bluuebunny/crossref_metadata_2025 huggingface dataset:
Attenuation of hydrogen radicals traveling under flowing gas conditions through tubes of different materials
Authors: Grubbs R. K. , George S. M.
Published: May 2006

Hydrogen radical concentrations traveling under flowing gas conditions through tubes of different materials were measured using a dual thermocouple probe. The source of the hydrogen radicals was a toroidal radio frequency plasma source operating at 2.0 and 3.3kW for H2 pressures of 250 and 500mTorr, respectively. The dual thermocouple probe was comprised of exposed and covered Pt∕Pt13%Rh thermocouples. Hydrogen radicals recombined efficiently on the exposed thermocouple and the energy of formation of H2 heated the thermocouple. The second thermocouple was covered by glass and was heated primarily by the ambient gas. The dual thermocouple probe was translated and measured temperatures at different distances from the hydrogen radical source. These temperature measurements were conducted at H2 flow rates of 35 and 75SCCM (SCCM denotes cubic centimeter per minute at STP) inside cylindrical tubes made of stainless steel, aluminum, quartz, and Pyrex. The hydrogen radical concentrations were obtained from the temperatures of the exposed and covered thermocouples. The hydrogen concentration decreased versus distance from the plasma source. After correcting for the H2 gas flow using a reference frame transformation, the hydrogen radical concentration profiles yielded the atomic hydrogen recombination coefficient, γ, for the four materials. The methodology of measuring the hydrogen radical concentrations, the analysis of the results under flowing gas conditions, and the determination of the atomic hydrogen recombination coefficients for various materials will help facilitate the use of hydrogen radicals for thin film growth processes.
Growth and characterization of GaAsSb metamorphic samples on an InP substrate
Authors: Mohammedy F. M. , Hulko O. , Robinson B. J. , Thompson D. A. , Deen M. J. , Simmons J. G.
Published: May 2006

Buffer layers of GaAs1−xSbx were grown on an InP substrate starting at x=0.49 (lattice matched to InP) and increasing x in steps of 0.03–1.0 (GaSb), followed by a 0.5μm thick GaSb metamorphic layer. A 10nm thick InAs quantum well was grown on top and capped with a 100nm GaSb. All layers were grown using gas source molecular beam epitaxy by reducing the As flux in small steps while keeping the Sb flux fixed. The substrate temperature during growth was ∼490°C. High-Resolution x-ray diffraction analysis showed that the metamorphic layer was almost fully relaxed. Cross-sectional transmission electron microscopy images show that a postgrowth anneal at 600°C for 30s successfully reduces the number of dislocations threading through the metamorphic layer. Hence the top layer can be used satisfactorily as a pseudosubstrate for subsequent layer growths assuming a GaSb lattice constant.
Characteristics of remote plasma atomic layer-deposited HfO2 films on O2 and N2 plasma-pretreated Si substrates
Authors: Choi Jihoon , Kim Seokhoon , Kim Jinwoo , Kang Hyunseok , Jeon Hyeongtag , Bae Choelhwyi
Published: May 2006

Characteristics of remote plasma atomic layer-deposited HfO2 on Si, which has a very thin SiO2 interlayer with and without remote plasma nitridation, have been investigated. The thin (∼1.5nm) intermediate layer containing nitrogen, which was prepared by sequential O2 and N2 remote plasma treatment of the Si substrate, can effectively suppress growth of the unintentional interface layer. In addition, it enhances the thermal stability and the resistance to oxygen diffusion during rapid thermal annealing. The HfO2 film containing the remote plasma nitrided SiO2 interlayer annealed at 800°C showed a lower equivalent oxide thickness of ∼1.89nm and a lower leakage current density (3.78×10−7Acm−2 at ∣VG−VFB∣=2V) compared to a non-nitrided sample of the same physical thickness. Also, we compared the characteristics of HfO2 films annealed in two different ambient environments, N2 and O2.
Numerical study on the scaling of a-Si:H thin film transistors
Authors: Fathololoumi Saeed , Chan Isaac , Moradi Maryam , Nathan Arokia
Published: May 2006

In this article we discuss the impact of channel length scaling on the above threshold current characteristics of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs). MEDICI simulation results of the interface surface potential show a lowering of the potential barrier for shorter channel lengths. This suggests a decrease in threshold voltage and increase in subthreshold slope with drain voltage particularly in submicron channel lengths, causing a nonsaturating output current. Simulation results of the above threshold current-voltage characteristics for short channel TFTs corroborate with measurement data of fabricated devices.
Nanoscale channel and small area amorphous silicon vertical thin film transistor
Authors: Chan Isaac , Fathololoumi Saeed , Nathan Arokia
Published: May 2006

