bluuebunny/crossref_metadata_2025 huggingface dataset:
Fast Consecutive Intramolecular Processes Involving Protein and Fe(III) in Ferri‐Cytochrome‐c in Aqueous Solution
Authors: Lichtin N.N. , Ogdan J. , Stein G.
Published: November 2013

The reaction of H atoms (produced by nanosecond pulse radiolysis) with ferri‐cytochrome‐c in aqueous solution results in the second order formation of an absorption spectrum due to the addition of H atoms to the enzyme protein. This spectrum is not specific to the iron moiety. The rate constant of the addition is 1×1010M−1 sec−1. This is followed by two first order intramolecular processes with specific rates of ∼ 1×105 sec−1 and ∼ 2×104 sec−1. In these processes, specific spectra related to the α and β bands of ferro‐cytochrome‐c appear. The results are interpreted to show reduction equivalent transfer through the protein to the iron moiety.
Interaction Between Montmorillonite and Benzidine in Aqueous Solutions II. A General Kinetic Study
Authors: Lahav N. , Raziel S.
Published: November 2013

A kinetic study of the formation of benzidine blue on montmorillonite was carried out by measuring the absorbance of the suspension at 580 nm. A preliminary study of the first stage of the reaction was conducted by stopped flow apparatus. The main change in the light absorbance took place during the first minutes of the reaction, but the reaction proceeded at a very slow rate for hours and days. The reaction rate became slower upon increasing the pH and upon addition of calgon (Na‐hexametaphosphate).
Topochemical Reactions of Monomers with Conjugated Triple‐Bonds VII Mechanism of Transition from Monomer to Polymer Phase During Solid‐State Polymerisation
Authors: Kaiser J. , Wegner G. , Fischer E. W.
Published: November 2013

The lattice‐controlled solid‐state polymerisation of three different modifications of 2,4‐hexadiin‐1,6‐diol‐bis(phenylurethane) was investigated by X‐ray and optical methods. The polymerisation is a homogeneous reaction. The polymer grows in the form of single chains within the crystal of the monomer. The chains extend along a definite crystallographic direction. Monomer and polymer are isomorphous and monomer‐polymer single crystals of various compositions are obtained up to a quantitative conversion in the case of modification I or III. Phase separation into a mesomorphic polymer and oriented monomer phase was observed on annealing partially polymerised single‐crystals of modification II below the transition point to modification III.
New Observations on Tautomerism and Ionization Processes in Hypoxanthines and 6‐Thiopurines
Authors: Lichtenberg D. , Bergmann F. , Neiman Z.
Published: November 2013

Ultraviolet spectra show that hypoxanthine and 6‐thiopurine are present in aqueous solution predominantly as 1H, 9H‐tautomers. However, a 3‐methyl group causes the imidazole ring to assume the 7‐NH form. The sequence of anion formation in hypoxanthine is 9(7) → 1 and in 6‐thiopurine 1 → 9(7). The NMR spectra suggest that the mono‐anion of hypoxanthine is a mixture of tautomers. Protonation always takes place in the imidazole ring, with the exception of 3‐methyl derivatives. In the latter, formation of mono‐cations involves the pyrimidine moiety.
Interaction Between Montmorillonite and Benzidine in Aqueous Solutions III. The Color Reaction in the Air Dry State
Authors: Lahav N.
Published: November 2013

The transformation of the blue color of bentonite‐benzidine pastes to green or yellow‐green upon air drying was studied by means of reflectance measurements and by comparison of the samples under study with color charts. The relative amounts of benzidine and bentonite and the relative humidity had a decisive effect on the color of the air dried films. Hygroscopic compounds such as LiCl and CaCl2 produced results similar to that produced by increasing the relative humidity. The explanation of the color reactions is based on the assumption that the (blue) monovalent and the (yellow) divalent cations coexist in the air dry benzidine‐bentonite films.
The Use of Free Radical Probes in the Study of Mechanisms of Enzyme Inactivation
Authors: Adams G. E. , Redpath J. L. , Bisby R. H. , Cundall R. B.
Published: November 2013

Some inorganic radical‐anions, which have very strong absorption spectra, have some oxidative reaction specificity with amino acids. Combined pulse radiolysis and inactivation studies on the reactions of these radicals with several enzymes lead to identification of amino acid residues essential to enzymic activity. Data are reviewed for the enzymes, lysozyme, ribonuclease, trypsin, α‐chymotrypsin, papain and carboxypeptidase. The reducing radical, CO2−, is more selective than the hydrated electron in its reactions with enzymes and proteins. Results from pulse radiolysis studies in electron‐transfer and other reactions involving CO2− are compared with results from inactivation studies with lysozyme and ribonuclease.
Primary Events in the Radiolysis of Nucleic Acids and their Components
Authors: Scholes G.
Published: November 2013

The radiolysis of dilute aqueous solutions of DNA and its components is discussed, with particular reference to the rates of reaction and the sites of reaction of the primary reactive species, OH, H and e−aq. Recent work on the formation and subsequent reactions of OH− and electron‐adducts of pyrimidines is described.
DNA Lesions and Mammalian Cell Killing: Cause and Effect?
Authors: Elkind M. M. , Ben‐Hur Ehud
Published: November 2013

Although indirect, studies of repair mechanisms afford a useful approach to the questions: Where and what are the sensitive sites in a cell related to division? This approach has been used with mammalian cells based on the fact that sublethal damage must be accumulated to kill a cell and the observation that cells can repair sublethal damage. Actinomycin D was found to inhibit this repair, an observation which justified a focus upon nuclear DNA as the site of damage and repair. Sedimentation studies with alkaline gradients revealed two repair processes. Repair of single‐strand breaks (also found by others) and repair to a complex of DNA, lipid, and protein. In cells sensitized to fluorescent light because of prior growth in the presence of 5‐bromodeoxyuridine, the data suggest that single‐strand breaks is the lethal lesion. In contrast, damage to a DNA‐membrane structure appears a more likely target for X‐ray cell killing.
Ion Transport and Oscillatory Phenomena Across Adsorbed Monolayers at the Amalgam/Water and Mercurous Halide/Water Interfaces
Authors: Masters B. , Miller I. R.
Published: November 2013

