bluuebunny/crossref_metadata_2025_split huggingface dataset:
Polymerization of Mono‐ and Bi‐functional Diacetylene Derivatives in Monolayers at the Gas–Water Interface
Authors: Day D. , Hub H. H. , Ringsdorf H. 
Published: November 2013

AbstractThe spreading and polymerization behaviour of long chain acids and phospholipid analogues containing diacetylene groups is described. Especially the polymerization in monolayers is followed spectroscopically with a new device used in connection with a commercial UV spectrometer. The polymerization is topochemically controlled. Sonification of the phospholipid analogues in water results in the formation of liposomes with multilayered membranes. Polymerization of the liposomes in water forms aqueous solutions of polymer vesicles.
The Valence Bond Orbital Model as an Interpretive Framework for Understanding Electronic Structure
Authors: Kirtman Bernard , Palke William E. , Chipman Daniel M. 
Published: November 2013

AbstractFor purposes of interpretation and understanding, the valence bond orbital model of electronic structure provides a number of advantages over the more common molecular orbital model. This derives primarily from the unique description of valence bond orbitals in terms of highly localized bond pairs, lone pairs, etc. Optimization of the orbitals by a self‐consistent‐field procedure brings out features which were not evident from the semi‐empirical treatments of earlier workers. Calculations on CH4, NH3, H2O and H2S show that optimized valence bond orbitals are often bent even at the equilibrium nuclear configuration and generally do not follow the nuclei when bond angles are varied. Analysis of a large basis set calculation on H2O shows that the valence bond wavefunction can be fitted well with a minimal basis, making it possible to interpret more clearly the connection between hybridization and interorbital angles. It is shown how the valence bond method provides a rigorous basis for factorization even of difficult second order physical properties, such as polarizability, into separated bond and lone pair contributions. These contributions can be further decomposed into sums of atomic valence state values plus corrections due to bond polarization effects. Finally, the advantages of using the valence bond model as a starting point for the perturbation treatment of electron correlation are pointed out. Calculations on H2 and LiH indicate that valence bond theory need only be carried through second order to obtain results comparable in accuracy to a third order treatment based on molecular orbitals.
Intramolecular Interactions Through Triple Bonds. Internal Rotation in Aminoborylpolyynes, NH2(C≡C)n BH2
Authors: Radom Leo , Vincent Mark A. 
Published: November 2013

AbstractAb initio molecular orbital theory has been used to examine interactions through triple bonds in polyynes [H‐(C≡C)n‐H], aminopolyynes [NH2‐(C≡C)n‐H], borylpolyynes [H‐(C≡C)n‐BH2], and aminoborylpolyynes [NH2‐(C≡C)n‐BH2], with n = 1,2,3 and 4. The rotational barrier in aminoborylpolyynes is a particularly effective probe of interactions transmitted through the triple bond network. The barrier for NH2‐C≡C‐BH2 is substantial (25.1 kJ mol−1) but is reduced by a factor of approximately 3 by the interposition of each additional C≡C linkage. There is a residual barrier (of 1.0 kJ mol−1) in NH2‐C≡C‐C≡C‐C≡C‐BH2 despite the fact that the interacting groups are separated by a distance of greater than 11 Å.
Empirical Correlations Between Conformational Parameters in β‐D‐Furanoside Fragments Derived from a Statistical Survey of Crystal Structures of Nucleic Acid Constituents Full Description of Nucleoside Molecular Geometries in Terms of Four Parameters
Authors: De Leeuw Harry P. M. , Haasnoot Cornelis A. G. , Altona Cornelis 
Published: November 2013