This article reports the design of vertical thin film transistors (VTFTs) in hydrogenated amorphous silicon (a-Si:H) technology. This transistor structure offers an elegant method of defining the channel length in nanoscale dimensions by means of dielectric film thickness. In addition, the device area of the vertical TFT structure is less than ∼1∕3 that of the ubiquitous lateral TFT structure. We study the deposition mechanisms to gain insight into the fabrication of VTFTs from a conceptual standpoint. The a-Si:H VTFT reported here advances current state of the art, by demonstrating the first 100nm channel length VTFT with an on/off current ratio of 108, threshold voltage of 2.8V, and a subthreshold slope of 0.8V∕decade. This is the shortest and truly vertical channel a-Si:H TFT reported, hitherto. We propose an application of a-Si:H VTFTs in high-resolution flat-panel electronics with TFT size independent fill factor, promising immense benefits in medical x-ray imaging.
Atmospheric pressure plasma analysis by modulated molecular beam mass spectrometry
Authors: Aranda Gonzalvo Y. , Whitmore T. D. , Rees J. A. , Seymour D. L. , Stoffels E.
Published: May 2006

Fractional number density measurements for a rf plasma “needle” operating at atmospheric pressure have been obtained using a molecular beam mass spectrometer (MBMS) system designed for diagnostics of atmospheric plasmas. The MBMS system comprises three differentially pumped stages and a mass/energy analyzer and includes an automated beam-to-background measurement facility in the form of a software-controlled chopper mechanism. The automation of the beam modulation allows the neutral components in the plasma to be rapidly and accurately measured using the mass spectrometer by threshold ionization techniques. Data are reported for plasma generated by a needle plasma source operated using a helium/air mixture. In particular, data for the conversion of atmospheric oxygen and nitrogen into nitric oxide are discussed with reference to its significance for medical applications such as disinfecting wounds and dental cavities and for microsurgery.
Formation and X-ray structure of the hostguest adduct [Pd3(dppm)3(CO)(NO3)]+ and the mechanism of its electrochemical reduction
Authors: Brevet David , Lucas Dominique , Richard Philippe , Vallat Alain , Mugnier Yves , Harvey Pierre D
Published: May 2006

[Pd3(dppm)3(CO)]2+, [Pd3]2+, reacts with NO3 to form the hostguest adduct [Pd3(dppm)3(CO)(NO3)]+, [Pd3(NO3)]+. The presence of NO3 inside the cavity is confirmed by X-ray crystallography. Using a qualitative relationship between λmax (UVvis) and the association constant of various adducts, the relative binding between NO3 and [Pd3]2+ is in the same order of magnitude of that between CF3CO2 and [Pd3]2+. Based on Pd···O distances, the NO3 ion is relativity more weakly associated to [Pd3]2+ in comparison with the known [Pd3(CF3CO2)]+. Using variation of [NO3] in the presence of [Pd3](PF6)2 and variation of scan rates, the resulting cyclic voltammogramms (CV) were simulated to extract both kinetic and thermodynamic constants associated to a square scheme. Both the anodic and cathodic scans of the title cluster exhibits ECE mechanisms (E = electrochemical process; C = chemical process). For the anodic scan, the EEC event is not ruled out, at least as a minor process.Key words: nitrate, palladium cluster, adduct, electrochemistry, X-ray structure, rate constant, equilibrium constant.
The effect of pressure and heating rate on the melting behavior of indium and tin
Authors: Jones David E.G , Wang Ruiping , Turcotte Anne-Marie
Published: May 2006

The dependence of the phase transition behavior of indium and tin on pressure and heating rate has been investigated using heat flow calorimetry (HFC). The work has shown a significant influence of heating rate on the form of the melting curve, onset temperature for both indium and tin, as well as the enthalpy of fusion for indium. Based on the result for the dependence of onset temperature on pressure, the volume changes on fusion have been estimated using the Clausius–Clapeyron equation and compared with literature results.Key words: indium, tin, high pressure, heat flow calorimetry calibration, fusion temperature, enthalpy of fusion.
LGA-Dock/EM-Dock  Exploring Lamarckian genetic algorithms and energy-based local search for ligandreceptor docking
Authors: Wiley E Ashley , MacDonald Michael , Lambropoulos Andreas , Harriman D Joseph , Deslongchamps Ghislain
Published: May 2006

We report the development of LGA-Dock and EM-Dock, two SVL-based docking programs for flexible ligand  rigid receptor docking applications. LGA-Dock is comprised of a stochastic population generator, a docking routine based on a Lamarckian genetic algorithm, and a local search function based on molecular mechanics (MM) energy minimization. Subsequent modifications of LGA-Dock to address performance issues produced EM-Dock, which proved to be as accurate and much faster than its predecessor despite the deletion of the genetic algorithm component. The basic performance of LGA-Dock and EM-Dock, compared with AutoDock and MOE™ 2004.03 docking routines is presented.Key words: docking, Lamarckian genetic algorithm, molecular mechanics, simulated annealing, tabu search, local search.
Studies of structure and dynamics in a nominally symmetric twisted amide by NMR and electronic structure calculations
Authors: Bain Alex D , Chen Hao , Harrison Paul H.M
Published: May 2006