This paper presents a method for the study of the transport of ions (K+, Na+, Cs+, Cl−, Br−) across adsorbed monolayers at the respective amalgam/water and mercurous halide/water interfaces. The method involves cathodic formation and anodic stripping (inverse polarography) of amalgams and in the reversed order (anodic formation) of mercury halides. Galvanostatic measurements were performed using a stationary mercury drop electrode, a platinum counter electrode and a Ag/AgCl reference electrode. Phase sensitive ac polarography was employed to determine the differential capacity of the electrical double layer in the presence of adsorbed monolayers. Adsorbed monolayers of octadecyltrimethylammonium and of eicosanyltrimethylammonium ions have different permeabilities towards the various cations; cesium ions are the most permeable. The transport of Cl− and Br− was studied across adsorbed monolayers of long chain quaternary ammonia and carboxylic acids and of polyglutamic acid at the mercurous halide/water interface. The anionic permeability depends on the type of anion, the composition of the monolayer, and the charge density of the monolayer and the interface. Ion transport is correlated with the packing in the monolayer. During the anodic stripping of potassium and sodium amalgams in the presence of octadecyltrimethylammonium chloride, and potassium, sodium and cesium amalgams in the presence of eicosanyltrimethylammonium chloride, sustained oscillations of the electrode potential were observed. The frequency varies from about 0.3 to 1.0 Hz, and depends on the current density, the duration of the anodic stripping, the composition of the monolayer, and the temperature. The amplitude of the oscillations varies from 100 to 500 mV. These oscillations involve the following processes: potential build‐up across the monolayer, monolayer rupture (hole formation), followed by hole closure. The oscillations were detected also by instabilities in the measurements of the differential capacity.
Rectifier Characteristics of BaSo4 Precipitation Membranes
Authors: Katzir‐Katchalsky A. , Hirsch‐Ayalon P. , Michaeli I.
Published: November 2013

The behavior of precipitate‐membranes of BaSO4 in cellophane is discussed in terms of the formation process of the system and the known permeability to Ba++, SO=4 and to water components. It is shown that the available information leads to an interpretation in terms of an electrical bipolar model. The specific case of Ba(OH)2 and H2SO4 solutions separated by the above membrane is discussed in detail. The rectification effect and the non‐zero stationary currents observed are discussed in terms of the flows of H+, OH− and the flow of the chemical reaction of water formation. The logarithmic term derived for the voltage current relationship is shown to be directly linked with the depolarizing effect of water formation, and thus quantitatively related to the kinetic constants of water formation and dissociation.
Inorganic Clays as Possible Prebiotic Peptide Templates
Authors: Paecht‐Horowitz Mella
Published: November 2013

Experiments on copolymerizations of various pairs of amino acid adenylates in the presence of montmorillonite are described. Only amino acids small enough to be able to enter in between the layers of the clay are able to polymerize. Those amino acids which do so, copolymerize according to certain preferences.
Quantum Mechanical Consistent Force Field (QCFF/PI) Method: Calculations of Energies, Conformations and Vibronic Interactions of Ground and Excited States of Conjugated Molecules
Authors: Warshel A.
Published: November 2013

The extension of the Consistent Force Field approach to π electron systems is outlined. The quantum mechanical energy of π electrons is given as an analytical function of the molecular coordinates. It is shown that with this analytical description one can calculate efficiently equilibrium geometries and vibrational frequencies of ground and excited states of conjugated molecules. Application of the calculated geometry and vibrations to the analysis of vibronic structure is described. A preliminary account of the use of observed vibronic structure for determination of the geometry of excited electronic states is given.
Analysis of Conformations of Amino Acid Residues and Prediction of Backbone Topography in Proteins
Authors: Burgess A. W. , Ponnuswamy P. K. , Scheraga H. A.
Published: November 2013

Methods for describing a discrete number of conformational states of amino acid residues in proteins are presented and used to investigate the topography of chain folding. The relative importance of short‐range, medium‐range and long‐range interactions is discussed in the light of an analysis of the conformational states for the different amino acid residues in eight proteins of known structure. A prediction algorithm, which assigns four states to each residue of a protein chain (α‐helix, extended structure, bend, or coil), has been developed from a consideration of both short‐ and medium‐range interactions and applied to thirteen proteins of known three‐dimensional structure. The prediction algorithm is simple to apply, and the assignment of α‐helix and extended structure is considerably better than in most other predictive schemes. The prediction of chain reversal or bend regions was also better than with previous algorithms, but these assignments were not as good as those for α‐helix and extended structure. The motivation for the development of this algorithm is not only to demonstrate the relative importance of short‐ and longer‐range interactions but, more important, to begin to develop procedures for obtaining an approximate starting conformation for subsequent energy minimization to predict the three‐dimensional structure of a protein. This procedure, as well as various other methods for the prediction of the backbone topography and conformational states of residues in proteins from the amino acid sequence, have been reviewed and evaluated by comparing the success of the methods to the success expected from a random assignment of conformational states.
Salicylates Attached to Polymers
Authors: Weiner Ben‐Zion , Havron Abraham , Zilkha Albert
Published: November 2013

