bluuebunny/crossref_metadata_2025 huggingface dataset:
(Polystyrylbipyridine)palladium(0): Heterogeneous Catalyst for the Mild Hydrogenation of Dienes to Monoenes
Authors: Card Roger J. , Neckers Douglas C.
Published: November 2013

(Polystyrylbipyridine)palladium(0) is an active catalyst for the selective hydrogenation of dienes to monoenes under mild conditions. Swelling solvents, such as tetrahydrofuran or benzene, are useable under 1 atm hydrogen at 25°C. The hydrogenation of linear isomers of hexadienes results in the formation of 1‐, 2‐ and 3‐hexene in each case. The hydrogenation occurs at the same rate for each of the hexadienes and < 1% hexane is observed until after consumption of the first equivalent of hydrogen. Cyclohexadiene undergoes hydrogen transfer to form cyclohexene and benzene. The chemistry observed is distinct from that observed in Bailar's use of (℗‐Ph‐CH‐2P(Ph2)2PdCl2as a hydrogenation catalyst.
The Three Phase Test: Intramolecular Nucleophilic Catalysis
Authors: Rebek Julius , Horton Joseph A. , Brown David
Published: November 2013

An unambiguous method for the detection of reaction in intramolecular nucleophilic catalysis intermediates is described. The three phase test involves generation of the intermediate from an insoluble, polymer‐bound precursor and its trapping by a second solid phase suspended in the same solution. The hydrolysis of monoesters of phthalic acid and of certain substituted phenyl esters is discussed.
Molecular Models for Solvent Structure at Polarizable Interfaces
Authors: Fawcett W. R.
Published: November 2013

Molecular models developed to account for the dielectric properties of the polarizable metal/solution interface are reviewed. Experimental data obtained for both aqueous and non‐aqueous systems in the absence of specific adsorption are examined within the context of these models. It is concluded that a relatively simple model can describe the properties of the inner layer in aprotic solvents where intermolecular forces can be described on the basis of permanent and induced dipole—dipole interactions.
Raman Scattering from Adsorbed Molecules in Electrochemical Systems
Authors: Efrima S. , Metiu H.
Published: November 2013

The classical theory of Raman scattering from adsorbed molecules on solid surfaces is outlined, and modifications aimed at discussing electrochemical systems are presented. We propose a mechanism for intensity enhancement and compute the polarization and the angular distribution of the scattered radiation. It is shown that for some limiting cases the position of the molecule with respect to the surface may be inferred from experiments.
Accuracy of Determining Electrode Properties from Impedance Measurements
Authors: Despić Aleksandar R. , Jović Vladimir D. , Djorović Mihajlo V.
Published: November 2013

An attempt is made to estimate the accuracy which can be achieved in the evaluation of parameters of an electrode with an activation controlled electrochemical process from measurements of electrode impedance. Ferrous/ferric couple at platinum in perchloric acid was taken as a model system and the impedance components have been measured as functions of frequency at different concentrations. Double layer capacitance, CDL, faradaic resistance, θ, and the ohmic resistance of the cell R, have been identified from the characteristic functions, and the indeterminacies in their values evaluated from the probable errors of measurements. The problem of indeterminacy was generalized and regions in the CDL – θ – R space were defined outside which those parameters cannot be determined within present limits of accuracy.
Solid Electrolyte Interphase (SEI) Electrodes. Part 1. The Kinetics of Lithium in LiAlCl4‐SOCl2
Authors: Peled E. , Yamin H.
Published: November 2013

The lithium electrode is always covered by a film of lithium chloride which acts as solid electrolyte interphase (SEI). Macropolarization curves indicate that the Tafel slope is greater than 3 V for electrodes having SEI thicker than 400 Å. This confirms that the rds for the deposition‐dissolution process is the migration of lithium cations through the SEI. Increasing the electrolyte concentration increases the interfacial capacitance and decreases the resistivity of the SEI. This is explained in terms of the effect of LiAlCl4 concentration on the concentration of the lattice defects. The effect of the concentration of the electrolyte on the resistivity of the SEI decreases as the thickness of the SEI is increased. The growth rate of the SEI increases as the concentration of the electrolyte is increased.
Electrocatalysis on Surfaces Modified by Foreign Metal Adatoms
Authors: Adžić Radoslav R.
Published: November 2013

An approach to the problem of electrocatalysis, which consists of modification of electrode surfaces by foreign metal adatoms, is presented. The paper reviews the work done in the author's lab on the effects of foreign metal adatoms, formed by underpotential deposition, on kinetics of several electrode reactions. Catalytic effects have been illustrated on examples of oxidation of some organic molecules, oxygen reduction, and electrodeposition of metals. The inhibition has been found with hydrogen evolution on noble metals. Small catalytic and inhibitory effects have been observed with redox reactions. The effects of foreign adatoms on the surface properties of electrode catalyst and the origins of catalytic and inhibitory effects have been analysed. The use of such adatoms as a probe in investigation of the mechanism of electrode reactions is discussed.
Biomimetic Chemistry in Oriented Systems
Authors: Breslow Ronald
Published: November 2013

Biomimetic chemistry is the field which attempts to imitate the accelerations and selectivities characteristic of enzyme‐catalyzed reactions. Enzyme chemistry is not slavishly mimicked in detail, but instead furnishes an intellectual stimulus to the design of novel catalysts and reagents. Work in this field at Columbia University is reviewed in the area of selective steroid transformations, reactions of flexible chains (in solution, micelle, and bilayer), and cyclodextrin lock and key chemistry inspired by chymotrypsin and by ribonuclease. Careful attention to geometrical factors leads to good mimics of the rates and selectivities of some enzymes.
Polymerization of Oriented Monomers. IV. Polymerization ofp‐Acryloyloxybenzylidene‐p‐butylaniline within a Liquid Crystalline Phase
Authors: Paleos C. M. , Voliotis S.
Published: November 2013

