bluuebunny/crossref_metadata_2025_split huggingface dataset:
Multielectron Transfer Across the Metal Electrolyte Interface in Electrochemical Systems
Authors: Efrima S. 
Published: November 2013

AbstractThe theory for the rate of multielectron transfer across the metal electrolyte interface in electrochemical systems is developed. The two‐electron case is specifically considered and compared to the one‐electron case. It is found that the two‐electron path may become more important than the parallel one‐electron channel under certain well‐specified circumstances.
Theoretical Study on the Origin of the Behavior of Ketones and Thiones in Hydrogen Photoabstraction Reactions
Authors: Bigot B. 
Published: November 2013

AbstractNatural correlation diagrams have been established to describe hydrogen photoabstraction in ketones and thiones for three different reaction paths (nx, πx and πc approaches). Their conclusions have been confirmed by ab initio SCF‐CI calculations. This procedure leads to the following conclusions: Energy barriers exist on the potential energy surfaces of all the potentially reactive excited states. The height of these barriers is strongly dependent on the distance separating the hydrogen acceptor and hydrogen donor atoms in the transition state. For the n approach, the reactive channel only involves nπ* excited states while, for the π approach, nπ* (hydrogen capture by the carbon atom) and ππ* (hydrogen capture by the oxygen or the carbon atom) excited states must also be taken into consideration. A proposal for resolving the ketone‐thione dichotomy is presented on the basis of these results.
Torsional Barriers Resulting from Two‐electron and Four‐electron Interactions. The Hydrazyl Cation and Anion Models
Authors: Kost D. , Aviram K. , Raban M. 
Published: November 2013

AbstractThe spectrum of rotations about single bonds has been divided into two different groups of processes, termed TA and TC. These describe interconversion of planar ground state conformations (as in amides), and interconversion of enantiomers (as in sulfenamides and hydroxylamines), respectively. A frontier molecular orbital analysis of the hydrazyl cation and anion reveals that a change in electron occupancy alone can bring about a change from one torsional process to the other. Thus, the four‐electron hydrazyl anion undergoes a TC process, while the corresponding two‐electron cation undergoes a TA rotation. Nonempirical SCF‐MO calculations of the model cation and anion with geometry optimization at the 4‐31G level confirm the results of the PMO analysis. The calculations also reveal a preference for an inversion (IA) mechanism for the topomerization of the cation system over a torsional process.
On the Electronic Origins of a Novel Transoid Structure of 1,3‐Diene‐zirconocene Complexes
Authors: Tatsumi Kazuyuki , Yasuda Hajime , Nakamura Akira 
Published: November 2013

AbstractCoordinations of conjugated 1,3‐dienes to a transition metal had been considered to occur only in a s‐cis geometry, until the recent discovery of Cp2Zr(s‐trans‐diene), diene = butadiene and trans, trans‐1,4‐diphenylbutadiene. A molecular orbital analysis of metal‐butadiene interactions is carried out, for the specific cases of Fe(CO)3 (butadiene) and Cp2Zr(butadiene), to discern the electronic origin of the novel transoid geometry. Stability of the s‐cis and s‐trans structures of Cp2Zr(butadiene) is found to be well balanced and we show how the geometrical choice is affected by substitution on butadiene.
Pyrrole Exchange Reactions in the Bilirubin Series
Authors: Bonnett Raymond , Buckley Dennis G. , Hamzetash Dariush , McDonagh Antony F. 
Published: November 2013

AbstractAcid treatment of a mixture of bilirubin IXα (1) and octaethyl‐10,23‐dihydro‐21H,24H‐bilin‐1,19‐dione (7), followed by dehydrogenation and esterification gives biliverdin IXα, biliverdin XIIIα and biliverdin IIIα (as the dimethyl esters, 2, 3, 4, respectively), and octaethylbilindione (8), together with two minor verdins of intermediate polarity. These are identified as the products of the crossed pyrrole exchange reaction, which are 8‐carboxyethyl‐12,13,17,18‐tetraethyl‐2, 7‐dimethyl‐3‐vinyl‐21H,24H‐bilin‐1,19‐dione methyl ester (9) and 8‐carboxyethyl‐12,13,17,18‐tetra‐ethyl‐3,7‐dimethyl‐2‐vinyl‐21H,24H‐bilin‐1,19‐dione methyl ester (10). An improved preparation of bilirubin XIIIα (5) and bilirubin IIIα (6) is described and the products are crystallised and characterised. The NMR spectra of bilirubin XIIIα and IIIα are recorded in a polar aprotic solvent (DMSO‐d6) at 400 MHz, and certain unusual features are discussed.
Hydrogen Bonding of Bilirubins and Pyrromethenones in Solution
Authors: Trull Francesc R. , Ma Jin‐Shi , Landen George L. , Lightner David A. 
Published: November 2013

