenone have been carried out, and it has been confirmed that, in the gas phase, attack on a hydrocarbon by the carbonyl results in the expected hydrogen abstraction. Also absolute rates in both the gas phase and solution (acetonitrile) are quite comparable. For a given carbonyl the differences in the rates of abstraction are largely accounted for by activation energy changes. The activation energies vary from a high of 4.8 kcal mol−1 for abstraction of a primary hydrogen by acetophenone to a low of 1.4 kcal mol−1 for abstraction of tertiary hydrogen by benzaldehyde. At room temperature the relative reactivities of primary, secondary and tertiary C—H bonds for all the carbonyls are 1:23:190. In solution benzophenone is two times more reactive than acetophenone, while benzaldehyde is ten times more reactive. In the gas phase benzaldehyde is about 18 times more reactive than acetophenone. Molecular orbital calculations indicate that the greater reactivity of benzaldehyde as compared to acetophenone stems from steric hindrance in the latter. The quenching of acetophenone triplets by various other substrates including naphthalene, benzene, isopropanol and triethylamine is also discussed.
Pentafluorosulfur Peroxides: Synthesis, Properties and Vibrational Spectra of SF5OOH, SF5OOF, SF5OOCl and Evidence for the Trioxide, SF5OOOSF5
Authors: Desmarteau Darryl D. , Hammaker Robert M.
Published: November 2013

The synthesis and properties of the novel pentafluorosulfur peroxides, SF5OOH, SF5OOF and SF5OOCl are described. The hydroperoxide obtained by the hydrolysis of SF5OOC(O)F is the parent compound of this series and the halogen derivatives are obtained by reactions with fluorine and chlorine monofluoride. The preparation of SF5OOF results in the formation of another unstable compound, believed to be SF5OOOSF5.

The vibrational spectra of SF5OOX and the related SF5OX (X = H,Cl,F) are analyzed in a consistent manner by assigning the most intense Raman band to the symmetric SF4 square stretching mode. Previous assignments for pentafluorosulfur peroxides had assigned the most intense band to the O‐O stretch.
The Reaction of Thiazylfluoride with Hexafluoropropene
Authors: Bludssus W. , Mews R. , Glemser O. , Alange G. G.
Published: November 2013

NSF (1) reacts with CF3CF=CF2 in the presence of CsF to give the known sulfides (CF3)3CF‐S‐S‐CF(CF3)2 (3) and (CF3)2CF‐S‐CF(CF3)2 (4) and the linear thiazenes (CF3)2C=N‐S‐CF(CF3)2 (5), (CF3)2CF‐S‐N=S=N‐CF(CF3)2 (6), [(CF3)2CF‐S‐N=]2S (7), and (CF3)2CF‐S‐NSF2 (8); (CF3)2CF‐S‐NSO (9) results from the decomposition of (8) with glass. Yields of (3)‐(9) depend on the reaction conditons; an attempt is made to explain the formation of the products.
Fluorine‐Containing Dithioethers
Authors: Sharp David W. A. , Miguel Hugo Torrens
Published: November 2013

The dithioethers CH3SCF2CH2SCH3, CH3SCH(CF3)CH2SCH3, CH3SCH(CF3)CH(CF3)SCH3, CH3SCH(CH3)CH2SCH3, CH3SCH(CH3)CH(CH3), C6F5SCH2‐CH2SC6F5, CF3SCH2CH2SCH3, and C6F5SCH2CH2SCH3 have been prepared by photochemical addition to olefines or by metathetical reactions. Physical and some chemical properties are described.
Synthetic Methods and Reactions.43. Preparation of Fluorohydrins from Epoxides with Pyridinium Poly‐hydrogen Fluoride
Authors: Olah George A. , Meidar David
Published: November 2013

The use of pyridinium poly‐hydrogen fluoride for the convenient production of fluorohydrins is described. Pyridinium poly‐hydrogen fluoride as the hydrofiuorinating agent for epoxides allows the reaction to be carried out under mild conditions without heating the reaction mixture. The method is applicable to both aliphatic and aromatic epoxides.
Mass Spectrometric Identification of New Derivatives of PGA's and 19‐hydroxy‐PGA's Specific for Multiple Ion Detection
Authors: Roselló J. , Sunol C. , Gelpi E.
Published: November 2013