AbstractA data file was created containing internal coordinates and derived parameters for 178 β‐D‐furanoside fragments, occurring in X‐ray structures of ribo‐, 2′‐deoxyribo‐ and arabinonucleoside derivatives. The variability in bond angles and torsion angles can be described by four conformational parameters; the kind and degree of ring puckering as expressed in phase angle of pseudorotation P and puckering amplitude vm, the conformation of the side‐chain δ and the glycosidic torsion angle χ. Comparison between out‐of‐plane and torsion angle methods for calculation of pseudorotation parameters revealed that the latter method is better suited to describe the internal coordinates of the non‐equilateral five‐membered ring. Inspection of the distribution of phase angles and puckering amplitudes lends additional support to the hypothesis that conformational interconversion of the familiar N and S type ring conformers is a process of hindered pseudorotation, proceeding through the O4′endo conformation (P = 90°) and maintaining a constant puckering amplitude. Both syn‐anti distribution and preferred ranges of χ are correlated with furanose conformation in ribonucleosides; the distribution is also affected by the type of base, purines showing higher syn/anti ratios than pyrimidines. Three modes are observed for the backbone torsion angle δ(C4′–C5′); the most abundant rotamer is δ+, followed by δa. The δ− rotamer has so far been found only in S‐type furanosides. Regressions based on existing or modified equations were used to express variations in internal coordinates in terms of the four parameters involved. As a consequence of its anomeric character, the glycosidic bond length can be described by a cyclic function of χ. Endocyclic bond angles' and torsion angles show small but systematic deviations from the pseudorotation equations derived for equilateral rings. Based on observed correlations with the four parameters, reasonably accurate coordinates for exocyclic atoms, including hydrogen atoms, could also be obtained. Coordinates for model β‐D‐furanoside fragments with preferred χ and δ orientations and variable pseudorotation parameters, covering a part of the N ⇌ S interconversion pathway, are presented. Their usefulness in future theoretical and experimental studies on nucleic acid conformations, both in the solid state and in solution, is discussed.
Stereospecific Methanol Elimination and Hidden Hydrogen Transfer in Cyclic Diesters under Electron Impact
Authors: Bar‐Shai Rivka , Bortinger Arie , Sharvit Joseph , Mandelbaum Asher 
Published: November 2013

AbstractElimination of CH3OH under electron impact occurs in trans‐1,2‐ and 1,3‐dicarbomethoxycycloalkanes to a greater extent than in the cis‐isomers. In the trans‐diesters, one of the α‐hydrogen atoms is abstracted in the course of the elimination. In the methyl esters of cyclohexane 1,4‐dicarboxylic acids, the elimination of CH3OH is also specific to the trans‐isomer, but the abstracted hydrogen atom does not originate at the α‐position. This result is explained by a stereospecific hidden hydrogen migration step, preceding the elimination.
Two Syntheses of [2.2.2] (1,2,3)Cyclophane
Authors: Neuschwander Brent , Boekelheide V. 
Published: November 2013

AbstractTwo syntheses are presented for [2.2.2] (1,2,3)cyclophane (7), the remaining member of the symmetrical [2n]cyclophanes whose preparation has not previously been described. The first method involved the dimerization of 2,6‐bis (4′, 4′‐dimethyl‐2′‐oxazolinyl)toluene (1 ) to give 2 which, on hydrolysis, produced the tetracarboxylic acid 3. Conversion of 3, via the acid chloride (4 ), to the tetraol 5, followed by treatment with phosphorus tribromide yielded the tetrabromide 6. The overall scheme was then completed by reaction of 6 with phenyllithium to give [2.2.2] (1,2,3) cyclophane (7 ). An alternate, more convenient, method began with the pyrolysis of 2,6‐bis (chloromethyl)toluene (8 ) to give 3‐chloromethylbenzocyclobutene (9 ). Formation of the Grignard of 9, followed by reaction with ferric chloride, yielded the dimer 10. Pyrolysis of 10 then gave [2.2.2] (1,2,3)cyclophane (7).
Pyrolysis and Isomerization Reactions of Multiply‐bridged Cyclophanes
Authors: Hopf Henning , Kleinschroth Jürgen , Murad Abo El‐Fetouh El‐Sayed 
Published: November 2013

AbstractWhen subjected to pyrolysis in para‐di‐isopropylbenzene (270°C, 40h) [2.2.2] (1,2,4)cyclophane (5 ) is cleaved to 2,9‐dimethyldibenzo[a,e]cyclooctadiene (8 ). Under comparable conditions (290°C, 65h), [2.2.2.2] (1,2,3,5)cyclophane (6 ) is converted to 6,11‐dimethyl[2.2.2] (1,2,3)cyclophane (9 ), the first derivative of a symmetrical [2n]cyclophane carrying three consecutive ethano bridges. The isomeric [2.2.2.2]‐ (1,2,3,4)cyclophane (7 ), on the other hand, cannot be induced to undergo carbon‐carbon‐bond cleavage followed by hydrogen abstraction. Under the influence of aluminiumtrichloride/hydrogenchloride hydrocarbon 5 isomerizes to the unsymmetrical [2.2.2] (1,2,4) (1,2,5)cyclophane (10 ), whereas 6 and 7 are stable under these conditions. The thermal stability and propensity of multiply‐bridged cyclophanes to undergo isomerization reactions is discussed.
Diels‐Alder Reactivity of Polycyclic Aromatic Hydrocarbons. III. New Experimental and Theoretical Results
Authors: Biermann D. , Schmidt W. 
Published: November 2013