The effect of organization of a liquid crystalline monomer polymerized within a mesomorphic phase on its polymer morphology has been investigated. Specifically,p‐acryloyloxybenzylidene‐p‐butylaniline has been polymerized in the mesomorphic phase, which is obtained by melting a mixture of this monomer withp‐propanoyloxybenzylidene‐p‐butylaniline. Destruction of the liquid crystalline phase was avoided during polymerization by employing a monomer to solvent molar ratio equal to 1:3. In addition, the same monomer was polymerized in the isotropic melt of the same mixture. Polymers derived from both modes of polymerization were characterized by X‐ray diffraction and exhibited the same degree of crystallinity (about 70%). It is therefore concluded that monomer orientation does not affect polymer crystallinity and also that crystallinity originates from the strong interaction of polymer side groups or alternatively from the intrinsic tendency of polymers to organize.
Quasi‐Crystals
Authors: Krongauz V. A.
Published: November 2013

Irradiation of solutions of photochromic spiropyrans in nonpolar solvents initiates a succession of spontaneous chemical and physical transformations, resulting in the appearance of colloidal spherical particles, globules, of very complex structure. They are composed of highly dipolar crystalline nuclei surrounded by amorphous envelopes. Due to their large permanent dipoles the globules are aligned in linear beaded structures in a constant electric field. Macroscopically these structures look like colored threads, extending along the electric lines of force. The globules and threads reveal properties associated both with crystals (optical and electric anisotropy, discrete X‐ray diffraction) and with amorphous materials (droplike form, fluidity). The globules grow stepwise, resulting in the appearance of several generations of threads with discrete increasing sizes of globules. The quasi‐crystalline globules mimic the organization of biomembranes.
Dipolar Micelles. 9. The Mechanism of Hydrolysis of Cationic Long Chained Benzoate Esters in Choline and Homocholine‐type Micelles
Authors: Pillersdorf A. , Katzhendler J.
Published: November 2013

An investigation was carried out on the hydrolytic reactions of the para‐substituted long chain esters N‐decyl‐N, N‐dimethyl‐N‐(2‐benzoyloxyethyl) ammonium bromide (CBE), N‐decyl‐N, N‐dimethyl‐N‐(3‐benzoyloxypropyl) ammonium bromide (HCBE) and decyl benzoates (DBE) in the cholinium type detergent N‐decyl‐N, N‐dimethyl‐N‐(2‐hydroxyethyl) ammonium bromide (I), and in the homocholinium type detergent, N‐decyl‐N, N‐dimethyl‐N(3‐hydroxypropyl) ammonium bromide (II). The first order rate constants (kc) of the trans‐benzoylation stage for HCBE in micelle I, CBE in micelle II and DBE in micelles I and II as catalysts were determined. The second order rate constants (k'OH) for the alkaline hydrolysis of these esters, were also evaluated. The results showed that the partition ratio (k−n/ko) of the tetrahedral intermediate formed during the reaction of I + HCBE, to the reactants and products, is relatively small when compared to the known values for oxygen esters in non‐micellar media. This phenomenon is discussed in terms of: (a) the stabilization of the anionic transition state by the positively charged micellar surface; (b) the electrophilic assistance to the expulsion of the leaving group; and (c) the favourable entropy of the substrate‐micelle intracomplex‐formation. Substituent effects on the rates (kc, k'OH) of CBE, HCBE and DBE were analyzed using the Hammett equation. When compared to the ρ values displayed in aqueous solution, the increased ρ values observed in micellar and dioxane‐water systems indicate that the hydrolytic reactions of HCBE and CBE in micelles proceed close to the less polar surface area. This requires a folded conformation of the head groups of the above mentioned esters. The low effectiveness of the hydroxylic micelles (I, II) in the hydrolysis of esters is also discussed.
Quantum Topology: Theory of Molecular Structure and its Change
Authors: Bader Richard F. W. , Tal Yoram , Anderson Steven G. , Nguyen‐Dang T. Tung
Published: November 2013

In this paper we review and exemplify a new and rigorous approach to the problem of molecular structure and its morphogenesis: the theory of quantum topology. The basis for this approach is provided by the topology of the total charge density in a given molecular system. The essential observation is that the only local maxima of a ground state distribution occur at the positions of the nuclei. The nuclei are therefore identified as point attractors of the gradient vector field of the charge density. The associated basins partition the molecular system into atomic fragments. Each atom is a stable structural unit defined as the union of an attractor and its basin. The common boundary of two neighbouring atomic fragments, the interatomic surface, contains a particular critical point, which generates a pair of gradient paths linking the two neighbouring attractors. The union of this pair of gradient paths and their endpoints is called a bond path. The network of bond paths defines a molecular graph of the system.

Having defined a unique molecular graph for any molecular geometry, the total configuration space is partitioned into a finite number of regions. Each region is associated with a particular structure defined as an equivalence class of molecular graphs. A chemical reaction in which chemical bonds are broken and/or formed is therefore a trajectory in configuration space which must cross one of the boundaries between two neighbouring structural regions. These boundaries form the catastrophe set of the system which, like a phase diagram in thermodynamics, denotes the points of “balance” between neighbouring structures. A general analysis of the structural changes in an ABC type system is given in detail together with specific examples of all possible structural elements in a molecular system.