Several salicylate derivatives were attached to low molecular weight polyethylene oxides through an ether or carbonate linkage utilizing the respective halide or chlorocarbonate derivative of the polymers as reactive intermediates. Aspirin and 0‐salicoyl salicylic acid were attached to polyvinyl alcohol and cellulose acetate by ester linkage through melt esterification of the reactants. Preliminary pharmacological evaluation of some of the polymeric salicylates showed that they have a longer duration of antiinflammatory and analgesic activities as compared with aspirin.
Sulfonation of Polyamides
Authors: Vaya Jacob , Zilkha Albert
Published: November 2013

The sulfonation of polyamides (Nylons‐3,6,11 and 6,6) was investigated using fuming sulfuric acid and solutions of SO3 in CCl4. The sulfonated polymers were soluble in water and insoluble in ethanol. Acid hydrolysis of the polymers and quantitative determination of the amino acids in the hydrolysate showed the presence of α‐sulfo amino acids. This showed that sulfonation in the polymer occurred in the active α‐position of the amide group.
Asymmetric Reduction of Ketones with NaBH4 in the Presence of Lecithin
Authors: Doiuchi Tohru , Minoura Yuji
Published: November 2013

The heterogeneous reduction of various water‐insoluble ketones (i.e., alkyl phenyl ketones and alkyl methyl ketones) was carried out with an aqueous NaBH4 solution in the presence of lecithin. It was found that the rate of the reduction reaction from ketone to carbinol was enhanced by lecithin, which is known as an optically active and zwitterionic surface active substance. Furthermore, when the unsymmetric ketones were reduced in this system the carbinols obtained were found to be optically active, though their optical yields were low. From the above results, it was concluded that the reduction reaction proceeds mainly at the micelle–water interface.
Oxygen Exchange Reactions of Cationic Carbonyl Derivatives of Manganese and Rhenium with H218O and Related Processes
Authors: Darensbourg Donald J.
Published: November 2013

Reactions proposed to involve the intermediacy of metal species containing the hydroxycarbonyl group, [M—COOH], are reviewed. Particular attention is given to reversible reactions of Group VIIb metal carbonyl derivatives with isotopically labelled water, which presumably proceeds via a [M—COOH] intermediate, to afford oxygen labelled metal carbonyl derivatives. The current status of the scope and mechanistic details of this oxygen‐exchange reaction is summarized.
Magnetic Circular Dichroism Spectra of Compounds Containing Quadruple Metal–Metal Bonds. Polarized Single Crystal Absorption Spectra of (n‐Bu4N)2[Re2Br8] at 5 K
Authors: Cowman Charles D. , Trogler William C. , Gray Harry B.
Published: November 2013

It is found that only weak MCD B terms are associated with the 1A1s → 1A2u (δ → δ*) absorption in Re2X2‐8 (X = Cl, Br), Mo2Cl4‐8, and Mo2Cl4[P(C2H5)3]4. At 15K (KBr disk sample), the 585 nm absorption system of Mo2Cl4[P(C2H5)3]4 shows 320 cm−1 vibronic fine structure in a1g (MoMo). By analogy, this sytem is assigned to the spin‐allowed δ → δ* transition. The lowest energy strong MCD signals exhibited by Re2Cl2‐8, Re2Br2‐8, and Mo2Cl4‐8 are A terms at 310, 420, and 270 nm, respectively. These are associated with the 1A1g → 1Eu (X(π) → δ*) transitions. Relatively intense MCD signals at 275 and 380 nm for Re2Cl2‐8 and Re2Br2‐8 and corresponding features in the single crystal polarized absorption spectra are assigned in each case to two overlapping transitions, 1A1g → 1A2u (X(π) → δ*) and 1A1g → 1Eu (X(σ) → δ*). The 440 nm absorption systems of Mo2 (O2CH)4 and Mo2(O2CCF3)4 exhibit strong MCD B terms.
Iron‐Nitrosyl Bond Configuration in Nitrosyl‐Hemoproteins: A Comparative EPR Study of Hemoglobin A and Hemoglobin Kansas
Authors: Chevion M. , Salhany J. M. , Peisach J. , Castillo C. L. , Blumberg W. E.
Published: November 2013

Hemoglobin A and hemoglobin Kansas can exist in alternative forms in which the affinity of heme iron for NO is mediated either by pH or the presence of allosteric modifiers. In this study, we show that the superhyperfine (SHF) pattern observed in the EPR which arises from the heme iron‐NO complex can be correlated with a change in the affinity of the ligated tetrameric protein for NO. When a combination three‐line and nine‐line SHF pattern is observed, the nitrosyl hemoprotein is in a low affinity form while a nine‐line pattern alone is an expression of high affinity. A theory is presented relating SHF patterns to a molecular orbital picture of the nitrosyl heme complex. It is suggested that the change of SHF pattern correlated with an alteration of the affinity state of ligated hemoglobin is related to a structural change of the proximal imidazole ligand to the heme.
IV. Symmetry and its Implications
Authors: Kurki‐Suonio Kaarle
Published: November 2013

Significance of symmetry in representing real‐ and reciprocal‐space properties of a crystal and of its constituents is discussed in terms of symmetry eigenfunctions. The formation of the site‐symmetrized multipole expansion for the 32 crystal point symmetries is shown, and the mutual relations between the real‐ and reciprocal‐space expansions are discussed. The origin of the symmetry‐based phase relations, equivalences and extinction rules of structure amplitudes is pointed out. Distinction is made between the accurate and approximate ones arising from the true space‐group symmetry and from the approximate free‐particle symmetry of the constituents, respectively. Finally, the symmetry imposed by reality of charge density and its violation through anomalous dispersion are discussed in terms of symmetric and antisymmetric contributions of atomic charge densities.
The Influence of Weak Magnetic Fields on Radiationless Processes
Authors: Küttner H. G. , Selzle H. L. , Schlag E. W.
Published: November 2013