AbstractSecond‐order rate constants for the Diels‐Alder reaction of aromatic hydrocarbons comprising up to 13 condensed benzene rings with maleic anhydride have been measured under standardized conditions. The position of attack was deduced from the UV spectra of the adducts. A number of theoretical models was examined as to how far they reproduce the observed rate data and positional selectivities. All except Fukui's frontier orbital concept were successful. These include Brown's para‐localization model, second‐order perturbation theory using Hückel orbital energies and AO coefficients, Clar's sextet theory and Herndon's structure count method. The latter two models provide a particularly attractive basis for discussing structure/reactivity relationships. Inter alia, they lead to the discovery of “isocanonical” series, i.e., to homologous series in which corresponding members have the same theoretical reactivity index and therefore the same rate constant, despite widely differing topologies.
The Relationship Between Zinc Protoporphyrin Levels in Blood and Lead‐Induced Anemia
Authors: Blumberg W. E. 
Published: November 2013

AbstractThe production of zinc protoporphyrin by the erythrocyte has been shown to be a good indicator of lead intoxication. The relationship between zinc protoporphyrin levels in blood and lead‐induced anemia are examined. Serum iron parameters show that the anemia is not related to the lack of availability or transport of iron, but to faulty utilization by the reticulocyte. Although zinc protoporphyrin is produced in response to both lead intoxication and iron deficiency, it is pointed out that plasma iron and lead levels provide a method for differentiating between them.
The Electronic Configurations of Catalases and Peroxidases in their High Oxidation States: A Definitive Assessment
Authors: Dolphin David 
Published: November 2013

AbstractThe highest oxidation states of catalase and horseradish peroxidase contain two oxidizing equivalents more than the resting ferrihemoproteins. While numerous electronic configurations for these primary complexes have been suggested, the most widely accepted formulation is that of an Fe(IV) (S = 1) porphyrin π‐cation radical (S = 1/2). The evidence for and against these electronic configurations is reviewed and assessed.
Influence of Sex Hormones on Relative Quantities of Multiple Species of Cytochrome P‐450 in Rat Liver Microsomes: A Possible Answer to the Sexual Differences in Metabolic Activities of Rat Liver Microsomes
Authors: Fujita Shoichi , Chevion Mordechai , Peisach Jack 
Published: November 2013

AbstractEPR spectra of rat liver microsomes from male, female and hormonally‐treated castrated hepatectomized rats were studied. The spectra, especially in the region of gmax, suggested a heterogeneity of local environments of the low spin ferric heme indicative of multiple structures for cytochrome P‐450. Certain features in the spectrum correlated with sexual differences. It is suggested that the changes in the relative amplitudes of the EPR features represent differences in the relative abundance of the individual proteins in the mixture that, in turn, are related to the sexual differences of metabolic patterns for reactions catalyzed by cytochrome P‐450.
Photosynthetic Oxygen Evolution and the Water Splitting Enzyme
Authors: Barber J. , Nakatani H. Y. , Mansfield R. 
Published: November 2013

AbstractA brief review is given of the present knowledge of the mechanism by which higher plants and algae bring about the photooxidation of water. Emphasis is placed on the role of manganese in this process and a summary is given of recent efforts from several laboratories to isolate the “water‐splitting enzyme”. Experimental evidence is presented that, in contrast to earlier beliefs, the Mn not removed by washing chloroplast thylakoids with high concentrations of Tris (hydroxymethyl) amino methane could be involved in O2 evolution. This manganese is equivalent to about 2 Mn per photosystem two reaction centre. Also presented are details of the isolation of a heme‐protein from spinach chloroplasts which may be intimately associated with the water splitting process. This protein has a native molecular weight of approximately 232 k Daltons and has characteristics similar to those of a catalase type enzyme. The possibility of involvement of peroxide formation during the water splitting process is discussed.
Orientation and Energy Transfer in Chlorophyll Monolayers Diluted with Hexadecane. Fluorescence and Sensitized Fluorescence
Authors: Gonen Oded , Levanon Haim , Patterson L. K. 
Published: November 2013