AbstractAnalysis of bilirubin and pyrromethenone N—H 1H‐NMR chemical shifts indicates that in CDCl3 solvent, where intermolecularly H‐bonded dimers or intramolecularly H‐bonded conformations are involved, the lactam N—H groups are more deshielded than the broader pyrrole N—H groups. In d6‐DMSO, disaggregation and unfolding occurs with H‐bonding to the solvent and the sharper lactam N—H resonances becoming more shielded than the pyrrole N—H signals.
Isophorcarubin — A Conformationally Restricted and Highly Fluorescent Bilirubin
Authors: Kufer W. , Scheer H. , Holzwarth A. R. 
Published: November 2013

AbstractThe biliverdins 2b, 5 and 1 have been reduced with NaBH4 to the bilirubins 8, 7 and 6, respectively. The conformation of these pigments is increasingly extended and the flexibility is increasingly restricted in the sequence given above, which does not influence the ease of reduction. The fluorescence of 6 and 7 has been studied at ambient temperature and 77 K in ethanol and 2‐methyltetrahydrofuran. 6 has an unusually high fluorescence yield which has been related to the conformational rigidity of its ring C, D chromophore. Both 6 and 7 show evidence of solvent and temperature dependent inhomogeneities in their fluorescence spectra.
Potassium‐39 Nuclear Magnetic Resonance Studies of Counterion Binding in Lyotropic Liquid Crystals
Authors: Boden N. , Jones S. A. 
Published: November 2013

AbstractThe binding of K+ at the lamella bilayer surface in the lamellar phase of the potassium oleate–water system has been studied by 39K NMR. The effects of temperature, water content and solubilizate (octanol, benzyl alcohol, benzene and cyclohexane) are investigated. A simple two‐site equilibrium model is used to interpret the measurements. The manner in which the solubilizate interacts with and perturbs the structure of the lamella bilayer surface is implicit in the results.
Proton Spin Relaxation Dispersion for Some Nematic Homologues of the Liquid Crystal PAA
Authors: Nagel G. , Wölfel W. , Noack F. 
Published: November 2013

AbstractIn order to elucidate the nature of spin relaxation in nematic liquid crystals in the vicinity of the nematic–isotropic phase transition temperature, we present extensive measurements and a quantitative analysis of the proton spin, T1, relaxation dispersion for some homologues of the PAA series (PAA, PAB, HAB). The studies were made for the nematic state over a broad range of Larmor frequencies. By a detailed inspection of the frequency, temperature and chain length dependences we conclude that in all cases at least three kinds of molecular reorientations with rather different relaxation dispersion contribute to the total T1, i.e., order fluctuations of the director (OFN), self‐diffusion (SD) and an additional mechanism, which we attribute to order parameter fluctuations (OFS). The combined OFN–SD–OFS model extends existing interpretations and allows a quantitative description of the observed phase transition effects. The effects of both OFN and OFS on T1 clearly increase with the molecular length and we deduce from this that not only the ring protons, but also some chain protons undergo these types of fluctuation. Due to the rather high temperature of the mesophases, the diffusional contribution is hard to evaluate exactly and hence there are no significant variations between the homologues within the limits of experimental error. However, a small systematic discrepancy with respect to independent diffusion measurements, available for PAA, indicates that this relaxation model is still incomplete and that a fourth mechanism should be included in the analysis. Possibly, this could be some kind of rotation as previously reported for low‐temperature nematics.
The Ozonolysis of Haloalkenes in Solution
Authors: Gillies Charles W. , Kuczkowski Robert L. 
Published: November 2013

AbstractA review of the ozonolysis of low molecular weight haloalkenes in solution is presented. The reaction products from the fluoroethylene series are discussed in detail. Secondary ozonides can be obtained from these alkenes although the yields decrease markedly with increased fluorination and diminish to negligible quantities with C2F4. Compared to alkylethylenes, a greater variety of products are obtained including some epoxides and fluorocyclopropanes. Yield data are supplemented by isotopic labeling results to elucidate the ozonolysis mechanism. The evidence indicates that the final ozonides are formed by the three step Criegee reaction mechanism although the stereoselectivity of the reaction differs from nonhaloalkenes.
Oxy‐Radicals and the Radiobiological Oxygen Effect
Authors: Greenstock Clive L. 
Published: November 2013