Magnetic effects on the fluorescence from singlet excited states represent an interesting new method of studying the behaviour of excited states. Here the intersystem crossing in a weak magnetic field in both protonated and deuterated glyoxal was studied. Fluorescence quenching was observed at various pressures with different collision partners as a function of the magnetic field. Time resolved emission and excitation spectra were observed and quenching rate constants were determined with and without magnetic field. The collision free fluorescence lifetime of glyoxal was found to be unaffected by the magnetic field. The magnetic fluorescence quenching effect is only observed in the presence of colliders and is seen as an enhancement of intersystem crossing. In protonated glyoxal all phosphorescence is collision dependent, whereas in the deuterated species there exists zero pressure phosphorescence. The magnetic effect is here interpreted as being due to the interaction of the singlet with the triplet state, which is magnetically sensitive. In particular additional intersystem crossing mechanisms are postulated to be operative in the presence of a magnetic field. A grouptheoretical method predicts which new frequencies must be involved in this new process, and this activity is experimentally confirmed in the emission spectrum.
Some Recent Approaches to the Study of Heterogeneous Redox Kinetics of Metal Complexes. Comparisons with Homogeneous Redox Processes
Authors: Weaver Michael J.
Published: November 2013

Some experimental and conceptual aspects of the electrode kinetics of metal complexes are reviewed, with emphasis on the comparison between heterogeneous and homogeneous redox behavior, using cobalt(III)/(II) and chromium(III)/(II) couples as illustrative examples. Experimental methods for determining the position of the heterogeneous reaction sites and for distinguishing between outer‐sphere and ligand‐bridged electrode reactions are examined, along with the interpretation of electrochemical activation parameters for outer‐sphere pathways. The possible roles of bridging ligands in catalyzing electrode reactions are discussed in terms of “intrinsic” and “thermodynamic” contributions to the observed rate constants.
On the Mutual Bonding of Particles of Quantum Systems
Authors: Golden Sidney
Published: November 2013

Conceptual, logical and dynamical restrictions on the criteria characterizing the mutual bonding of particles are examined. In accord with these restrictions, observables consisting of fixed linear combinations of the particles' position and conjugate momentum are used to construct appropriate “boundedness” criteria. As a result, comparable localization of relative position and relative momentum are obtained for mutually bound particles; neither excessively compact nor excessively diffuse localization can result for bonding to occur.
Scattering Theory Concepts and Chemical Bonding
Authors: Calais Jean‐Louis , Schwarz Karlheinz
Published: November 2013

Scattering methods used to calculate the electronic structure of molecules and solids are reviewed, and it is suggested that scattering theory concepts should be further used and developed for extracting information about the chemical bonding in molecules and solids.
A Theoretical Aspect of the Formation and Breaking of Chemical Bonds in Reactions
Authors: Fujimoto Hiroshi , Fukui Kenichi
Published: November 2013

A partitioning analysis of the electron reorganization in chemical reactions is presented. The mechanisms of the formation and breaking of chemical bonds, the rehybridization of the reaction sites and shift of electronic charges have been discussed in terms of the exchange, delocalization and polarization interactions. The specific roles of these interactions in causing the rearrangements of the constituent nuclei of the composite reacting system along a reaction pathway have been elucidated. The effective collaboration between the delocalization interaction and the polarization interaction has been suggested to be of key importance in promoting the progress of chemical reactions.
The Conformations of Cyclononane Dynamic Nuclear Magnetic Resonance and Force‐Field Calculations
Authors: Anet Frank A. L. , Krane Jostein
Published: November 2013

Iterative force‐field calculations for cyclononane are reported for four conformations and five transition states for conformational interconversions. The strain energies (in kcal/mol) relative to the [333] conformation are as follows: energy minima, [333], 0; [144], 1.2; [225], 1.7; [234], 3.2; conformational transition states, [1323], 4.4; [1224], 6.0; [1233], 7.0; [18], 8.3; [1134], 9.9. The 13C NMR spectrum of cyclononane shows two dynamic NMR effects at low temperatures and reveals the presence of three different conformations. At – 173.°C, the major conformation (≃ 95%) is [333] and the minor conformation (≃ 5%) is [225]. The third conformation, [144], is present in a very small amount (≃ 1% at – 95°C) and is only barely detectable by its broadening effect near – 95°C. The [225] has an entropy which is 3.5 eu greater than that of the [333], mainly because of the high symmetry of the latter conformation. As a result of the entropy difference, ΔG0 for the [333] to [225] equilibrium is quite temperature dependent and the equilibrium constant is very strongly temperature dependent. It is estimated that at room temperature there are roughly 40% of the [333], 50% of the [225], and 10% of the [144] in cyclononane.
Additions of Chlorosulfonyl Isocyanate to 6‐Substituted Benzonorbornadienes
Authors: Mazzocchi Paul H. , Harrison Aline M.
Published: November 2013

The addition of chlorosulfonyl isocyanate (CSI) to 6‐substituted benzonorbornadienes gives pairs of isomeric syn and anti β‐lactam products. Previously published molecular orbital calculations indicate that the benzonorbornadiene π‐bond is not polarized. Relative rate data are inconsistent with either a concerted addition process or a process which proceeds by the direct formation of a carbenium ion intermediate. A mechanism proceeding through the initial formation of a benzonorbornadiene‐CSI π‐complex which decays to a carbenium ion in a product determining step is suggested.
Energy Transfer at 1.5 K in Some Photosynthetic Bacteria Monitored by Microwave‐Induced Fluorescence (MIF) Spectra
Authors: Hoff A. J. , De Vries H. Gorter
Published: November 2013