Amides that are twisted around the C—N bond show unusual spectroscopy and reactivity when compared with planar amides. The diacyl derivatives of 3,4,7,8-tetramethyl-2,5-dithioglycoluril are intriguing examples of this class, since the crystal structures show that the two acyl groups are twisted by different amounts on either side of the molecule owing to a combination of steric and electronic effects. However, the 1H NMR spectra in solution at room temperature exhibit only one acyl resonance, so there must be fast interconversion among pairs of equivalent structures of each compound. We have prepared a number of derivatives with different acyl groups, both on the glycoluril framework as well as on its dithio analogue. The chemical exchange in solution was slowed down sufficiently by cooling to see individual sites for only two compounds: the dithiodipivaloyl and the dithiodiadamantyl derivatives. The barriers were estimated at 41 kJ mol–1 for the dipivaloyl derivative and 45 kJ mol–1 for diadamantyl derivative. The results show that rotation around the twisted amide bond is slowed by both the steric size of the acyl group and the presence of the thioureido group vs. the ureido group in the glycoluril core. In the solid-state 13C NMR spectra, there is no evidence for any dynamics, even for the diacetyl derivative at ambient temperature. Electronic structure calculations predict a geometry for the dipivaloyl derivative very close to that observed in the crystal structure. These results indicate that the crystal confines, but does not distort the molecule. A mechanism for the exchange is proposed. The relevance of these results to the mechanism of Claisen-like condensations in diacylglycolurils is also discussed.Key words: 1H and 13C NMR, exchange, dynamics, CP/MAS, solids, line shape analysis, amides, twisted amides, atropisomers, glycoluril.
Chiral shift reagent catalyzed, diastereoselective imino DielsAlder reaction  Synthesis of pyrazolyltetrahydroquinolines
Authors: Sridhar Radhakrishnan , Perumal Paramasivan T
Published: May 2006

A new, target oriented, highly diastereoselective synthesis of pyrazolyltetrahydroquinolines was attempted successfully. The chiral shift reagent Eu(tfc) was found to be a better choice for inducing diasteroselectivity.Key words: imino DielsAlder reaction, chiral shift reagent catalyst, pyrazolyltetrahydroquinolines, diastereoselectivity, antibacterial activity.
Synthesis of pyrimidine annulated furothiopyrans  An efficient sequential and tandem catalyzed Claisen rearrangement  intramolecular hydroaryloxylation
Authors: Majumdar K C , Chattopadhyay S K
Published: May 2006

Regioselective synthesis of a hitherto unreported furothiopyran moiety fused at the C-5 and C-6 positions of a pyrimidine heterocycle was achieved by the application of sequential Claisen rearrangement in which a second aromatic Claisen rearrangement and intramolecular hydroaryloxylation were catalyzed by aluminum chloride. The second aromatic Claisen rearrangement step was also studied under thermal conditions to give mostly isomerized exocyclic compounds. The precursor endocyclic compounds were synthesized by thermal [3,3] sigmatropic rearrangement of the corresponding sulfide.Key words: aluminum chloride, sequential Claisen rearrangement, hydroaryloxylation, furothiopyran, pyrimidine.
Preparation and application of 4′-carboxybenzo-15-crown-5, a drift-type crown ether phase transfer catalyst
Authors: Chou Ming-Seng , Shih Jeng-Shong
Published: May 2006

A new "drift-type" crown ether phase transfer catalyst, 4′-carboxybenzo-15-crown-5 adsorbed on poly(styrene/divinylbenzene)-NR2 resin, was synthesized and applied to catalyze the oxidation of benzhydrol with NaMNO4 as an oxidant. Before the reaction, the drift-type crown ether catalyst was adsorbed on the resin with ion-pair (resin-NR2H+−O2C-monobenzo-15-crown-5) formation. During the reaction period the crown ether catalyst was released from the resin into the reaction solution as a homogeneous catalyst by adjusting the acidity of the solution to 10−1 M HCl to destroy the ion pair between the catalyst and the resin. After the reaction, the crown ether catalyst could be readsorbed on the resin by adjusting the concentration of HCl to 10−5 M and could be readily recovered by filtration. The catalyst, 4′-carboxybenzo- 15-crown-5, was found to be a more effective catalyst than other crown ethers, such as 15-crown-5, 18-crown-6, monobenzo-15-crown-5, and dibenzo-18-crown-5.
A convenient procedure for the efficient preparation of alkyl (Z)-3-iodo-2-alkenoates
Authors: Piers Edward , Wong Timothy , Coish Philip D. , Rogers Christine
Published: May 2006

Reaction of alkyl 2-alkynoates with sodium iodide (1.6–5.5 equiv.) in acetic acid (6.2–13 equiv.) at 115 °C provides good to excellent yields of the corresponding alkyl (Z)-3-iodo-2-alkenoates.
Crystal chemistry of tetraradial species. Part 5. Interaction between cation lone pairs and phenyl groups in tetraphenylborates: crystal structures of Me3S+,Et3S+,Me3SO+,Ph2I+, and 1-azoniapropellane tetraphenylborates
Authors: Knop Osvald , Cameron T. Stanley , Bakshi Pradip K. , Linden Antony , Roe Stephen P.
Published: May 2006