AbstractThe fluorescence and force‐area behavior of chlorophyll a (Chl a) monomolecular films diluted with hydrocarbon on an aqueous subphase is reported. It is found that the diluent hexadecane, instead of conventional fatty alcohols, results in a substantial increase in the fluorescence yield of Chl a. The luminescence intensity varies nonmonotonically with respect to the intermólecular distance. A model for the mutual orientation of the chlorophyll pigments and the consequent energy transfer processes in terms of a Förster mechanism is proposed. For a random distribution of the Chl a molecules in the monolayer (k2 = 2/3) an average value of 43° is calculated for the angle between the molecular planes and the water surface. This model yields a value of 23 Å for the Förster R0. Also, a value of 58° is derived for the angle between the Q‐band transition moment and the line of intersection between the molecular plane and water surface. When a mixture of Chl a and Chl b in the monolayers is being excited at the absorption band of the latter pigment, typical fluorescence of Chl a is being detected. This sensitization process can also be interpreted in terms of the orientational model of the Chl molecules as presented in this work.
Thermal Decomposition of Copper(II) Squarate
Authors: Brown Michael E. , Galwey Andrew K. , Beck Michael W. 
Published: November 2013

AbstractA kinetic and thermochemical study has been made of the dehydration, decomposition and oxidation reactions of copper(II) squarate dihydrate, CuC4O4 · 2H2O. In contrast to results obtained for the nickel(II) salt, the copper salt undergoes preliminary dehydration in a temperature range (410–440 K) appreciably below that for decomposition (590–620 K). The anhydrous material is then more stable than nickel squarate (560–600 K). The major decomposition products in nitrogen are Cu and CO and the reaction may proceed further to CuO and CO2 in an oxyen atmosphere. Values for the enthalpies of dehydration, decomposition and oxidation are reported. Isothermal DSC was used to study the kinetics of dehydration and of decomposition and results were analyzed using relative‐rate reduced‐time (RRRT) curves. The dehydration conforms to the Avrami—Erofe'ev expression, whereas the decomposition approximates closely to the contracting area model. Apparent Arrhenius parameters are reported.
Importance of the Phthalocyanine Ring Carbon π‐orbitals in the Electrical Conduction of Metal Phthalocyanine Polymers
Authors: Whangbo Myung‐Hwan , Stewart Kevin Randall 
Published: November 2013

AbstractThe electronic structures of metal phthalocyanine polymers PcMX and MPc were examined by performing tight‐binding band calculations on PcMX (MX = SiO, GeO, AlF, GaF, CrF) and MPc (M = Ni, Co, Fe) chains. The present band structure study suggests that the electrical conductivities of halogen‐doped PcMX and MPc chains are governed by their valence bands a composed of the HOMO a1M of each Pc ring. It is the valence band a that becomes partially empty upon halogen‐doping. The Pc ring π‐orbital a1M is made up of only the carbon p orbitals. By symmetry, none of the s, p and d orbitals of a metal atom M can mix into the a1M orbital. Consequently, the width of the valence band a is solely determined by such factors as the inter‐ring separation and the inter‐ring rotation. It was noted that there should be one, two and three unpaired electrons on each metal atom of CoPc, FePc and PcCrF chains, respectively. Thus halogen‐doped CoPc, FePc and PcCrF are expected to have magnetic properties associated with the unpaired electrons.
Delta Bilirubin: The Fourth Fraction of Bile Pigments in Human Serum
Authors: Wu T.‐W. 
Published: November 2013

AbstractDelta bilirubin is a strong, possibly covalent complex between bilirubin and an albumin‐like protein, found in the serum of most jaundiced adults. The bili‐albumin complex reacts directly diazo‐positive. An examination of its azo pigments indicates that only one half of the tetrapyrrole is linked to the protein, presumably via an amide bond formed between a propionic acid side chain of bilirubin and an amine group (e.g., ϵ‐NH2 of a lysine) in the albumin backbone; the other (distal) propionic acid chain is largely unesterified. Preliminary analyses of the peptides generated by chemical or enzymic cleavage of the delta fraction suggest a bilirubin bonding region within positions 128–297 of albumin and another involving positions 298–585. Thus, delta bilirubin may represent the first true human biliprotein. The implications of this biliprotein in both the fundamental and applied aspects of bile pigment research are discussed.
Deuterium NMR of Biaxially Ordered Liquid Crystals
Authors: Doane J. William 
Published: November 2013

AbstractDeuterium NMR spectra and spectral patterns obtained from several different biaxially ordered liquid crystalline phases are reviewed. An interpretation of these spectra in terms of structural and orientational order is given and a comparison is made between some of the phases in order to examine different aspects of biaxiality. In all of the phases presented, the deuterium spectra are interpreted as being time averaged in the fast motion regime. Liquid crystalline phases reviewed include the ribbon phase of lyotropic soaps, the cholesteric, smectic C and smectic G phases of thermotropic materials.
Pd(O) Promoted Transformation of 1,1‐Dialkoxycarbonyl‐2‐(1,3‐butadienyl)cyclopropanes into 2‐Ethenyl‐3‐cyclopentenes
Authors: Morizawa Yoshitomi , Oshima Koichiro , Nozaki Hitosi 
Published: November 2013