With an optically detected magnetic resonance technique, Microwave Induced Fluorescence difference (MIF) spectra have been recorded at 1.5 K for a number of photosynthetic bacteria under reducing conditions. It is found that the MIF spectra coincide with the longest wavelength fluorescence band. A relation for energy transfer at very low temperature is derived, leading to a (r0R)−3 dependence of the rate of energy transfer on the distance r0 of closest approach of donor and acceptor pigment and the diameter 2R of the antenna complex. It is tentatively concluded that the photochemical trap in the photosynthetic reaction center has a distribution of 0‐0 transitions similar to that of the antenna pigments.
Homogeneous Catalysis of the Photoreduction of Water by Visible Light. 3. Mediation by Polypyridine Complexes of Ruthenium(II) and Cobalt(II)
Authors: Krishnan C. V. , Creutz Carol , Mahajan Devinder , Schwarz Harold A. , Sutin Norman
Published: November 2013

Irradiation of polypyridineruthenium(II) complexes (RuL2+3) in aqueous solutions containing ascorbate (HAs−) and Lî (2,2′‐bipyridine or other polypyridines) with visible light yields reduced polypyridine LîH2 and H2. The quantum yield of H2 is increased greatly when cobalt(II) is also present and is optimal, Φ = 0.13, with L = bpy, Lî = 4,4′‐(CH3)2bpy (2 × 10−3 M), [Co(II)] = 0.01 M, [HAs−] = 0.7 M at pH 4.5–5.5. The quantum yield of LîH2 is ≧ 0.01 when Lî = bpy at ≧ 10−3 M, [HAs−] = 0.7 M at pH ∼ 5 but diminishes greatly at pH ≧ 6 or when cobalt(II) is present. The photochemical experiments and pulse‐radiolysis studies indicate that the quantum yields reflect the relative rates of some twenty elementary reactions. The rate constant for the first, quenching of *RuL2+3 by HAs− to give RuL+3 and ascorbate radical, spans the range 0.2 × 107 M−1 s−1 (L = 4,4‐(CH3)2bpy) to 1.1 × 109 M−1 s−1 (L = 5‐(Cl)phen). In the absence of Co(II), RuL+3 reduces LîH+ and formation of LîH2 (in competition with oxidation of LîH by ascorbate radical) ensues. When Co(II) is present, Co(Lî)2+n complexes are reduced by RuL+3 to cobalt(I) complexes Co(Lî)+n. These may react with H+ to form cobalt hydride complexes which undergo further reaction with H2O, H+ or Co(Lî)2H2+ to produce H2. At high pH, however, these reactions are too slow to compete with oxidation of cobalt(I) (k ∼ 1010 M−1 s−1) and the hydride complexes by ascorbate radical. By contrast, at pH < 4 the H2 yield drops because of oxidation of Co1(Lî)+n (or its hydride) by LîH+.
Dehydration Energy of the Exchangeable Cations in Montmorillonite — a DTA Study
Authors: Poinsignon C. , Yvon J. , Mercier R.
Published: November 2013

Homoionic montmorillonites, with exchangeable alkaline and alkaline earth cations, have been studied by means of DTA. The activation energy of dehydration of cations was deduced by applying kinetic considerations to thermograms, obtained experimentally. The results are in good agreement with the literature.
The Effects of Thermal Treatment on Associations between Fatty Acids and Montmorillonite
Authors: Yariv S. , Shoval S.
Published: November 2013

The sorption from CCl4 solutions of acetic, lauric and stearic acids by montmorillonites saturated with H, Li, Na, K, Cs, Ca, Mg, Cu, Al and Fe, was studied by IR and X‐ray methods combined with various thermal treatments. Two distinct species were detected in the clay phase, the carboxylic acid (RCOOH) and the carboxylate anion (RCOO−). The ratios of the two sorbed species were found to be dependent both on the nature of the exchangeable cation and on the chain length of the acid. In Al−, Fe− and Cu—montmorillonites, the ionic species predominates whereas in Cs—montmorillonite the acidic species predominates. Stearic acid shows the highest tendency to dissociate. The following associations were identified as occurring in the interlayer space: (I) and (II) — linkages between COOH group and oxygen sheet of silicate layer or with hydrophobic structured water; (III) — linkage between a COOH group and an exchangeable cation through a “water bridge”; (IV) — direct linkage between a COOH group and an exchangeable cation; (V) — linkage between a COO− group and an exchangeable cation through a “water bridge” and (VI) — direct linkage between a COO− group and an exchangeable cation. Although several types of associations are associated with every homoionic montmorillonite, some are more predominant. Thus, with Cs, association (I) predominates. With Li, Ca and Mg, association (III) predominates and is transformed to (IV) after thermal dehydration; whereas with Na and K association (IV) appears in freshly prepared sample. Al, Fe and Cu—montmorillonites mainly produce association (V). This structure is transformed into (VI) in the case of Al—montmorillonite by stearic and lauric acids, only. In the case of Cu—montmorillonite, all the three acids are transformed into (VI), but at 200°C decarboxylation of lauric and stearic acids occur.
Scattering of Low Energy Atoms by Surfaces: The Helium—Copper Systems
Authors: Armand G. , Lapujoulade J. , Manson J. R. , Perreau J. , Salanon B.
Published: November 2013

A number of experimental results for the diffraction of low energy helium by closely packed and stepped copper surfaces is presented. Observation of selective adsorption resonances shows that the surface average of the interaction potential is the same for all crystal faces. The thermal attenuation of the scattered intensities is given by a modified Debye—Waller factor. These results are interpreted in terms of interaction potentials with a soft exponential repulsion and an attractive well using an exact method based on Neumann iteration of the transition matrix equation. Good agreement with experiment is obtained in several systems over a large range of scattering energies.
The Ionization of Alkanes
Authors: Bouma Willem J. , Poppinger Dieter , Radom Leo
Published: November 2013