AbstractThe radiobiological oxygen effect can be separated into its biological and chemical components, using time‐resolved or kinetic studies. Rapid mix techniques show that the full oxygen enhancement ratio (OER) for radiation‐induced cell killing is expressed when oxygen is present or admitted 40 ms prior to irradiation. This predominant chemical action of oxygen, including direct and indirect action, is too fast to implicate processes other than those involving free radical species. The two principal hypotheses, “oxygen fixation” and “active oxygen”, are discussed. Pulse radiolysis studies of model systems provide kinetic data to indicate the principal damaging species and radiation targets, the kinetics and sequence of damaging events, and the role of redox processes involving oxygen, oxy‐radicals and radiation modifiers. Complementary steady‐state radiolysis studies enable the nature and extent of the physico‐chemical damage to be quantified. Mechanistic aspects of the radiobiological oxygen effect are inferred from in vitro studies of irradiated mammalian cells, comparing the different biological end‐points with specific forms of chemical damage, under a variety of physical, chemical and biological conditions.While this review stresses the role of oxygen in sensitizing the critical cellular target DNA, responsible for the acute biological effect of cell killing, it must be remembered that for cancer and other chronic effects, having a long latent period between radiation exposure and the biological expression of deleterious damage, there may not be an oxygen effect or if there is, it may be quite different in terms of the damaging species, principal radiation targets, and the sequence and kinetics of the damaging events. Although oxygen and oxy‐radicals may be involved in the initiation and development of cancer, their overall roles in influencing the chronic effects of radiation may be complex and dependent upon cellular and multicellular factors, making it difficult to unambiguously elucidate possible mechanisms.
Conjugate Addition of Phosphine‐Complexed Organocopper Reagents to α,β‐Unsaturated Ketones
Authors: Suzuki M. , Suzuki T. , Kawagishi T. , Morita Y. , Noyori R. 
Published: November 2013

AbstractOrganocopper reagents formed from equimolar amounts of copper(I) iodide and organolithiums and 2 to 3 equiv of tri‐n‐butylphosphine undergo smooth reaction with α,β‐unsaturated ketones, and after aqueous quenching afford β‐substituted ketones in high yields. This method is economical (enone/organolithium=1:1.05 to 1:1.25) and applicable to a wide variety of entering groups and enone substrates, and compares well with the existing procedures.
Infrared Multiphoton Induced Isomerization of cis‐3,4‐Dichlorocyclobutene. III. Pressure Dependence of the Yield
Authors: Ringwelski Andrzej Z. , Presser Nathan , Gordon Robert J. 
Published: November 2013

AbstractThe multiphoton induced isomerization of cis‐3,4‐dichlorocyclobutene (DCCB) was studied as a function of pressure. Three experiments indicated the existence of a nonthermal mechanism at neat pressures below 60 mtorr. These were: (i) the leveling off of the fractional yield of isomerization products at low pressures; (ii) the absence of dissociation products from 2‐iodo‐2‐methyl propane, which served as a chemical thermometer in the presence of a large excess of DCCB; and (iii) a break in the power‐law dependence of the optoacoustic signal produced by neat DCCB. The increased yield at higher DCCB pressures is believed to be due partly to a thermal reaction and partly to increased absorption caused by rotational or vibrational hole filling. Optoacoustic measurements also showed that at constant fluence the absorbed energy was greater for long, low intensity pulses, as might be expected for inhomogeneous excitation. However, the present experiments did not reveal an intensity dependence of the reaction yield. Addition of argon as a buffer gas monotonically decreased the yield, indicating that vibrational quenching more than offsets any hole‐filling effects.
The Preparation and Kinetic Properties of Fully‐Functionalized Imidazole Vesicles
Authors: Moss Robert A. , Kim Kwang Yoo 
Published: November 2013

AbstractThe 4‐imidazolecarboxylic acid ester of N,N‐dihexadecyl‐N‐methyl‐N‐(β‐hydroxyethyl)ammonium chloride (1) and an analogous monohexadecyldi‐methylammonium ion surfactant (2) were prepared from readily available starting materials. Surfactant (1) formed fully‐functionalized imidazole vesicles which are powerful catalysts for the cleavage of p‐nitrophenyl acetate, p‐nitrophenyl hexanoate and 4‐acetoxy‐3‐nitrobenzenesulfonate. Maximum observed rate constants ([1] = 3.4 × 10−3 M, pH 8.0, 0.01 Tris buffer, μ = 0.01, 25°C) are 2.82, 16.3 and 14.3 s−1, respectively, for the three substrates. These and related kinetic parameters are compared with those of imidazole‐functionalized micellar surfactants, such as 2.
The Borane Adventure — Past, Present and Future
Authors: Brown Herbert C. 
Published: November 2013