Determination of the crystal structures of the tetraphenylborates of the trimethylsulfonium (Me3S+,P21/n), triethylsulfonium (Et3S+,Cmcm), trimethylsulfoxonium (Me3SO+,P21/m), diphenyliodonium (Ph2I+,Pnma), and 1-azoniapropellane (C10H18N+,P21/n) cations has shown that when regions of high local electron density (lone electron pairs) are present in the cation, the cation orients itself so as to minimize the repulsion between the lone pair(s) and the nearest anion phenyl groups, i.e. the cation orientation responds to the lone pair – aromatic π system interaction. This behaviour contrasts with the formation of H …π hydrogen bonds in tetraphenylborates of organic ammonium cations containing N-hydrogens and provides corroborative evidence for our previous finding: (1) given the opportunity, a cation N-hydrogen will form a hydrogen bond to a phenyl group(s) of the anion; (2) the orientation of the cation to form the N—H(N) …π bond is thus a manifestation of a positive hydrogen-bonding tendency rather than a passive response to overall packing requirements in the crystal. The geometries of the above cations are compared to those reported in the literature and those of Me3S+ and Me3SO+ have also been optimized by ab initio (RHF/6-31G*) methods.
Formation of isomeric monohalo-n-alkanoic acids in the reactions of γ- and δ-n-alkanolactones with hydrogen halides.
Authors: Granata Alessandro , Perlin. Arthur S.
Published: May 2006

not available
Substituent dependency in pentaamminemetal(II) complexes (M = Ru, Os) of styrenes probed by using carbon-13 nuclear magnetic resonance spectroscopic method and voltammetrical analysis
Authors: Lin Shaw-Tao , Jong Jin-Tsuey
Published: May 2006

The 13C NMR spectra of [M(NH3)5(styrene)]2+ (M = Ru, Os) demonstrate the substituent dependency of the interaction between the vinyl group and the metal moiety which suggests the modes of bonding. This phenomenon is also observed from the voltammetric analytical results which confirm the substituent dependency of the oxidation potential.
Concerning the internal rotational barrier and the experimental and theoretical nJ(13C,13C) and nJ (1H,13C) in ethylbenzene-β-13C
Authors: Schaefer Ted , Chan Wing K. , Sebastian Rudy , Schurko Robert , Hruska Frank E.
Published: May 2006

The 1H nuclear magnetic resonance spectra of ethylbenzene-β-13C in CS2/C6D12 and acetone-d6 solutions yield long-range 1H,1H and 1H,13C coupling constants. The 13C {1H} NMR spectra yield 13C, 13C couplings. The conformational dependence of some of these coupling constants is compatible with two values of the barrier to internal rotation about the exocyclic Csp2—Csp3 bond. If the fourfold component of the internal rotational potential is not larger than about 20% of the twofold component and the perpendicular conformer is most stable, then the barrier height is probably less than 6 kJ/mol. However, if the stable conformer has a torsion angle of 60° for the exocyclic C—C bond, then the coupling constants are consistent with a twofold barrier of about 23 kJ/mol. Experimental values of [Formula: see text] and [Formula: see text], where ψ and θ are the torsion angles for the exocyclic C—C and C—H bonds, respectively, are compared to those obtained from INDO MO FPT computations of the angular dependence of nJ(H,C), and nJ(H,H), nJ(C,C) for n ≥ 3. For example, the computations very likely give the correct qualitative ψ dependence of 5J(C,C), yet overestimate its extremum by about a factor of two, either because of an overestimate of the σ–π exchange integrals or because of too large a valence orbital density at the carbon nucleus. Other nJ values are discussed in a similar manner and, because optimized geometries are used in the computations, a somewhat more reliable treatment arises for some coupling constants; an example is 3J(C,C) at small torsion angles.
Structural studies of ligands for the glycine binding site of the excitatory amino acid receptor complex: 2. 6-bromo-, 6-methyl-, and 6-styryl-1,8-ethano-4-hydro-2,3-quinoxalinedione
Authors: Kubicki Maciej , Kindopp Teresa W. , Capparelli Mario V. , Codding Penelope W.
Published: May 2006

The crystal structures of three tricyclic quinoxalinedione derivatives, 6-bromo-1,8-ethano-4-hydro-2,3-quinoxalinedione (1), 6-methyl-1,8-ethano-4-hydro-2,3-quinoxalinedione hydrate (2), and 6-styryl-1,8-ethano-4-hydro-2,3-quinoxalinedione (3), are reported. For 1 and 2, the space groups are P21/n with the unit cell parameters for 1: a = 7.4003(5) Å, b = 8.5799(5) Å, c = 14.3127(9) Å, β = 90.639(6)°, and for 2: a = 7.0590(2) Å, b = 10.7483(3) Å, c = 13.9509(7) Å, β = 103.290(3)°. For 3, the space group is P21/c, with a = 19.3683(10) Å, b = 8.0962(16) Å, c = 19.5801(16) Å, β = 114.028(6)°. Compound 3 crystallizes with two molecules in the asymmetric part of the unit cell; in one of them the styryl group is disordered. The geometries of the 1,8-ethano-4-hydro-2,3-quinoxalinedione fragments are similar in all observations, with the differences mainly caused by the different nature of the substituents in the 6-position. Hydrogen bonds connect the molecules into three-dimensional networks. Head-to-tail π-stacking between molecules connected by a center of symmetry determines the packing modes in 1 and 2 but there is no π-stacking in the crystal structure of 3. The crystal structures of the three quinoxaline derivative ligands for the glycine receptor suggest a mode of recognition that involves an [Formula: see text]receptor hydrogen bond, a three-centre hydrogen bond to the neighboring carbonyl groups on the ligand, and π-stacking between ligand and receptor. This mode is consistent with the geometric constraints of the current binding site model but places greater emphasis on hydrogen-bond interactions.
Acetal formation from methyl formate, pinacolone, and benzophenone: equilibrium constants in methanol and water determined by a chain of transacetalization equilibria
Authors: Guthrie J. Peter , Guo Junan
Published: May 2006