Ab initio molecular orbital theory has been used to study the ionization of methane, ethane, propane, n‐butane, isobutane, cyclopropane and cyclobutane. For methane radical cation, the preferred structure has C20 symmetry and a pair of long C—H bonds, resembling a complex of CH+2 with H2. This appears to be the only significant minimum on the CH+4 surface. For the remaining acyclic systems, the calculated structures of lowest energy (e.g., CH3‐CH2—CH+3, CH3‐CH2—CH2‐CH+3) are characterized by the extreme lengthening of a single C—C bond. However, alternative structures with two or more elongated C—C bonds have very low relative energies. The main structural conclusion for the alkane and cycloalkane radical cations is that the potential energy surface connecting various possible isomeric structures is generally very flat. Facile wide amplitude distortions and scrambling processes are therefore expected in these ions. The large structural changes which accompany ionization are manifested in large differences between vertical and adiabatic ionization potentials.
Theoretical Determination of Molecular Structure and Conformation. XI. The Puckering of Oxolanes
Authors: Cremer Dieter
Published: November 2013

Structural, conformational and energetic properties of cyclopentane (1) and the seven oxolanes monoxolane (2), 1,3‐dioxolane (3), 1,2‐dioxolane (4), 1,2,4‐trioxolane (5), 1,2,3‐trioxolane (6), tetroxolane (7), and pentoxolane (8) are investigated employing the polarized 6‐31G* basis set at the Hartree‐Fock level of theory. Extensive geometry optimization is carried out within the model of the semirigid pseudorotor. The conformational potentials V of compounds 1–8 are evaluated as a function of the puckering amplitude q and the pseudorotation phase angle Φ. Ring molecules 1 and 8 are free pseudorotors, while pseudorotation is hindered by barriers ≤ 3.3 kcal/mol for oxolanes 2–7. Puckering and inversion barriers increase with the number of O‐O bonds but decrease with the number of ether bridges. Puckered C2‐symmetrical twist forms are the most stable conformations for compounds 2–7 but 6, where highest stability is found for the C2‐symmetrical envelope forms. At room temperature a multitude of conformers of 1–8 coexists either because of free pseudorotation (barriers < RT) or large amplitudes of pseudolibration. These results are rationalized in terms of the rotor potentials of appropriate reference compounds (C2H6, CH3OH, H2O2). A more elegant approach leads to a simple π electron count and an analysis of bonding and antibonding overlap in the π‐type HOMO's. In this way the effects of substituents can be predicted and conformational preferences of the furanose ring in nucleotides, nucleosides and carbohydrates explained. The relative stability of the oxolanes is analyzed by calculating O‐O bond energies, bond‐bond interactions and ring strain for each compound. The lability of the higher oxolanes is traced back to increased ring strain. A new method of the conformational analysis of ring compounds is outlined.
NMR Determinations of Molecular Geometries in Liquid Crystal Media
Authors: Long Robert C. , Goldstein J. H.
Published: November 2013

The application of NMR spectroscopy in oriented media to the determination of molecular geometries is illustrated by results for three distinctly different molecules: naphthalene, ethylene oxide and spiropentane. The need for systematic application of vibrational corrections is emphasized and some of the effects associated with the different choices of solvent media are considered.
The 1H and 13C Magnetic Shielding Tensors of Ethylene‐1,2–13C Determined by NMR of the Oriented Molecule
Authors: Diehl P. , Moia F.
Published: November 2013

We have determined the elements of the 1H– and 13C‐chemical shift tensors of ethylene‐1,2–13C dissolved in various liquid crystals with methane –13C as a reference substance. The 1H‐results have the same relative order of magnitude as the theoretical predictions. The 13C‐results agree perfectly with theory but differ slightly from solid state data. Deviations must be attributed to variations in local effect contributions to the shifts in the various solvents which cannot be eliminated.
Dynamics of Molecular Processes by NMR in Liquid Crystalline Solvents
Authors: Luz Z.
Published: November 2013

The effect of dynamic processes on the lineshapes of NMR spectra in liquid crystalline solutions is discussed. The discussion includes dynamic 1H spectra of s‐trioxane, cyclooctatetraene and bullvalene undergoing, respectively, ring inversion, bond shift and bond rearrangement. Also discussed are dynamic deuteron spectra using the examples of cyclohexane‐d12 and p‐dioxane‐d8 undergoing ring inversion. Finally the formalism required to simulate dynamic NMR spectra in liquid crystalline solution is reviewed and the possible factorization of the problem by symmetry considerations is discussed.
Infrared Photochemistry with Shaped Laser Pulses
Authors: Ashfold M. N. R. , Atkins C. G. , Hancock G. , Macrobert A. J.
Published: November 2013

A relatively simple design for a hybrid CO2 laser and electrooptic crystal switch is described, which can be used to produce pulses with fast (< 10 ns) rise and fall times with constant power over their duration (50–1000 ns). Gaussian beam profiles allow separate and quantitative determination of both fluence (J cm−2) and intensity (W cm−2) dependences of processes induced by IR multiple photon absorption (MPA). Visible luminescence accompanying MPA of OsO4 is shown to depend on intensity, and the dependences of MPA cross sections in SF6 on fluence and intensity are derived from optoacoustic and long path absorption measurements.
IR Multiphoton Excitation of Small Asymmetric Top Molecules: Rotational and Vibrational Inversions for Laser Action in Ozone
Authors: Quack M. , Sutcliffe E.
Published: November 2013

Detailed quantum‐dynamical calculations for the time‐evolution matrix of the IR multiphoton excitation of a small asymmetric molecule, including vibrational and rotational couplings, are presented. It is shown that rotationally and vibrationally specific multiphoton excitation is possible and gives inversions that are potentially useful for lasers in the IR and FIR.
Infrared Multiple Photon Photochemistry of Gas Phase Ions
Authors: Tumas William , Foster Robert F. , Brauman John I.
Published: November 2013