The mass spectra of the new piperidyl‐PGA (TMS), derivatives, formed by reaction of prostaglandin A1 or A2 with BSTFA/piperidine, are characterized by a dominant M − 173 ion, remarkably specific for MID applications. Similar derivatives have also been obtained with mixtures of BSTFA and other heterocyclic amines (hexamethylenimine, pyrrolidine, morpholine and propylenimine), which give the respective heterocycle‐PGA (TMS), derivatives. Likewise, the corresponding 11‐piperidyl‐19‐OH‐PGA (TMS)4 seem specially suited for the selected ion detection of PGA “artifacts” in extracts of human semen. The postulated structure of fragments at M − 173 and M − 423 (series I) and M − 425 (series II) is in favour of a pushpull type of reaction mechanism leading to these derivatives by nucleophilic addition of the amine on the C‐11 atom of the PGA's and concurrent formation of an enolate ion from the 9‐keto group with silanization of all reactive groups by BSTFA. An example of an application to the determination of PGA's and 19‐OH‐PGA's in a biological extract is given.
Analysis of Drugs by Chemical Ionization and Electron Impact Mass Spectrometry
Authors: Zitrin S. , Yinon J.
Published: November 2013

The value of direct chemical ionization mass spectrometry in combination with electron impact for both screening and confirmation purposes has been demonstrated. It was found to be an analytical tool for the identification of drugs and related compounds.
Luminescent Excited States of Phthalideisoquinolines: Studies on Low Temperature Spectra with some Observations on the Photochemistry and Mass Spectrometry of these Compounds
Authors: Turnbull James H. , Bowd Alexander
Published: November 2013

The phthalideisoquinolines narcotine and hydrastine show anomalous blue fluorescence emissions in glassy media at 77 K, whereas the closely related compound bicuculline, in which the configuration of the C–1 proton is reversed, shows no such effect. We propose that this blue emission arises from a phthalidestilbene system formed by a reversible excited‐state isomerisation.
On Some New Oxidation—Reduction Polymers
Authors: Manecke Georg , Ehrenthal Einhart , Finck Wilhelm , Wunsch Friedrich
Published: November 2013

Redox polymers are among the most well‐documented polymer reagents, and tailored redox polymers are obtainable. The synthesis and the redox properties of some new silica‐anchored quinones are described. A high redox reaction velocity was found with a modified pore‐controlled glass. Polymer reagents with N‐chlorobenzotriazole units were obtained from suitable benzotriazole derivatives or by polymer‐analogous reactions starting from p‐aminostyrene polymers. With these products chlorinations as well as mild oxidation reactions were performed with good yields. Reusability and mechanical properties were good.
Matrix Effects on Size‐Selectivity of Polymeric Reagents and Catalysts
Authors: Regen Steven L. , Lee Dan P.
Published: November 2013

Selectivity has been investigated for the catalytic hydrogenation of alkenes using a polymeric catalyst derived from a polystyryldiphenylphosphine resin and tris(triphenylphosphine)chlororhodium (I). These data together with the selectivity observed for a closely related polymeric phosphine reagent capable of converting alcohols to alkyl chlorides provide compelling evidence that a polymer support can impose a modest degree of size‐selectivity to an attached reagent and catalyst.
The Properties of Oxide Layers Formed on Iridium, Rhodium and Ruthenium Electrodes During Potential Cycling
Authors: Woods R.
Published: November 2013

Cycling the potential of iridium, rhodium or ruthenium can result in the growth of a metal oxide on the electrode surface. The formation of the oxide layer depends on both the upper and lower potential limits of the cycle and on the composition of the electrolyte solution. Reversible changes in valency of the metal atoms in the oxide, accompanied by proton transfer, occurs during a potential cycle and give rise to voltammetric peaks which are symmetrical about the potential axis. The nature and composition of the oxides and their electrocatalytic activity for oxygen evolution are discussed.
Influence of Electrosorption on Kinetics and Selectivity of Electroorganic Synthetic Reactions
Authors: Köster K. , Riemenschneider P. , Wendt H.
Published: November 2013