Transacetalization equilibrium constants can be measured in methanol solution. This allows a ladder of equilibrium constants to be constructed from acetophenone, for which the equilibrium constant for acetal formation has been measured in methanol, to methyl formate, for which it has not. This is the first direct measurement of an equilibrium constant for formation of an acetal of an acyclic ester. We have also determined equilibrium constants for acetal formation from benzophenone and pinacolone. The value for benzophenone is inconsistent with the value reported for the diethyl acetal by Pfeiffer and Adkins (G.J. Pfeiffer and H. Adkins. J. Am Chem. Soc. 53, 1043 (1931)). We must conclude that their analytical method was subject to systematic errors.
Oxygen transfer reactions. 2. A comparison of the reactions of ruthenium tetroxide, chromyl chloride, and permanganate with thianthrene 5-oxide
Authors: May Bonnie L. , Yee Henry , Lee Donald G.
Published: May 2006

Products obtained from the oxidation of thianthrene 5-oxide, SSO, have been used to compare oxygen transfer mechanisms for three high-valent transition metals. Oxidation of SSO by benzyltriethylammonium permanganate in methylene chloride gives the corresponding sulfone, thianthrene 5,5-dioxide (SSO2), as the exclusive product. Oxidation of SSO by ruthenium tetroxide also gives SSO2 as the predominant product along with minor amounts of the disulfoxide, thianthrene 5,10-dioxide (SOSO). However, the converse is observed when chromyl chloride is used as the oxidant; SOSO is the major product. It is suggested that oxygen transfers from permanganate and ruthenium tetroxide are initiated by complexation between the central metal atom and the oxygen end of the S=O dipole, while oxidation by chromyl chloride is likely initiated by an alternative mechanism, possibly a single electron transfer.
Antineoplastic agents 285. Isolation and structures of cephalostatins 14 and 15
Authors: Pettit George R. , Xu Junping , Ichihara Yoshitatsu , Williams Michael D. , Boyd Michael R.
Published: May 2006

The tube-inhabiting marine worm, Cephalodiscus gilchristi has been found to produce a series of cytotoxic and antineoplastic disteroidal alkaloids. A substantial study concerned with discovery of new cell growth inhibitory constituents of this marine animal has led to isolation of cephalostatins 14 (5a) and 15 (5b). The structures were determined by interpretation of spectral data, principally high-field (400 and 500 MHz) NMR and HRFABMS combined with NMR spectral relationships to cephalostatin 1 (4) where the structure was previously assigned by X-ray crystal structure analysis. The stereochemical assignments were established employing the ROESY NMR technique. Both new cephalostatins significantly inhibited growth of the P388 lymphocytic leukemia and a selection of human cancer cell lines. The two new cephalostatins further enhance knowledge of structure–activity parameters for these potentially important steroidal alkaloids.
Structural and conformational analysis of six-, seven-, and eight-membered cycloalkanes in tris(5-acetyl-3-thienyl)methane inclusion compounds
Authors: Pang Li , Brisse François
Published: May 2006

Tris(5-acetyl-3-thienyl)methane (TATM) forms inclusion compounds with cyclohexane, -heptane, and -octane in a host/guest ratio of 2:1. The clathrate compounds have been studied by X-ray crystallography. The inclusion compound with cyclohexane crystallizes in two distinct triclinic unit cells of space group [Formula: see text] in the A form, the unit cell dimensions are a = 8.622(2), b = 10.194(2),c = 12.795(2) Å, α = 79.09(1)°, β = 72.74(1)°,γ = 84.89(1)°, V = 1053.9(4) Å3(220 K); for the B form, a = 11.668(4), b = 13.729(3), c = 14.227(4) Å, α = 89.40(3)°, β = 77.20(4)°, γ = 76.24(4)°, V = 2157(1) Å (room temperature). In the A form the cavity is of the channel type but the guest molecule is located on a centre of inversion while in the B form the cavity is also of the channel type but the guest molecule is in a general position. The inclusion compounds of cycloheptane and cyclooctane crystallize only in the B form. The guest molecules in the B form are disordered. Conformational analyses of the guest molecules have also been performed by a combination of molecular dynamics and energy minimization.
Realistic, detailed images of proteins and tertiary structure elements: ab initio quality electron density calculations for bovine insulin
Authors: Walker P. Duane , Mezey Paul G.
Published: May 2006