An ion cyclotron resonance (ICR) spectrometer can be used to generate, trap, and detect gas phase ions which are irradiated with an infrared laser. Infrared multiple photon (IRMP) photochemistry can be conveniently studied under these conditions. Results of several recent studies are discussed.
The Structure of Valinomycin Cesium Picrate
Authors: Steinrauf L. K. , Folting Kirsten
Published: November 2013

The structure by X‐ray crystallography of the valinomycin–cesium picrate complex has been determined by the heavy atom method. The crystal class was trigonal, space group P31, with unit cell dimensions a = b = 14.028(4) â, 31.888(10) Ǎ, Z = 3, filtered Mo radiation; the final value of R was 0.111 for 4088 reflections measured at − 170 °C. The enclosed cation was found to be too large for the cavity, causing the valinomycin to be incompletely closed and forming a weak bond with the picrate at the D‐valyl end, which was the same end as had been previously found to be involved in the sodium complex. This result is in agreement with the proposal that the cation enters and exits by the D‐valyl end.
The Redox Chemistry of Manganese(III) and ‐(IV) Complexes
Authors: Yamaguchi Kenneth S. , Sawyer Donald T.
Published: November 2013

The oxidation‐reduction thermodynamics for the manganese(III), ‐(IV), and ‐(II) ions, and their various complexes, are reviewed for both aqueous and aprotic media. In aqueous solutions the reduction potential for the manganese(III)/(II) couple has values that range from +1.51 V vs. NHE (hydrate at pH 0) to −0.95 V (glucarate complex at pH 13.5). The Mn(IV)/(III) couple has values that range from +1.0 V (solid MnIVO3 at pH 0) to −0.04 V (tris gluconate complex at pH 13.5). With anhydrous media the propensity for the Mn(III) ion to disproportionate to solid MnIVO2 and Mn(II) ion is avoided. For aprotic systems the range of redox potentials for various manganese complexes is from +2.01 V and +1.30 for the Mn(IV)/(III) and Mn(III)/(II) couples (bis terpyridyl tri‐N‐oxide complex in MeCN), respectively, to −0.96 V for the Mn(IV)/(III) couple (tris 3,5,‐di‐tert‐butylcatecholate complex in Me2SO). The redox reactions between manganese complexes and dioxygen species (O2, O2, and H2O2) also are reviewed.
Aggregate Structures of Carboxylic Acids and Amides
Authors: Etter Margaret C.
Published: November 2013

A configurational analysis of hydrogen‐bonded aggregates of carboxylic acids and amides has been developed using a notation derived from graph theory. This concept of aggregate structure, independent of crystal structure, is useful for making correlations between intermolecular interactions in different phases, and for comparing physicochemical properties of isostructural aggregates. The analysis also provides a means for assessing the role of intermolecular bonds on the course of reactions or phase transformations which take place in aggregate structures.
Phase Transfer Catalyzed Bromide—Chloride Exchange: Dependence of Equilibrium Position and Selectivity Constant on Nature and Composition of Aqueous Phase
Authors: Yonovich‐Weiss Minda , Sasson Yoel
Published: November 2013

The position of the equilibrium in the PTC exchange reaction of alkyl chloride with metal bromide salts RCl + MBr ⇌ RBr + MCI under various condtions is utilized for calculation of the selectivity constant KselCl/Br. This constant is found to be dependent on the nature of the metal cation and the inorganic phase composition. KselCl/Br can be defined for any point on the phase diagram MCl—MBr—H2O at a given temperature, solvent and quaternary cation.
Computer Simulation of Enzymatic Reactions
Authors: Warshel A. , Russell S. , Sussman F.
Published: November 2013

Our strategy in modeling ezymatic reactions is demonstrated by considering two complimentary approaches, which are based on the powerful EVB method. The first approach uses the X‐ray coordinates at their face value and estimates the reaction potential surface by using a simplified and efficient microscopic dipolar model for the solvent. The second approach (which became practical only upon the emergence of supercomputers) uses a surface constrained all atom solvent model, molecular dynamics (MD) and the umbrella sampling method to evaluate the actual free energy barrier. Both approaches indicate that the correct evaluation of the “solvation energy” of the ionic resonance structures is the key for understanding enzyme catalysis. The MD version of the EVB method is used in a preliminary study of the validity of linear free‐energy relationships in enzymatic reactions. This study supports the ad hoc assumptions introduced in the simplified method.
1,3‐Diketones as Active Ligands in the Nickel‐Catalyzed Linear Oligomerization of Olefins
Authors: Behr A. , Falbe V. , Freudenberg U. , Keim W.
Published: November 2013

The novel complex (4‐cyclooctene‐l‐yl)‐ bis(trifluoroacetamido) nickel oligomerizes ethene to olefins of about 85% linearity. Unusual high activities of 11500 mol ethene/mol nickel were obtained. Also higher α‐olefins such as propene, butene or octene could be oligomerized yielding 70‐80% linear dimers. By using (4‐cyclooctene‐1‐yl)(1,1,1,5,5,5‐hexafluoro‐2,4‐pentanedionato) nickel activated by BF3 the bulky olefin neohexene was converted selectively into dimers, which are linear, considering the C‐C‐linkage.
Reduction of n‐C5H11N3 Catalyzed by [Mo2Fe6S8(OMe)3(SC6H4‐p‐n‐C8H17)6]3– in an Aqueous Micellar Solution
Authors: Moriya Koji Tanaka Makoto , Tanaka Toshio
Published: November 2013