Necessary conditions for the use of the influence of electrosorption on electrode kinetics, consecutive chemical kinetics and selectivity of electroorganic synthetic reactions are discussed. The role of electrosorption for the performance of four selected examples of electroorganic reactions is demonstrated. Three of these reactions are anodic and the fourth is cathodic. In the two cases of anodic formation of disaryliodonium cations and anodic oxidation of olefins, it is shown that the adsorptive character of the anode material is decisive for the success of the respective synthesis. For the first reaction, high current yields with only little loss of the precious iodoarene by formation of polymers, as well as avoidance of self coupling can only be obtained at Pt‐anodes in acetonitrile. Anodic dimer formation by olefin oxidation only gives high yields at graphitic carbon anodes in methanol. Here semiquantitative data for electrosorption of olefins and supporting electrolyte anions have been obtained by electrode‐kinetic measurements. They explain the influence of the supporting electrolyte anion on the dimer yield by the different spacial demands of the adsorbed anions of the supporting electrolyte. Due to very poor adsorption of radicals at Pt‐anodes and strong radical adsorption at carbon‐anodes, the anodic radical addition to olefins yields quite different product mixtures at these two anode materials. The optical inductive effect of electrosorbed strychninium cations on the cathodic reduction of acetylpyridine are shown to be caused by the enormous concentration enhancement of the alkaloid due to adsorption at the interphase. This is the precondition for complex formation between the negatively charged carbonation formed by disproportionation of the 1‐e reduction product and the positively charged inductor cation. The preference of one of the two possible steroisomers is obtained only by this complex formation.
Preparation and Catalytic Behaviour of Polymers with Pendant Oligoethyleneoxy‐groups (Polymers of Non‐cyclic Crown Ethers)
Authors: Hiratani Kazuhisa , Reuter Peter , Manecke Georg
Published: November 2013

Vinyl monomers with oligoethyleneglycolether moieties as pendant groups, having different lengths of the oligoether chain and different end groups such as the ethyl‐ and quinolinyl‐group, have been prepared. Homopolymers, a copolymer with styrene and crosslinked copolymers with p‐divinylbenzene were obtained by radical polymerization of the vinyl monomers. The catalytic activity of these polymers in comparison with that of the low molecular weight model compounds was investigated in the nucleophilic substitution of benzylchloride with the acetate ion. The influence of the counter cation of the acetate ion, the lengths of the oligoethyleneglycol chain, the nature of its end group and the effect of different solvents on the catalysis were studied.
Asymmetric Reactions in Chiral Hydrophobic Binding Sites
Authors: Sugimoto Toyonari , Baba Naomichi
Published: November 2013

With a number of illustrations from the asymmetric reactions conducted in chiral hydrophobic binding sites provided by chiral micelles, cyclodextrins and non‐enzyme globular proteins, this research account presents a cross section of recent biomimetic research related to stereospecificity in enzyme reactions.
The Photochemistry of the Polymerization of Diacetylenes in Multilayers
Authors: Fouassier J. P. , Tieke B. , Wegner G.
Published: November 2013

The quantum yield Φ of the photopolymerization of the amphiphilic diacetylene derivate pentacosa‐10,12‐diynoic acid (1) in form of multilayers was investigated. The multilayers were obtained by the Langmuir–Blodgett technique. The quantum yield for monochromatic irradiation at Λ = 254 nm depends strongly on conversion and slightly on temperature. It is independent of light intensity and number of multilayers. The quantum yield Φ extrapolated to zero‐conversion was found to be 10.3 ± 0.2 at 25°C. It increases linearly with increasing temperature. The activation energy of the photopolymerization was determined as EA = 4 ± 1 kcal mol−1. Φ seems to decrease nearly linearly with increasing conversion and reaches about half its initial value at 65 percent conversion. 3,3′‐Distearyl thiacarbocyanine iodide (2) was found to act as a sensitizer of the photopolymerization, if it was included into the multilayers by cospreading with 1. Photopolymerization occurs upon irradiation into the absorption band of the dyestuff‐monomer up to 600 nm. At higher concentration of 2, when dimers or multimers are formed, the photopolymerization is quenched.
Micellar Catalyzed Dephosphorylation The Role of Hydrophobicity of the Nucleophile
Authors: Bunton Clifford A. , Sepulveda Luis
Published: November 2013