Detailed, ab initio quality electron density calculations are reported for the elements of the tertiary structure of bovine insulin, a protein containing 773 atoms in 51 residues. The high resolution 3D images of the molecular body and tertiary structure elements of insulin are computed using the new Molecular Electron Density Lego Assembler (MEDLA) method. The high, intermediate, and low electron density images show detailed, intricate shape features not revealed before. The MEDLA technique serves as a computational microscope for chemical, biochemical, pharmaceutical, and drug design applications.
Investigation of the effect of ring size on the product distribution for the Schiff base reaction of 2-acetylcycloalkanones with diamino alkanes
Authors: Enriquez Raul G. , Fernandez-G Juan M. , Leon Ismael , Reynolds William F. , Yang Ji.-Ping , Yu Margaret
Published: May 2006

The Schiff base condensation reaction of 1,2-diaminoethane with a series of 2-acetylcycloalkanones (from cyclopentanone to cyclooctanone) has been investigated and the products characterized by two-dimensional nuclear magnetic resonance. The site of attack of the amino groups, i.e., ring ketone or acetyl ketone, is determined primarily by ring size. 2-Acetylcyclohexanone yields two products in ca. 9:1 ratio, the major product where the two amino groups attack at the ring ketones of two different cyclohexanone molecules, and the minor product where one amino group attacks one ring carbonyl of one cyclohexanone while the second amino group attacks the acetyl group of another. 2-Acetylcyclopentanone yields all three possible products with the major product involving attack at the acetyl groups of two different cyclopentanones. The corresponding reactions for 2-acetylcycloheptanone and 2-acetylcyclooctanone each give a single product corresponding to attack at the acetyl groups of two different cycloalkanones. Similar product distributions are observed for the reactions of the different 2-acetylcycloalkanones with 1,4-diaminobutane. Keywords: Schiff base reactions, diketones, 2D NMR.
Synthèse de divers types d'hétérocycles fonctionnels par décomposition de peroxydes insaturés: réactions compétitives
Authors: Montaudon E. , Lemée L. , Bourgeois M-J.
Published: May 2006

Decomposition of γ-unsaturated substituted peroxides in a good hydrogen donor solvent can lead to two diastereoisomeric oxetans. An intramolecular 1,5 hydrogen shift followed by a cyclization may compete with the homolytic intramolecular substitution following the first addition step. The factors governing this never-reported competition are determined. Keywords: unsaturated peroxides, oxetans, homolytic intramolecular substitution, intramolecular 1,5 hydrogen shift.
Cyclobutene photochemistry. Substituent effects on the photochemistry of 1-phenylcyclobutene
Authors: Leigh William J. , Postigo J. Alberto
Published: May 2006

The photochemistry and photophysics of 1-phenylcyclobutene and five aryl-substituted derivatives have been studied in various solvents at room temperature. All six compounds fluoresce with quantum yields in the 0.2–0.3 range in cyclohexane and acetonitrile solution. 1-Phenylcyclobutene undergoes [2+2]-cycloreversion [Formula: see text] to yield phenylacetylene upon photolysis in either hydrocarbon or acetonitrile solution, and undergoes (Markovnikov) solvent addition upon irradiation in methanol solution [Formula: see text] in addition to cycloreversion. Triplet sensitization and quenching experiments indicate that cycloreversion and methanol addition are both excited singlet state processes. None of the six compounds studied undergo ring opening to the corresponding 2-aryl-1,3-butadiene in detectable yield. Quantum yields for cycloreversion in cyclohexane, acetonitrile, and methanol solution and methanol addition have been determined for the six compounds, along with excited singlet state lifetimes. The quantum yields and rate constants for cycloreversion and methanol addition are both enhanced by substitution with electron-donating groups. The variation in the rate constant for [2+2]-cycloreversion with substituent indicates that there is substantial dipolar character developed in the cyclobutenyl σ-bond framework during the reaction, in almost exact correspondence with that developed in the π system during photoprotonation. No deuterium scrambling is observed in 1-phenylcyclobutene-2,4,4-d3 after photolysis in pentane solution to ca. 80% conversion, indicating that skeletal rearrangements leading to cyclopropyl carbenes do not occur in the direct photolysis of arylcyclobutene derivatives. A pericyclic mechanism for the photocycloreversion reaction is suggested. Triplet–triplet absorption spectra and triplet lifetimes of 1-phenyl-, 1-(para-methylphenyl)-, and 1-(para-trifluoromethylphenyl)cyclobutene in hydrocarbon solution are also reported. Keywords: photochemistry, cyclobutene, fluorescence, [2+2]-cycloreversion, substituent effects, nanosecond laser flash photolysis, lifetime, triplet state, styrene, photoaddition
Kinetic study of the reactions of methoxy-substituted phenacyl radicals
Authors: Jovanovic S.V. , Renaud J. , Berinstain A.B. , Scaiano J.C.
Published: May 2006

The photochemistry of various mono- and dimethoxy-substituted α-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-α-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have π,π* character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of 6 with 1,4-cyclohexadiene occurs with a rate constant of 2.6 × 105 M−1 s−1, while for addition to the double bond in 1,1-diphenylethylene the rate constant is 9.4 × 107 M−1 s−1. Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k ≤ 105 M−1 s−1). In contrast, reaction with oxygen occurs with kq = 2.5 × 109 M−1 s−1. Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin. Keywords: methoxyphenacyl radicals, α-bromoacetophenones, laser flash photolysis.
Solvent effects on the reactivity of solvated electrons with in C1 to C4 alcohols
Authors: Zhao Yixing , Freeman Gordon R.
Published: May 2006