The properties of the topologically defined atoms and their temporal changes are identified within a general formulation of subspace quantum mechanics. It is shown that the quantum mechanical partitioning of a system into subsystems coincides with the topological partitioning: both are defined by the same set of “zero flux” surfaces. Consequently the total energy, or any other property, is partitioned into additive atomic contributions.

We show that, in general, a definite structure can be assigned to a given molecular system. Quantum mechanically this structure is associated with an open neighbourhood of the most probable nuclear geometry. Finally we generalize the notion of molecular structure to non‐isolated molecules and, in contrast to recent work by Woolley, we conclude that molecular structure exists in spite of intermolecular interactions and not as a result of them.
Models for Active Sites of Metalloenzymes: Comparison of Zinc and Beryllium Containing Complexes
Authors: Osman Roman , Weinstein Harel
Published: November 2013

Metal containing models of the active site of the metalloenzyme carboxypeptidase were studied by quantum chemical ab initio calculations. The comparative properties of [Zn(OH)(NH3)2]+ and [Be(OH)(NH3)2]+ indicate some of the characteristics of the role of transition metals in biological systems: the Zn complex, but not the Be complex, generates directionality constraints for an incoming ligand that are compatible with the geometrical features identified in the crystal structure of the active site — substrate complex. Moreover, an incoming ligand (H2O) is perturbed more strongly by the Zn complex than by the Be complex. This perturbation (a charge transfer and a characteristic polarization of the electronic density along the metal–ligand axis) is in good agreement with the proposed effect of the metal in the active site on the reactivity of a carbonyl carbon in a peptide bond. The perturbation caused by the incoming ligand in the electronic structure of the metal containing complex is also larger for the Zn complex than for the Be complex. These findings indicate that while the Be complex may simulate some of the purely electrostatic effects of the Zn containing model of the active site, it does not represent as well as the Zn complex some of the reactivity elements that have been implicated in the role of the metal in the catalytic mechanism of carboxypeptidase.
Observations on the Significance of the Electrostatic Potentials at the Nuclei of Atoms and Molecules
Authors: Politzer Peter
Published: November 2013

The first portion of this paper reviews the development of a series of relationships, both exact and approximate, between the energies of atomic and molecular systems and the total electrostatic potentials at the nuclei in these systems. Mathematical formulations are presented for such properties as the energy of the valence electronic charge of an atom, and the energies associated with the individual atoms in a molecule. An equation is obtained that relates the energy of a molecule to the changes in the electrostatic potentials at the nuclei of the free atoms when they combine to form the molecule. The second portion of the paper discusses some properties of the potentials at nuclei, including their remarkable insensitivity to the environments of the nuclei. It is pointed out that the potentials at nuclei serve as links between theory and various experimental techniques, including electron spectroscopy, nuclear magnetic resonance, and Mossbauer spectroscopy. A general theme of this paper is that the electrostatic potentials at nuclei are a fundamental and unifying property of atomic and molecular systems, which can help to clarify and generalize some very basic concepts.
Quantum‐Mechanical Calculations of Molecular Force Fields at the SCF Ab Initio Level Methyl Halides as Test Cases
Authors: Berthier Gaston
Published: November 2013

The possibilities of the present quantum‐chemistry methods for calculating the vibrational properties of polyatomic molecules have been investigated, taking the CH3X series (X = F, Cl, Br, I) as bench tests. Finite‐difference computations carried out with various basis sets at the SCF ab initio level on methyl fluoride bring out the performance of the LCAO‐MO method as concerns the prediction of molecular force constants and integrated infrared intensities. Extension of these calculations to other halides is made possible using pseudo‐potentials for the interaction between the atomic cores and the valence‐shell electrons of the molecule. The atomic force field parameters defined in terms of Cartesian displacement coordinates suffer from a certain lack of transferability, apparently due to the variations of the chemical bonding effects when passing from fluorine to iodine.
An Improved Molecular Mechanics Force Field for Alcohols and Ethers
Authors: Allinger Norman L. , Chang Scott H.‐M. , Glaser Donna Hindman , Hönig Helmut
Published: November 2013

The MM2 force field has been extended to the title class compounds. Generally, the force field does a good job of calculating structures and energies. A number of interesting conformational problems are examined, including: conformational energies of methyl groups on oxane and 1,3‐dioxane rings, the syn‐axial methyl–hydroxyl interaction and a number of hydrogen–bonding problems, including the bifurcated hydrogen bond in axial‐5‐hydroxyl‐1,3‐dioxane.
Is the Effective Size of an Alkyl Group a Gauge of Dynamic Gearing?
Authors: Hounshell W. Douglas , Iroff Linda D. , Iverson D. J. , Wroczynski R. J. , Mislow Kurt
Published: November 2013

This paper presents a critical discussion of the perceived similarity between alkyl groups and mechanical gears. Specifically, we examine the notion that the availability of “free spaces” between the ligands of the central atom in an alkyl group leads to a reduction in the steric requirements of the group and, hence, to an effect on the rate of conformational interconversion. It is concluded that there is no compelling experimental evidence to support the operation of this type of dynamic gear effect.
The Configurational Stability of Triorganostannyl‐Transition Metal Compounds
Authors: Gielen Marcel , Eynde Ivan Vanden
Published: November 2013

The configurational stability of triorganostannyl–iron, –manganese and –cobalt complexes has been determined by studying the coalescence of the NMR signals of diastereotopic groups and by looking at the epimerization of diastereomeric mixtures of these compounds. Possible substitution reactions, transforming the studied derivatives into one another, have been checked.
Carbon‐13 Variable Temperature NMR Studies of Some Dimethyl‐1‐oxa‐3,4‐diazacyclohexanes1
Authors: Katritzky Alan R. , Patel Ranjan C. , Brito‐Palma Fernando M. S. , Riddell Frank G. , Turner Eric S.
Published: November 2013