Cationic micelles of hexadecyltrimethyl ammonium bromide (CTABr) increase the rate of dephosphorylation of p‐nitrophenyldiphenyl phosphate (PNPDPP) by aryloxide ions by factors of almost 104. The rate‐surfactant profiles for reactions of p‐ethyl‐, p‐propyl‐, p‐t‐butyl‐ and p‐t‐amyl‐phenoxide ion and of 2‐naphtholate ion can be interpreted quantitatively in terms of the distributions of the two reactants between aqueous and micellar pseudophases. The second order rate constants in the micellar pseudophase are slightly smaller than in water, showing that the rate enhancements are due largely to concentration of reactants at the water–micelle interface. Similar conclusions can be drawn from analysis of the micellar catalysis of the dephosphorylation by oximate ions of p‐nitrobenzaldehyde and 2‐quinoline carbaldehyde.
Polymeric Matrices in Chemical Reactions — Silent or Active Partners?
Authors: Warshawsky Abraham
Published: November 2013

The evidence for the participation of the polymer in the reactions of polymer‐bound reagents is reviewed.
Organized Monolayers by Adsorption. III. Irreversible Adsorption and Memory Effects in Skeletonized Silane Monolayers
Authors: Sagiv Jacob
Published: November 2013

Oleophobic mixed monolayers of n‐octadecyltrichlorosilane (OTS) and surfactant dyes are adsorbed on polar solid surfaces. The dye molecules are then removed by solvent treatment leaving holes entrapped within a stable network of chemisorbed and polymerized silane molecules. Surfaces coated with such skeletonized OTS monolayers are shown to exhibit adsorption properties related to specific geometrical features of the displaced molecules. This provides a new artificial means for directing molecular organization taking place at a solid‐liquid interface. Practical applications to the study of liquid crystals, chemical reactions of oriented molecules and chromatography are suggested.
Metalloid Heterocycles for Organic Synthesis
Authors: Shanzer Abraham
Published: November 2013

In this paper we discuss applications of cyclic intermediates for selective derivatization of difunctional substrates and controlled condensation to macrocyclic systems. Ring compounds that incorporate metalloid elements like silicon and tin have been chosen for this purpose and we demonstrate how the combination of their properties can be applied for the solution of specific problems.
Light Induced Redox Reactions in Simple and Functional Surfactant Systems and Their Relevance to Energy Conversion Problems
Authors: Grätzel Michael
Published: November 2013

Self‐assembly of surfactant molecules in aqueous solution yields micellar aggregates. These, through local polarity and electrostatic potential effects, can be used kinetically to control light‐initiated electron transfer reactions. Cooperative effects assisting charge separation are observed with functional micelles where the redox relays are linked to the surfactant molecule. The implications for energy conversion systems are discussed.
Quantum Mechanical Aspects of the Molecular Structure Hypothesis
Authors: Woolley R. G.
Published: November 2013

A discussion of the quantum mechanics of molecules with the emphasis on quantum mechanical principles is presented in the expectation that a better understanding of the assumptions tacitly embodied in conventional quantum chemistry will follow. The discussion centres mainly on the position of the chemical structure hypothesis in a quantum mechanical theory based on quantum states. Various different kinds of molecular states are identified according to characteristic properties of their wavefunctions, ranging from the completely delocalized (fully symmetrized) eigenstates to the “molecular structure states” of rigid molecules. I suggest that the adiabatic separation of electrons and nuclei can be avoided by working with the generator coordinate method (GCM) which promises to be a powerful non‐adiabatic formalism for molecular quantum theory.
The Quantum Mechanical View of Molecular Structure and the Shapes of Molecules
Authors: Trindle Carl
Published: November 2013

The idea that a single molecule has a shape—that is, a longlived nearly fixed (relative) arrangement of nuclei in space—has been shown by Woolley to be a classical idea imposed on the quantum mechanical picture of matter. In this report the argument that the Born–Oppenheimer approximation is semi‐classical is reviewed, and some pioneering computations free of the Born–Oppenheimer approximation are discussed. The problem of reconciling the results of these computations with the powerful and successful molecular structure theory is described, and a mode of computation is proposed which strikes a compromise between the sometimes inadequate Born–Oppenheimer Hamiltonian and the complicated full molecular Hamiltonian.
The Concept of Molecular Structure in Quantum Theory: Interpretation Problems
Authors: Claverie Pierre , Diner Simon
Published: November 2013