Reduction of n‐C5H11N3 by Na2S2O4 was performed in the presence of (n‐Bu4N)3 [Mo2Fe6S8(OMe)3(SC6H4‐p‐n‐C8H17)6] ((n‐Bu4N)3 [Mo‐Fe]) as a catalyst in aqueous Triton X‐100 micellar solutions. The rate of the reduction is enhanced efficiently by the addition of methyl viologen (MV2+). The methyl viologen radical cation (MV+) produced by the reaction of MV2+ with Na2S2O4 undergoes a disproportionation reaction to afford MV2+ and MV° in the micellar solution. The resultant MV° formed in the micelle transfers two electrons to [Mo‐Fe]3− effectively to give [Mo‐Fe]5−, which reduces n‐C5H11N3 with two electrons to produce n‐C5H11NH2 and N2.
EPR Characteristics of the Electron Acceptors A0, A1, and (Fe‐S)X in Digitonin and Triton X‐100 Solubilized Pea Photosystem I
Authors: Mansfield Royston W. , Evans Michael C.W.
Published: November 2013

Photosystem I (PS I) particles prepared from pea chloroplasts using either digitonin or Triton X‐100 as the solubilizing detergent were used to investigate the EPR characteristics of the electron acceptors A0, A1, and (Fe‐S)X. The signal from component A1 appears asymmetric in both digitonin and Triton X‐100 solubilized samples. However, the difference spectrum which represents the isolated signal from component A0 is symmetric in Triton X‐100 solution, but not in digitonin. Upon deuteration of digitonin solubilized PS I, the signal from A1 became partially narrowed to give a spectrum comparable to that of deuterated Triton X‐100 solubilized particles. However, deuteration did not produce new spectral features in the digitonin solubilized preparation, but emphasized those already present Upon storage of either type of particles at 77 K, it was found that the signal from component A1, but not that from A0, decayed. The difference spectrum of A1 obtained with g = 2.0058 and a peak‐to‐peak linewidth of 0.95 mT is compatible with the identification of A1 as an anionic semiquinone. In digitonin solubilized particles, component A0 and not A1 affected the EPR signal of the iron‐sulphur center X, which suggests a magnetic interaction between A0 and X. These results suggest that component A0 is closer to X than is A1, and that A1 may not be in a linear electron transport pathway between A0 and the iron‐sulphur centers. They also show that Triton X‐100 perturbs the structure of the reaction center.
Fourier Transform‐EPR Spectroscopy of Electron Transfer from Excited State of Chlorophyll and Porphyrin to Duroquinone
Authors: Bowman M.K. , Toporowicz M. , Norms J.R. , Michalski T.J. , Angerhofer A. , Levanon H.
Published: November 2013

Electron transfer reactions between the photoexcited triplets of several porphyrin and chlorophyll precursors, 3P (electron donors), and neutral duroquinone, DQ (electron acceptor), were studied by selective laser excitation‐Fourier transform (FT)‐EPR spectroscopy. Four different P, which are related to the photosynthetic apparatus, were examined in ethanol solutions at −243 K: pyrochlorophyll a (pChla), zinc‐pyrochlorophyll a (ZnpChla), zinc‐tetraphenylporphyrin (ZnTPP), and magnesium‐tetraphenylporphyrin (MgTPP). This highly time‐resolved EPR spectroscopy (∼10 ns delay time, ẗd, between the laser excitation and the detection pulse) permits differentiation between the various CIDEP mechanisms, i.e., triplet mechanism (TM) and radical‐pair mechanism (RPM), associated with the electron transfer process. The spectra of the DQ.− generated by the different triplet precursors strongly depend on two inherent parameters characteristic of the molecular system: (1) the zero‐field splitting (ZFS) and the selective singlet‐triplet (Ti l̊ S1) population rates, Ai (i = x, y, z); (2) the difference in g‐values and hyperfine components between the donor and acceptor radicals. Since the differences in g‐values and hyperfines are very nearly the same in all four P.+‐DQ.− systems examined, the different results, particularly in the time range 10 ns < ẗ<d < 300 ns, are attributed to triplet spin memory transfer via TM. A quantitative spectral analysis is presented to describe the time evolution FT‐EPR spectra. This method is used to determine the following dynamic processes: (1) spin lattice relaxation rates of the triplet precursors in solution; (2) donor‐acceptor electron transfer reaction rates; (3) spin lattice relaxation rates of the acceptor radical.
Techniques for the Measurement of Small Anisotropies in Magic Angle Spinning NMR Spectra
Authors: Raleigh D.P. , Kolbert A.C. , Levitt M.H. , Griffin R.G.
Published: November 2013

Two novel methods for enhancing the effect of small anisotropies on magic angle spinning (MAS) NMR spectra are described. The first method is a simple two‐dimensional (2D) spin echo experiment, which generates rotational sidebands at a fraction of the spinning frequency in the ω1 dimension, while maintaining the effective spinning speed in ω2. It is shown that the rotational sidebands in the 2D spectrum are much larger than in a normal one‐dimensional MAS spectrum obtained at the same spinning speed, and the relative intensities of the two‐dimensional sideband array can be analyzed to yield the anisotropy and asymmetry parameter of the coupling tensor. The second method is designed to measure homonuclear dipolar couplings. We show that the effect of a dipolar coupling between members of an isolated spin pair can be enhanced by adjusting the spinning speed such that nωr = ωΔiso, where n is an integer, ωr is the spinning speed, and ωΔiso is the difference between isotropic shifts. When this condition, termed rotational resonance, is satisfied, a broadening or a splitting of the normally sharp resonance lines is observed. In addition, rapid exchange of Zeeman order between the dipolar coupled spins is seen. This method holds promise for the measurement of through space dipolar couplings and thus internuclear distances in polycrystalline solids.