Conformational equilibria are elucidated and rationalised for 3,4‐dimethyl‐1‐oxa‐3,4‐diazacyclohexane, and a trans‐bicyclic analogue. The major conformer has 3ax‐NMe, 4eq‐NMe: two minor conformations are also significantly populated. Kinetic and thermodynamic parameters are correlated with those for analogous heterocycles.
13C‐NMR Spectroscopic and Conformational Studies on Rotations in Substituted Ethanes
Authors: Freitag Wolfgang , Schneider Hans‐Jörg
Published: November 2013

Possibilities and results of13C‐DMR line shape analysis are explored with 1X‐2,2‐dimethylpropanes (neopentanes,1), 2X‐3,3‐dimethylbutanes (2) and 2X‐2,3,3‐trimethylbutanes (3) with X = H, Hal, OR or CH3. The observed barriers ΔG* consistently increase in this sequence, e.g. for X = Cl from 6.5 (1) to 7.7 (2) to 10.4 (3) kcal/mol. Molecular mechanics force field calculations for1, although giving barriers too low by 0.5–1.5 kcal/mol, correctly reproduce the observed trends and explain the puzzling low barriers observed for X = CH3in comparison to X = Hal, as the result of the higher ground state energy of the methyl compounds. Conformational equilibria are investigated with the non‐topomeric 2X‐2,3‐dimethylbutanes. The ΔG0values obtained from low temperature signal integration agree with force field predictions only for X = H and F, which dominate, as X = OR does, in the all‐gauche conformation; in contrast, the predominance of the trans conformer for X = Cl, Br and J is not rationalized. The calculated ground state geometries show no significant geometry distortion upon change of the substituents X, which provides a basis for the13C shielding comparison to corresponding cyclohexanes of similar substitution pattern. The observed substituent effects on the13C shifts are in agreement with other models and mechanistic proposals for the screening constant variations.
1H‐NMR Conformational Study of 2‐Acyl‐3,5‐dialkylfurans Using Lanthanide Shift Reagents
Authors: Balaban Alexandru T. , Gheorghiu Mircea D. , Draghici Constantin
Published: November 2013

Pyrylium salts undergo oxidative ring contraction to 2‐acyl‐3,5‐dialkylfurans II. A systematic 1H‐NMR room‐temperature study was made for compounds II a‐e having methyl and/or ethyl substituents, using Eu(fod), as lanthanide shift reagent. The conformation always is trans (IIB), minimizing steric interactions. Calculations also indicate a preferred planar conformation IIC for the 5‐ethyl group. The corresponding oximes III were also studied similarly, but the effects are too complex to be understood at the present stage.
Peptide Conformation, IX Conformational Studies on Cyclotripeptides Containing N‐Benzyl‐Glycine
Authors: Kessler Horst , Krämer Peter , Krack Gerhard
Published: November 2013

Synthesis and NMR spectra of the cyclic tripeptides of the general structure cyclo[ProxBzl.Gly3‐x] (x = 0–2; Bzl.Gly. = N‐Benzylglycyl) are reported. Two slowly interconverting conformations of these tripeptides containing only cis peptide bonds have been observed in solution by NMR spectroscopy. Structural proof and assignments to a flexible boat conformation and a rigid crown conformation, respectively, have been achieved by interpretation of the NMR parameters and the correlation with the crystal structure of one compound. The conformational behaviour of cyclic tripeptides is discussed.
Understanding Benzannulenes
Authors: Mitchell Reginald H.
Published: November 2013

The effect on NMR data when annulenes are benzannelated is discussed for forty‐three compounds. It is shown that benzannelation reduces the diatropicity or paratropicity of the macrocyclic system and the reasons for this in terms of increased bond localisations in the macrocylic ring are indicated. Examples are discussed where no decrease in diatropicity is found to occur on dibenz (naphth)annelation, and these are shown to be consistent with no change in bond orders.
Exocyclic Charge Flight from Cyclopropenide Systems as a Criterion of Antiaromaticity
Authors: Mitchell R. David , Bauld Nathan L.
Published: November 2013

The unusually extensive exocyclic charge flight (from cyclopropenyl to organo moiety) proposed for the cyanocyclopropenide ion is affirmed by MINDO/3 calculation. Nevertheless, the MINDO/3 wave function for this ion is shown not to be absolutely antiaromatic. The basis for exocyclic charge flight is shown, using the “de‐perturbation” approach, to be the small (cyclopropenide) HOMO/ (cyano) LUMO gap.
Evaluation of Strain Effects on the Reactivity of Small Rings
Authors: Stirling Charles J. M.
Published: November 2013

The qualitative effect of ring strain on the reactivity of small ring systems is briefly reviewed. It is pointed out that nucleofugalities of groups cannot be measured for nucleophilic substitution but that such measurements are possible for activated eliminations. Application of the techniques for determination of nucleofugality to eliminative fission of small strained rings shows that nucleofugality is very greatly enhanced. The extent of enhancement cannot be determined for epoxides, but has been determined for cyclopropanes, the results showing that about 60% of the ring strain energy is released in the transition state of ring fission. Catalysis of ring fission requires very specific placement of proton donor groups; gem‐dimethyl substitution raises the enthalpy of the transition state for eliminative fission of cyclopropanes.
The Nature of Sulfur Monoxide from Thiirane Oxide: A Stereochemical Test
Authors: Aalbersberg W. G. L. , Vollhardt K. P. C.
Published: November 2013