AbstractForty‐eight years ago, diborane was a chemical rarity, available only in two laboratories in the world. The requirements of research during World War II led to the discovery of practical synthetic methods of diborane and to the discovery of sodium borohydride. These turned out to be excellent reducing agents in organic chemistry. Exploration of these reducing characteristics led to the discovery of hydroboration. Hydroboration made organoboranes readily available. These organoboranes have proven to be the most versatile intermediates now available for organic synthesis. A recent development makes possible the synthesis of essentially any compound containing a chiral center in both optical isomers in essentially 100% optical purity. Thus this research program has taken boranes from a material available only in trace quantities to a reagent widely used in organic synthesis, greatly assisting the chemist in overcoming synthetic difficulties.
Trivalent and Tetravalent Nickel in Oxime‐Imine‐Amine Coordination
Authors: Chakravorty Animesh 
Published: November 2013

AbstractModel complexes of nickel(III) and nickel(IV) are furnished by hexadentate and tridentate oxime‐imine‐amine ligands derived by the condensation of biacetyl monoxime with amines. The synthesis, characterisation, structure, redox equilibria and reactivity of these complexes are critically reviewed. Correlations and generalisations pertaining to oxidation state stability, electroprotic equilibria and reaction mechanisms are examined. The possible significance of the present complexes in the context of the nickel‐containing methanogenic enzymes is noted.
Kinetic Aspects of Redox Reactions of Nickel(III) Complexes
Authors: Mcauley Alexander , Norman Paul R. 
Published: November 2013

AbstractThe synthesis and characterisation of nickel(III) complexes are reviewed. Kinetic aspects of redox reactions including oxidation by Co(OH2)3+6 and Fe(phen)3+3 are described, as are the reactions involving oxidation of non‐metallic substrates. Reference is made to the evaluation of self‐exchange rates for the Ni(III)L/Ni(II)L couples, especially for systems where both oxidant and reductant retain an octahedral geometry. Using the reversible reaction between Ni(9‐aneN3)3+2 and Ni(10‐aneN3)3+2 (9‐ane = 1,3,7‐triazanonane, 10‐ane = 1,3,7‐triazadecane), data are presented for evaluation of the self‐exchange rate constant for the Ni(10‐aneN3)3+/2+2 couple [(7.5 ± 1) X 103 M−1s−1]. This value is compared with that for similar systems where changes in bond lengths on oxidation have been determined by X‐ray techniques.
Nickel(III) and Copper(III) Complexes with 13‐ and 14‐membered Tetra‐aza Macrocycles. Ring‐size and Medium Effects on the MIII/MII Redox Couple Potentials
Authors: Fabbrizzi Luigi , Licchelli Maurizio , Perotti Angelo , Poggi Antonio , Soresi Silvia 
Published: November 2013

AbstractThe one‐electron oxidation of the NiII and CuII complexes with 13‐ and 14‐membered macrocycles 1 and 2 to produce authentic NiIII and CuIII species has been electrochemically investigated and the E1/2 values associated to the reversible MIII/MII redox couple have been determined by means of the Differential Pulse Voltammetry technique. The 13‐membered ring favours the attainment of the CuIII state, whereas the 14‐membered ring favours the formation of the NiIII complex. This opposite behaviour is satisfactorily interpreted in terms of size of the metal ions and aperture of the macrocyclic cave. Moreover the effect of the concentration of the inert electrolyte (NaClO4, 0.1–7.0 M) on the E1/2(MIII/MII) values has been investigated: the increase of the NaClO4 concentration favours the formation of the CuIII complexes and disfavours the formation of the NiIII species. This behaviour is ascribed to the destabilization of NiIII and CuII complexes induced by perchlorate ion through the formation of hydrogen bonds with water molecules axially coordinated to the metal ions.
Reactivity of Intermediates in the Photochemistry of Copper(II) Macrocyclic Complexes
Authors: Ferraudi G. 
Published: November 2013