The rate constants [Formula: see text] in pure C1 to C4 alcohol solvents at 298 K increase with increasing viscosity and decreasing permittivity. Thus the reactivity increases with decreasing diffusivity and increasing coulombic repulsion, so the Debye–Smoluchowski model does not apply. The effective reaction radius κRr increases with decrease of effective trap depth Er/τ of the electrons in the solvent: κRr = CτRr(Er/τ)pτ. Values of κRr and Er/τ change with temperature, and values of Pτ fall in four categories: ∼0.0 for water and methanol; ∼1.3 for primary alcohols; 0.6 for secondary alcohols; 1.8 for tert-butanol. The C—H groups participate in the [Formula: see text] reaction. Keywords: alcohol solvents, solvated electron, nitrate ion, reactivity, solvent effects.
Crystal structure and thermal expansion of hexakis(phenylthio)benzene and its CBr4 clathrate
Authors: Michalski Darek , White Mary Anne , Bakshi Pradip K. , Cameron T. Stanley , Swainson Ian
Published: May 2006

The crystal structures of hexakis(phenylthio)benzene (HPTB) and its CBr4 clathrate have been determined by single crystal X-ray diffraction data collected at T = 18 °C and refined to final Rw of 0.036 and 0.047, respectively. Pure HPTB is triclinic, space group [Formula: see text] (No. 2), with a = 9.589(2) Å, b = 10.256(1) Å, c = 10.645(2) Å, α = 68.42(1)°, β = 76.92(2)°, γ = 65.52(1)°, and Z = 1. The CBr4 clathrate of HPTB is rhombohedral, space group [Formula: see text] (No. 148), with a = 14.327(4) Å, b = 20.666(8) Å, and Z = 3. The host–guest mole ratio of HPTB–CBr4 is 1:2. Neutron powder diffraction was carried out on powders of both compounds in the temperature range 25 K < T < 295 K. Thermal expansion coefficients were determined for HPTB and HPTB–CBr4 over this temperature range. Keywords: thermal expansion, crystal structure, clathrate.
Thermodynamic characteristics of NADH/NAD+ analogues in acetonitrile: 2-methyl, 4-methyl and 2,4-dimethyl 1-benzyl-dihydronicotinamides and the corresponding pyridinium species
Authors: Anne Agnès , Moiroux Jacques
Published: May 2006

Procedures were elaborated for the syntheses of the title compounds. The thermodynamic changes brought about by each methyl substitution were then determined quantitatively. In acetonitrile, the respective one-electron oxidation and one-electron reduction potentials of the NADH and NAD+ analogues were obtained by means of direct and indirect (using ferrocene mediators) cyclic voltammetry. The redox potentials of formal hydride transfers were deduced from the studies of equilibrated reactions occurring between the analogues. The pKa's of the cation radicals ensued. Keywords: NADH/NAD+ methylated analogues, one-electron transfers, hydride transfer, thermodynamics.
Ouverture sélective d'époxydes styréniques substitués à l'aide d'azoture de lithium en présence de β-cyclodextrine
Authors: Guy Alain , Doussot Joël , Dallamaggiore Agnès , Garreau Robert
Published: May 2006

Regio- and enantioselective opening of substituted styrenic epoxides by lithium azide in presence of β-cyclodextrin is described. The regioselectivity of the reaction is related to the hydrophobicity of the substrate, which governs its penetration inside the cavity of the cyclodextrin and is favorable to the β opening of the epoxide. The enantioselectivity observed during the kinetic resolution of the racemic epoxide is mainly dependent on the non-covalent interactions between the upper rim of the cyclodextrin and the epoxide. Keywords: β-cyclodextrin, inclusion complexes, regio- and stereocontrol, chemistry in aqueous medium.
Calculation of surface tension of salt solutions: effective polarizability of solvated ions
Authors: Stairs Robert A.
Published: May 2006

The Onsager–Samaras version of Wagner's theory of surface tension of solutions of 1:1 salts has been modified to include ion-induced dipole terms in the image potential. Effective solvated ionic polarizabilities are used, which are large and negative owing to local dielectric saturation around the ions. The modified theory agrees with the order of magnitude of the variation among various alkali halides (e.g., Δσ for 0.1 molar aqueous KI is about 70% of the corresponding value for KCl). Polarizabilities were estimated from Hasted's dielectric decrements. Keywords: surface tension, salt solutions, symmetric electrolytes, effective polarizability, theory.
Synthesis and structural characterization of barium complexes containing the bulky amino-siloxide hybrid ligand −OSi(tBu)2((CH2)3NMe2)
Authors: Shao Pengcheng , Berg David J. , Bushnell Gordon W.
Published: May 2006