In order to study the stereochemistry of thiirane oxide decomposition, syn‐ and anti‐2,3‐dideuterothiirane‐1‐oxides and trans‐2,3‐dideutero‐thiirane‐1‐oxide have been synthesized in three steps from cis‐ and trans‐dideuteroethylene via stereospecific formation of the corresponding oxiranes and thiiranes. The presence of the deuterium label was used so as not to perturb the course of the decomposition from that of the parent thiirane oxide. Pyrolysis of the labeled thiirane oxides in both gas and solution phase proceeded with greater than 90% retention of stereochemistry. Analyses were performed by comparison of the infrared spectra of the product mixtures with standard calibrated samples. Attempts to trap possible biradical intermediates with di(p‐anisyl)thioketone gave a low yield of products, possibly derived from addition to thiirane oxide. These results suggest a possible partial contribution of a biradical mechanism to the thiirane oxide decomposition. If bond rotation in the biradical is slow, this mechanism could be the sole mode of sulfur monoxide extrusion.
Strained Mono‐ and Dications. On the Structures of [(CCH2)7H]2⊕ and [(CCH3)7CH3]2⊕ Dications
Authors: Carnadi H. , Giordano C. , Heldeweg R. F. , Hogeveen H. , van Kruchten E. M. G. A.
Published: November 2013

The reactions of alcohol 1 with HF‐SbF5 and of alcohol 8 with FSO3H‐SbF5 at low temperatures afforded dications [(CCH3)7H]2⊕ and [(CCH3)7CH3]2⊕, respectively. Both NMR (1H and 13C) data and analysis of the products, obtained upon quenching of the cations, were used to discuss the possible structures of these cations. Both ions showed a remarkable NMR behaviour, viz. an equivalency of all methylgroups on the one hand and all skeleton carbon atoms on the other hand, indicating a high symmetry or a rapid (on the NMR time scale) process. Using model compounds the weighted average 13C‐NMR chemical shifts of reasonable structures for the ions were calculated and compared with the experimentally obtained values. The conclusion is drawn that the former reaction presumably leads to dications 34 and 36 and the latter reaction to dication 51.
Electron Spin Polarization in Photosynthetic Models: Chlorophyll a Mediated Photoreduction of 1,4‐Benzoquinone
Authors: Warden Joseph T.
Published: November 2013

The observation of Chemically Induced Dynamic Electron Spin Polarization (CIDEP) is reported for the chlorophyll a cation radical and 1,4‐benzosemiquinone anion generated by laser flash photolysis of ethanolic solutions of chlorophyll a and benzoquinone. The signs of the polarizations exhibited by the radicals are consistent with the predominant operation of the Triplet Mechanism for CIDEP. Chemical decay for the photoinduced radicals occurs by recombination via reverse electron transfer (Kt ∼ 2.9×0.8×109 M−1s−1).
Energetics of Light‐Induced Charge Separation Across Membranes
Authors: Warshel Arieh
Published: November 2013

The efficiency of light induced electron transfer across membranes is examined on a molecular level. The activation energies that determine this efficiency are evaluated by pure electrostatic considerations in terms of the membrane dielectric and the relaxation of the dielectric at the sites of the electron donors and acceptors. These considerations allow one, for the first time, to assess the efficiency of photosynthetic systems in terms of their molecular components. It is demonstrated that a conduction chain of stacked acceptors immersed in a low dielectric membrane cannot be expected to provide an efficient photosynthetic system. On the other hand, efficient photosynthetic systems can be obtained when the acceptors are placed in polar active sites of proteins and arranged in order of increasing redox potentials.
Photolysis of Saturated Four‐Membered Rings with Two Exocyclic Double Bonds: A Photoextrusion Route to (Hetero)Methylenecyclopropanes
Authors: Quast Helmut , Fuss Andreas
Published: November 2013

The photochemistry of cyclobutanes and saturated four‐membered heterocycles with two exocyclic double bonds (except bisalkylidenecyclobutanes) is reviewed. Emphasis is laid on photoproducts rather than on mechanism, with special regard for the formation of (hetero)methylenecyclopropanes via photoextrusion of small stable molecules.
Alternative Modes of Addition of Electrophilic Olefins to the Vinylcyclopropane Systems in 2,2‐Ethano‐1‐Methylene‐1,2,3,4‐Tetrahydronaphthalene and 1,1‐Ethano‐2‐Methylene‐1,2,3,4‐Tetrahydronaphthalene
Authors: Sarel Shalom , Felzenstein Abraham‐Meir , Weisz Moshe
Published: November 2013

The title compounds 2,2‐ethano‐1‐methylene and 1,1‐ethano‐2‐methylene‐1,2,3,4‐tetrahydronaphthalene (13 and 14 respectively) were synthesized by the Wittig reaction from the corresponding ethano‐tetralones (16) and (19) which were, in turn, prepared from the respective α‐ and β‐tetralones (15) and (18). It is shown that the reaction of (13) with TCNE at 0° yields the corresponding [2 + 2] cycloaddition adduct (20), isomerizing thermally to the formal [2 + 5] cycloaddition adduct (21). Compound (13) reacts with N‐tosylisocyanate (TIC) and with N‐phenylsulfonylisocyanate (PSI) to a 4:1‐isomeric mixture of (Z)‐ and (E)‐acryloamides (24) and (26), respectively. The reaction of (14) with TCNE at 0° leads to the [2 + 5] cycloadduct (30), while with (TIC) and (PSI) the corresponding tricyclic adducts (28a) and (28b) result. Bromination of (16) with NBS yielded the respective 4‐bromo‐2,2‐ethano‐1‐tetralone (31) which gave, upon dehydrobromination with the aid of DBU, the naphtho‐dihydrofurane derivative (34). The latter was formed most likely via facile cyclopropylketone upon dihydrofurane rearrangement of an initially formed 2,2‐ethano‐1‐oxo‐1,2‐dihydronaphthalene (32) which was desired for the formation of (33). Structures of all new compounds were assigned on basis of analytical and spectroscopic data (IR, UV, mass spectra, 1H‐NMR).
The Cycloaddition of Vinylaziridines with Unsaturated Substrates
Authors: Hassner Alfred , Chau William , D'Costa Rosario
Published: November 2013