AbstractThe reactivity of unstable copper macrocycles which act as intermediates in photochemical transformations of copper(II) complexes with synthetic macrocyclic ligands has been reviewed. Although most of these species were investigated by flash photolysis, a number of pulse radiolytic investigations of related species were also incorporated in this review.
Structural Parameters of Photochromic Dihydroindolizines. X‐ray Analysis and MINDO/3‐Calculations
Authors: Dorweiler C. , Spang P. , Dürr H. , Peters K. , Schnering H.G.V. 
Published: November 2013

AbstractTo elucidate the detailed geometry as well as the bonding situation in the new photochromic spirodihydroindolizines 3 an X‐ray structure analysis of 3a, an analogue of 3, as well as MINDO/3‐calculations for 4, a model for 3, were carried out. There is good agreement between the geometric data obtained by these two methods. The spiro bonds are long, due to spiro linking. The lengths of these bonds cannot be explained by spiro interactions, according to MINDO/3. They result, rather, from an increase of strain due to spiro linking. The longest of these bonds is the C(1)‐C(4) one, which is about 0.01‐0.20 Å longer than a normal C‐C single bond. This is due also to the neighbouring sp3‐hybridized N‐atom. The C(1)‐C(4) bond is therefore the weakest bond in the molecule, and can be broken very easily by UV or visible light, thus explaining the photochromic behaviour of this type of system.
Solid‐State Chemistry of Cyanine‐Type Dyes and the Effect of Two Novel Counterions on Their Crystal Properties
Authors: Etter Margaret C. , Holmes Brian N. , Kress Ruth B. , Filipoich George 
Published: November 2013

AbstractOrganic dyes undergo a variety of solid‐state chemical processes, including intra‐ and intermolecular reactions, gas‐solid reactions, and polymorphic transformations. The properties of dye solids are markedly affected by this chemistry. This paper reviews solid state dye chemistry from the literature and reports in detail the chemistry of two novel cyanine dye salts whose properties are controlled by the nature of their counterions. In cyanine‐oxonol salts, the oxonol counterion is a large planar dye which forms crystalline dye aggregates with cyanine ions. There is a multiplicity of polymorphic forms of these mixed dyes reflecting multiple favorable dye aggregate geometries. The cyanine‐borate salts undergo intermolecular solid‐state reactions. In either large single crystals or dispersions of the latter salts in polymer binders, alkyl transfer from the anion to the chromophore can be induced thermally or photochemically.
Solid State Hydrolysis of 2‐Methyl‐4H‐benzoxazin‐4‐one. Mode of Propagation of the Reaction Process During the Hydrolysis
Authors: Vicens Jacques , Decoret Claude , Royer Jean , Etter Margaret C. 
Published: November 2013

AbstractKinetic studies of hydrolysis and deuteriolysis of microcrystalline 2‐methyl‐4H‐benzoaxazin‐4‐one 1 into N‐acetylanthranilic acid 2 are presented. Hydrolysis of mixtures of microcyrstalline 1 and 2 are given. The results are interpreted and a mechanism is proposed for the solid gas reaction. A theoretical approach of the crystal packing of 1 permits an evaluation of the mode of diffusion of water molecules in the crystal. Both experimental and theoretical results lead to a proposal of the mechanism of water propagation and the hydrolysis process in the solid phase.
Stereochemical Discrimination at Organic Crystal Surfaces 1: The Systems Serine/Threonine and Serine/Allothreonine
Authors: Weissbuch I. , Shimon L.J.W. , Addadi L. , Berkovitch‐Yellin Z. , Weinstein S. , Lahav M. , Leiserowitz L. 
Published: November 2013

AbstractThe stereoselective interactions in aqueous solution between the crystal faces of centrosymmetric (R,S) serine 1 and additives resolved and racemic threonine 2 and allothreonine 3 were studied by four independent techniques: change in crystal habit on growth, anisotropic distribution of occluded additive during the crystal growth, etch pit formation on partial dissolution and atom‐atom potential energy calculations. The effects of the two diastereomers 2 and 3 are radically different due to differences in adsorption: threonine is adsorbed on the {011} crystal faces; the (R) enantiomer is adsorbed at, and eventually occluded through the (011) and (011) crystal faces replacing an (R) serine molecule. By symmetry, (S) threonine is adsorbed and occluded through the enantiotopic (011) and (011) faces replacing (S) serine. In contrast, (R) and (S) allothreonine are both adsorbed primarily at the (100) and (100) faces. The results indicate that the serine moiety of adsorbed threonine and allothreonine assume the same conformation and position as the substrate molecule. The additive is adsorbed only at specific sites such that the Cβ‐CH3 groups emerge from the crystal surface. The observed effects lead in the case of threonine, but not allothreonine, to the assignment of its absolute configuration.