The classical concept of molecular structure (namely, a set of atoms with a well‐defined geometrical arrangement in space) originated well before the advent of quantum theory, and, contrary to widespread views, these two conceptual schemes (classical molecular structure and quantum theory) are not easy to reconcile. We begin with a preliminary discussion of molecular structure in terms of space and time correlations in the framework of quantum theory. Then we summarize the salient features of some critical views raised‐during recent years (essentially by Woolley) concerning the concept of molecular structure. The essential claim of this viewpoint is that “molecular structure” is not an intrinsic attribute of an isolated molecule, and appears rather as an effect of the environment (other molecules? vacuum electromagnetic field, according to quantum electrodynamics?). Afterwards, we discuss these issues in detail: we show that it is necessary to distinguish between a “quantum” or “potential” structure (which may be deduced in a straightforward way from the quantum treatment of the molecule), and the “classical” molecular structure (according to which the molecule is considered as a set of rotating and vibrating material points, corresponding to the atoms); we briefly discuss some aspects of the Born–Oppenheimer approximation. From our discussion, we deduce that the “classical structure” concept is indeed non‐trivial from a purely quantum‐mechanical point of view; in actual fact, it is related to the problem of the “classical limit” of quantum mechanics, and it appears that, at the present time, none of these classical concepts and behaviours is convincingly derivable from a strict quantum mechanical basis. Finally, we consider these same problems from the point of view of stochastic electrodynamics (SED), which is one of the stochastic models for microphysics proposed so far as a possible alternative or extension of usual quantum theory. In the framework of such a theory, the “classical limit” and, in particular, the “classical molecular structure” no longer poses a problem, but aside from these satisfactory qualitative aspects, several quantitative failures of SED prevent us from accepting it, at least in its present state, as a genuine physical alternative to quantum theory. We therefore conclude that “classical molecular structure”, and classical concepts in general, remain an open problem in the framework of present‐day quantum physics.
Theoretical Error Estimates in Atomic and Molecular Structure Calculations
Authors: Cohen M. , Feldmann Tova
Published: November 2013

We present a brief account of methods of deriving lower and upper bounds to energy eigenvalues, overlap integrals, expectation values, transition elements and properties of atoms and molecules (such as dynamic polarizabilities) which are related to second order perturbation theory.
Potential Energy Surface of BH4− and Molecular Deformations Induced by an External Cation
Authors: Eisenstein Odile , Dunitz Jack D.
Published: November 2013

The part of the potential energy surface of BH4− which describes molecular deformations preserving C3v symmetry is calculated using a 6–31 G basis set of atomic orbitals and fitted by analytical power functions up to cubic terms in symmetry coordinates and cartesians. Minimum‐energy paths drawn on this energy surface are compared with the path describing the deformation of BH4− induced by approach of a cation along a threefold axis and also with a path derived by assuming bond‐number conservation. The possible relevance of the conclusions for interpreting experimental deformation paths in crystal environments is discussed.
The Reliability of Quantum Mechanical Predictions of Chemical Reactivity
Authors: Klopman Gilles
Published: November 2013

The reliability of the predictions of quantum mechanical methods is critically examined in selected areas of chemical reactivity. Several examples are discussed, including the mechanism and conformation of reaction products, the stability of carbocationic intermediates and the formation of hydrogen bonded species.
H–H Bond Breaking on Plane and Stepped Pt(111) Surfaces
Authors: Tantardini Gian Franco , Simonetta Massimo
Published: November 2013

A model potential has been constructed to study the interaction of H2 with Pt(111) and Pt(S) [6(111) × (111)] surfaces. An increase in bond breaking activity on the stepped surface can be envisaged.
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

The 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

Schemes 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

The 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

1,5,9‐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

The 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

The 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

The 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

Strained 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

Rigid 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

A 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

Addition 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

Absorption 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

Contrary 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

The 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

The 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

Photochemical 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

The 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

The 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

Photocurrent‐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.