Vinylaziridines undergo ring expansion reactions which in case of divinylaziridines lead to dihydroazepines. Addition of 2‐phenyl‐2‐vinyl‐aziridine 10a to dimethyl acetylene dicarboxylate provides a facile method of in situ generation of a divinylaziridine and leads to isolation of an azepine derivative. Likewise we show that cycloaddition of 10a proceeds even with olefinic nitriles, sulfones and esters with formation of tetrahydroazepines. However, when the substrates are unsaturated nitro compounds or ketones, a novel rearrangement to open chain enamine derivatives takes place. Deuterium labelling in the β‐nitrostyrene substrate and in the vinylaziridine provide evidence for a retro‐ene reaction mechanism leading to the rearrangement. These data provide a framework for assessing the factors that guide the reactions either towards formation of seven‐membered ring azepine derivatives or toward open‐chain rearranged products.
New Photosensitizers of the Ruthenium‐Polypyridine Family for the Water Splitting Reaction
Authors: Juris Alberto , Barigelletti Francesco , Balzani Vincenzo , Belser Peter , Von Zelewsky Alex
Published: November 2013

The photophysical and electron transfer properties of the following Ru(II) complexes have been characterized: Ru(bpy)3‐n (biq)2+n, Ru(bpy)2(Cl2‐bpy)2+, Ru(Cl2‐bpy)2+3, Ru(bpy)2(NO2‐bpy)2+, Ru(NO2‐bpy)2+3, Ru(bpy)2(opdi)2+, Ru(bpy)2(phedi)2+, where n = 1 to 3, bpy = 2,2′‐bipyridine, biq = 2,2′‐biquinoline, and opdi and phedi are o‐quinodiimine ligands. The properties of these and other Ru‐polypyridine type photosensitizers are discussed on the basis of the requirements needed to split water with solar energy in an efficient way. It is shown that the Ru(bpy)2(LL)2+ complexes, where LL is a polypyridine‐type ligand which is easier to reduce than bpy, are expected to be more efficient photosensitizers than Ru(bpy)2+3.
The Potentiality of Chromium(III)‐Polypyridyl Complexes Toward Use in Photochemical Solar Energy Conversion
Authors: Hoffman Morton Z. , Serpone Nick
Published: November 2013

The properties of polypyridyl complexes of Cr(III), Cr(NN)3+3, are examined with regard to the photochemical determinants of efficient solar energy utilization and compared with those of corresponding complexes of Ru(II), Fe(II), Os(II), Rh(III), and Ir(III). The following characteristics are discussed: absorption spectra, nature and lifetimes of excited states, intersystem crossing efficiency between lowest spin‐allowed and spin‐forbidden excited states, energetics and kinetics of excited‐state redox quenching, and the properties of redox‐generated energy‐rich charge‐carriers. Recent results dealing with the effect of solution medium on intersystem crossing efficiency and excited‐state lifetimes are presented. Cr(NN)3+3 complexes have been applied to solar energy conversion schemes and a brief review and critique are offered. The advantages and disadvantages of Cr(NN)3+3 as solar energy harvesters and electron‐transfer mediators are discussed in terms of future research directions.
Photogeneration of Hydrogen from Polymeric Viologen Systems
Authors: Lee P. C. , Matheson M. S. , Meisel D.
Published: November 2013

Polymers containing pendant viologen groups were prepared and studied for their quenching efficiencies of the Ru(bpy)2+3excited state, their back reactions following the quenching, their viologen radical yields, and finally for their effectiveness in hydrogen production in the presence of colloidal platinum. Results are compared with those from methyl viologen containing systems to evaluate the performance of the polymeric viologen systems.
An Excited State Photoelectrochemical Cell for the Production of O2 Based on Oxidative Quenching of Ru(bpy)2+3
Authors: Dressick Walter J. , Meyer Thomas J. , Durham Bill
Published: November 2013

A description is given of an excited state photoelectrochemical cell in which the excited state, Ru(bpy)2+*3 (bpy is 2,2′‐bipyridine) is quenched via irreversible electron transfer to Co(NH35Cl+2 which gives Ru(bpy)3+3. The oxidizing equivalents in Ru(bpy)3+3 are transferred via an external circuit to a second half‐cell in which OH− is oxidized to O2. The variation of photogenerated charge and limiting currents with changes in [Co(NH3)5Cl+2], time, and other variables are presented in the context of a kinetic model for the cell. Conditions are also described for the operation of the cell as an actinometer which is useful for the wavelength range 360 nm ≦ λ ≦ 580 nm and exhibits reproducibility comparable to that observed with conventional chemical actinometers.
The Mechanism of Photocatalysis on Semiconductor Powders in a Liquid Medium. New Experimental Methods and Their Results
Authors: Manassen Joost
Published: November 2013