Abstract1,3‐Butadienylcyclopropanes activated by two electron‐withdrawing groups (1) smoothly rearrange to vinylcyclopentene derivatives in the presence of a Pd(O) catalyst under mild conditions. For example, dimethyl 1,3‐butadienylcyclopropane‐1,1‐dicarboxylate 1a reacts with 3 mol% Pd(PPh3)4 in DMSO at 25–60 °C for 30 min to provide dimethyl 2‐ethenyl‐3‐cyclopentene‐1,1‐dicarboxylate 2a in 87% yield. Other data show that the new method is useful in the synthesis of cyclopentene derivatives including dolichodial and iridodiol.
Topochemical Photodimerization of 4‐(3‐Oxo‐3‐phenyl‐l‐propenyl)benzoic Acid and Its Esters
Authors: Hasegawa M. , Arioka H. , Harashina H. , Nohara M. , Kubo M. , Nishikubo T. 
Published: November 2013

AbstractThe photodimerization behaviors of 4‐(3‐oxo‐3‐phenyl‐1‐propenyl)benzoic acid (1) and its esters (2–7) in the crystalline state are investigated. These crystals, except that of the phenyl ester (7), dimerize photochemically to yield the corresponding cyclobutane derivatives in a typical topochemical fashion. Head‐to‐head type cyclobutane derivatives are obtained from crystals of the acid and ethyl ester (1 and 3), whereas head‐to‐tail type derivatives are formed from the other ester crystals (2, 4, 5, and 6). The temperature dependences of these reaction behaviors and the configurations of the photoproducts are correlated with the molecular arrangement in the reactant crystals. Reversible or irreversible behavior of as‐formed dimer crystals, derived from 2 or 4 crystals, is interpreted in terms of accumulated strain energy in the course of the topochemical process.
The Reaction of (Dicarbonyl)(η5‐Cyclopentadienyl)Cobalt with Bis(trimethylsilyl)ethyne. A One‐Step Synthesis of Tetrakis(trimethylsilyl)butatriene
Authors: Fritch John R. , Peter K. , Vollhardt C. 
Published: November 2013

AbstractReaction of the two title compounds in a nitrogen‐flushed boiling medium over several days produced a series of cyclobutadiene and trinuclear biscarbyne complexes in addition to the only organic substance, tetrakis(trimethylsilyl)butatriene, formed formally by catalytic dimerization of bis(trimethylsilyl)ethyne. Mechanistic pathways are discussed which might provide the connection between starting materials and products.
Selectivity in the Solvolysis of 1‐Adamantyl Halides in Binary Solvents
Authors: Samuel P. McManus , Zutaut Steven E. 
Published: November 2013

AbstractThe products of solvolysis of 1‐bromoadamantane in twenty‐six binary solvent mixtures were studied. The solvent systems included aqueous aliphatic alcohols, mixtures of trifluoroethanol with water, formic acid, acetic acid, dichloroacetic acid, and hexafluoroisopropanol (HFIP) and mixtures of HFIP with methanol and ethanol. Also, 1‐chloroadamantane was studied in three binary solvents. The relative rates of reaction of the components of each of the binary mixtures (selectivity values) were calculated. These selectivity values provided evidence supporting the hypothesis that both steric bulk and electrophilicity of the solvent components strongly affect the selectivities. Intrinsic nucleophilicity is of lesser importance. The front‐side nucleophilicities are shown to be composition dependent, especially when highly electrophilic solvents are used. It is suggested that solvent sorting may account for this but other possible causes are discussed.
Conformational Analysis of Polypeptides and Proteins for the Study of Protein Folding, Molecular Recognition, and Molecular Design
Authors: Scheraga Harold A. 
Published: November 2013

AbstractConformational energy calculations provide an understanding as to how interatomic interactions lead to the three‐dimensional structures of polypeptides and proteins, and how these molecules interact with other molecules. Illustrative results of such calculations pertain to model systems (α‐helices and β‐sheets, and interactions between them), to various open‐chain and cyclic peptides, to fibrous proteins, to globular proteins, and to enzyme‐substrate complexes. In most cases, the validity of the computations is established by experimental tests of the predicted structures.