Two new methods are described for following the reactions occurring on semiconductor powders, when illuminated in a solution of reactants. The reaction between nitroaromatics and hydrazine gives gaseous nitrogen as one of the products, which can be measured manometrically. The rate with CdSe as the catalyst is strongly dependent on the structure of the nitroaromatic, giving faster reactions for the better electron acceptors. The more polar the solvents the lower the yields. The reaction between K4Fe(CN)6 and air in aqueous solution is measured by electrochemical reduction of the product K3Fe(CN)6. Rates are strongly dependent on heat treatments given to the semiconductor before reaction and the dependence of rate on CdSe heat treatment is very similar for both the nitrobenzene/hydrazine reaction in methanol and the K4Fe(CN)6 oxidation in water. Electrochemical measurements with CdSe electrodes in benzene show the current onset to be at a potential similar to the first reduction potential of the nitroaromatics used, which explains the strong rate dependence on the reduction potential of the nitroaromatic. A mechanism is proposed for the n‐type case of a rapid reaction occurring between the minority carriers and hydrazine, by which the powder charges up negatively and the electron transfer from the charged powder to the nitroaromatic in solution is rate determining. With p‐type semiconductors, which give slower rates, the minority carrier reaction with the nitroaromatics is rate determining and only little positive charge accumulates on the particles.
Origin of Thermal Analysis
Authors: Mackenzie R. C.
Published: November 2013

International acceptance of a definition of “thermal analysis” should enable the originator of studies in this field to be located. A search of the literature suggests that the first to use a technique that could be so classified was Jakob Fredrik Emanuel Rudberg (1800–1839) in Sweden, who observed inverse cooling‐rate phenomena in 1829. He was followed by Moritz Ludwig Frankenheim (1801–1869) in Germany, who determined cooling curves in 1837, by James Ballantyne Hannay (1855–1931) in Scotland, who introduced mass‐change curves in 1877, and by Henry Louis Le Chatelier (1850–1936) in France, who devised quasi‐isothermal heating curves and heating‐rate curves in 1883 and 1887. As all of these studies were probably initially independent of the earlier work, perhaps the honour should be shared.
The Stability of Hydrates of Inorganic Salts in Cooling—Heating Cycles
Authors: Feilchenfeld H. , Fuchs J. , Kahana F. , Sarig S.
Published: November 2013

The stability of calcium chloride hexahydrate with an admixture of nucleating agent was tested during 1000 cooling—heating cycles and was found satisfactory. The experimental set‐up based on computer monitored heating and natural cooling in a cold room is described. Addition of lithium chloride decreases phase transition temperature, but affects the stability.
The Formation of Nickel Ferrite: A Thermo‐magnetic Study
Authors: Tzehoval Hannah , Steinberg Menachem
Published: November 2013

The preparation of NiFe2O4 in the solid‐state has been studied by means of thermo‐magnetic and thermo‐gravimetric continuous measurements and by X‐ray diffraction analysis. The method consists of the preparation of a crystallized organo‐metallic complex (NiFe citrate precursor), which on decomposition at moderate temperatures gave NiFe2O4. The yield of the product depends on the ambient atmosphere during decomposition. Optimal conditions for a high yield of nickel ferrite were pretreatment of NiFe citrate at 250°C in air followed by gradual heating under vacuum to 500°C. This is a relatively low temperature as compared to temperatures previously used in the preparation of NiFe2O4.
An Experimental Review of Thermoanalytical Techniques for the Measurement of the Oxidative Stability of Polyethylene
Authors: Collins L. W. , Wendlandt W. W.
Published: November 2013

Thermoanalytical techniques are commonly used to evaluate the oxidative stability of polyolefin resins. The most common technique for this evaluation is the oxidative induction time (OIT) as measured by DSC. This study compares the OIT values for a standard set of polyethylene samples with stability measurements made using other DSC and TG methods as well as a less common photothermal analysis method. The methods were all shown to give comparable results that could be correlated with oxidative stability. Interpretation of the measurements in terms of the resin properties is a significant problem in all thermoanalytical methods.
Estimation of Ca(OH)2 and Mg(OH)2 — Implications in Cement Chemistry
Authors: Ramachandran V.S.
Published: November 2013

Portland cement, hydrated Portland cement, magnesium oxychloride, magnesium oxysulphate and plasters are inorganic cementitious systems in which different amounts of MgO, CaO, Ca(OH)2 and Mg(OH)2 are present. Identifying and estimating the Ca‐ and Mg‐oxides and hydroxides by DTA provides information that can, in turn, be used for studying the mechanism of hydration of CaO, reactivity of calcium silicates, the relevance of the autoclave test for determining the unsoundness of cements, interactions occurring in polymer‐impregnated cement, the effect of various admixtures on the hydration kinetics of cement and cement compounds, the contribution of Ca(OH)2 to the physico‐mechanical properties of cement, the composition of the hydrated silicates, mechanism of hydration, chemical activity of blended cements, strength development in magnesium‐oxychloride and magnesium‐oxysulphate cements and causes of failure of gypsum plasters.
Study of Cholesterol—Sitosterol Crystallization Mixtures by DTA
Authors: Karpuj Liliana , Garti Nissim , Sarig Sara
Published: November 2013

DTA analysis was used to confirm evidence from X‐ray diffraction studies for the existence of 1:1 mixed sitosterol—cholesterol crystals. The curve of melting point against composition has an inflection point at about 1:1 ratio. The DTA peaks for 1:1 mechanical mixtures and for 1:1 mixed crystals are different with respect to symmetry and width. The curve of the heat of enthalpy against the composition of crystals also changes indicating a change in entropy. This can be attributed to the difference between melting mixed crystals (high order) and solid solution (low order). The solubility curve in the sitosterol—cholesterol—alcohol system has also a region of inflection confirming the previous conclusions.
Helium Diffraction as a Probe of Surface Structure
Authors: Engel Thomas
Published: November 2013

The diffraction of He atoms from solid surfaces has been recently used to determine the electron distribution of electrons at surfaces. From this, the position of underlying atoms can usually be inferred. Recent developments which test the validity of the hard corrugated wall method to calculate diffraction intensities are discussed. Structural studies which have been carried out to date, on various surfaces with He beams, are tabulated.
Studies of Structure and Dynamics in Heterogeneous Reactions
Authors: Bernasek Steven L.
Published: November 2013

The combination of methods from surface chemical physics with methods from studies of molecular reaction dynamics can provide a detailed understanding of the dynamics of heterogeneous reactions. Examples from work in the author's laboratory are used to illustrate this approach to the study of gas—solid reactions.
Physical Interactions Between Molecules and Supported Catalysts
Authors: Gersten Joel I.
Published: November 2013

Analytic expressions for the interaction potential of a molecule with a supported catalyst are obtained. Particular attention is focused on the region of the boundary line between the metal particle and the substrate.
The Electrostatic Potential Above the 100 Surface Plane of a f.c.c. Ionic Crystal with a Continuous Electron Chargè Distribution
Authors: Ephraim A. Ben , Folman M.
Published: November 2013

The electrostatic potential above the 100 surface plane of a f.c.c. ionic crystal has been calculated using a point charge distribution for the positive nuclei and a continuous charge density distribution for electrons. The continuous electron charge distribution has been obtained from the known results of a Fourier crystal structure analysis. The results for the electric field and its different gradients are compared with those obtained from the Lennard‐Jones and Dent model (point charge distributions for both the positive and negative ions). It has been found that the two models lead to significant differences of the electrostatic potentials and their derivatives.
Electronic Structure of Ion‐Radical Organic Solids and Polyenes
Authors: Soos Zoltán G.
Published: November 2013

The electronic structures of organic ion‐radical salts, molecular conductors, and charge‐transfer complexes are described by solid‐state models with one valence state per site. Similar models occur in π‐electron theories of conjugated molecules or in spin Hamiltonians for magnetic insulators. A localized or valence bond (VB) representation is qualitatively adequate in narrow band systems. Diagrammatic VB methods in finite models yield convenient exact solutions to the resulting configuration interaction (CI) problem as correlated states based on linear combinations of VB diagrams. Also found are exact charges, bond orders, ionicity, correlation functions, transition moments or other properties of correlated electronic states. Particular attention is given to qualitative failures of conventional one‐electron descriptions, which often reflect degeneracies lifted by correlations. Correlated states give the correct ordering of the 21Ag and 11Bu states of trans‐trans octatetraene, clarify the midgap absorption in polyacetylene, rationalize the partial ionicity and electrostatic energy of ion‐radical organic complexes, demonstrate an exactly soluble magnetic analog to the Mott transition, and quantitatively fit the thermodynamics of several random‐exchange Heisenberg antiferromagnets. Correlated states remain central in simultaneously modeling optical, magnetic, and electrical excitations in π‐radical solids.
Molecular Structures of Linear Polypyrrolic Pigments
Authors: Sheldrick W. S.
Published: November 2013

The results of X‐ray structural analyses of open‐chain polypyrrolic compounds are reviewed. Particular emphasis is placed on bilirubin and biliverdin derivatives and on the relevance of their observed conformations to the biological function and properties of natural bile pigments. Typical for bilirubin derivatives is a ridge‐tile conformation stabilised by six intramolecular hydrogen bonds, in which the individual pyrromethenone units are Z‐synperiplanar. Macrocyclic helical conformations are adopted by biliverdin derivatives and model bilatrienes‐abc. The bonding pattern and degree of bond delocalisation within the tetrapyrrole skeleton are discussed in comparison to the molecular geometries of model bi‐ and tripyrrolic compounds. The results indicate that the methine bridge at C5 may demonstrate considerable conformational flexibility in bilatrienes‐abc, whereas a marked degree of bond delocalisation is observed for the B, C and D rings. Protonation or metal complexation leads to pronounced changes in the degrees of stretch and widening of the helix and to corresponding changes in the electronic spectrum.
NMR Spectra of Molecules Oriented in Mixtures of Thermotropic Solvents of Opposite Diamagnetic Anisotropies
Authors: Khetrapal C. L. , Kunwar A. C.
Published: November 2013

Theory and applications of NMR spectra of molecules oriented in mixed liquid crystals of opposite diamagnetic anisotropies are presented. The theoretical treatment is based on the Landau theory. The applications include a novel method for the determination of chemical shift anisotropy, individual determination of the indirect spin‐spin and direct dipole–dipole couplings, determination of diamagnetic anisotropy of liquid crystals and precise determination of spectral parameters.
Alkyl Chain Flexibility in Liquid Crystals
Authors: Samulski Edward T.
Published: November 2013

Deuterium NMR results from labeled chains appended to nematogens and discotigens, chains linking mesogenic units in linear polymeric nematics and simplen‐alkanes and lipids solubilized in a nematic solvent are examined. A phenomenological single chain model is used as a vehicle for contrasting alkyl chain flexibility in various ordered phases. Agreement between the model and experiment suggests a commonality in the behavior of all of these types of alkyl chains originating from a repulsive (steric) constraint that is imposed on the isomerization of the chain by the local ordered environment.