The bulky amino-siloxide hybrid ligand −OSi(tBu)2((CH2)3NMe2) allows isolation of dinuclear, hydrocarbon-soluble barium complexes. The structures of two mono(ligand) complexes, {Ba[OSi(tBu)2((CH2)3NMe2)][N(SiMe3)2]}2 (1) and {Ba[OSi(tBu)2((CH2)3NMe2)][N(SiMe3)2][THF]}2 (3), have been determined by X-ray diffraction (crystal data: 1, monoclinic, C2/c, a = 20.451(4) Å, b = 14.277(3) Å, c = 22.258(5) Å, β = 101.14(2)°, V = 6376.1(2) Å3, Z = 4 (dimers), R = 0.069,Rw = 0.100; 3, monoclinic, P21/c, a = 11.025(4) Å, b = 22.414(3) Å, c = 13.925(2) Å, β = 104.05(2)°, V = 3338.0(1) Å3, Z = 2 (dimers), R = 0.090, Rw = 0.117). Complex 1 is stable towards ligand redistribution in d6-benzene or d8-toluene, but in d8-THF both 1 and 3 exist in equilibrium with the ligand redistribution products Ba[N(SiMe3)2]2[THF]2 and Ba[OSi(tBu)2((CH2)3NMe2)]2 (2). Complex 2 was prepared independently and appears to exist as a monomer–dimer equilibrium in d6-benzene solution. Complexes 1–3 do not sublime when heated under high vacuum (200 °C, 10−4Torr). Keywords: siloxide, barium, synthesis, structure, bis(trimethylsilyl)amide.
Structural studies of organoboron compounds LXIII. Reaction of ketoximes with aldehydes and diphenylborinic acid, and the crystal and molecular structures of two different types of boron chelates and one of the precursor aldehyde adducts
Authors: Kliegel Wolfgang , Riebe Ulf , Rettig Steven J. , Trotter James
Published: May 2006

The diphenylboron chelates of ketoxime adducts to formaldehyde and trichloroacetaldehyde (chloral) have been synthesized. The reactions were carried out as three-component one-pot reactions, and also as two-step reactions for the ketoxime chloral adducts, which were isolated and characterized before reaction with oxybis(diphenylborane) to form the diphenylboron chelates. One of the intermediate adducts and two of the diphenylboron chelates have been characterized by X-ray crystallographic analysis. Crystals of O-(2,2,2-trichloro-1-hydroxyethyl)-2-propanonoxime, 8d, are monoclinic, a = 7.010(2), b = 11.832(3), c = 11.684(2) Å, β = 90.48(2)°, Z = 4, space group P21/c; those of 4-isopropylidene-2,2-diphenyl-1,3-dioxa-4-azonia-2-boratacyclopentane, 9a, are orthorhombic, a = 11.098(2), b = 15.364(1), c = 8.715(2) Å, Z = 4, space group P212121; and those of 5-trichloromethyl-2-isopropylidene-3,3-diphenyl-1,4-dioxa-2-azonia-3-boratacyclopentane, 10d, are monoclinic, a = 9.164(2), b = 15.304(3), c = 13.859(2) Å, β = 105.28(1)°, Z = 4, space group P21/n. The structures were solved by direct methods and refined by full-matrix least-squares procedures to R = 0.037, 0.044, and 0.040 (Rw = 0.041, 0.016, and 0.042) for 1082, 1256, and 3062 reflections with I ≥ 3σ(F2), respectively. The X-ray analyses show that the products from the reaction between acetonoxime, oxybis(diphenylborane), and formaldehyde or trichloroacetaldehyde, respectively, possess different boron chelate structures. With formaldehyde a COBON chelate ring is obtained and with trichloroacetaldehyde a BOCON chelate ring is formed. Keywords: boron chelates, boron compounds, organboron compounds, crystal structures.
Aryldiazenido complexes: structure, fluxionality, and properties of the iridium ethylene aryldiazenido complex [(η5-C5Me5)Ir(C2H4)(p-N2C6H4OMe)][BF4] and a comparison with the analogous nitrosyl complex [(η5-C5Me5)Ir(C2H4)(NO)][BF4]
Authors: Yan Xiaoqian , Einstein Frederick W.B. , Sutton Derek
Published: May 2006

[Cp*Ir(C2H4)(N2Ar)][BF4] (1BF4; Ar = p-N2C6H4OMe) has been synthesized by reacting [ArN2] [BF4] with Cp*Ir(C2H4)2 at low temperature. An initial electrophilic attack of the incoming diazonium ion at iridium, followed by expulsion of C2H4, is postulated to account for the mild reaction conditions that are in sharp contrast to the usual inertness of the bis(ethylene) compound toward ligand substitution. The IR and nitrogen NMR data for 1BF4 and its 15Nα derivative unambiguously establish that the ArN2 ligand has the singly bent geometry in this complex in solution. The X-ray crystal structure confirms this for the solid state, and establishes that the plane of the aryldiazenido ligand is approximately perpendicular to the plane defined by the Ir atom and the centers of mass of the Cp* and ethylene ligands. An extended Hückel molecular orbital analysis of the singly bent aryldiazenido ligand has been carried out and satisfactorily accounts for the observed orientation of the ArN2 ligand. An analysis of the variable-temperature 1H and 13C NMR of 1BF4 indicates that both restricted rotation of the C2H4 ligand and a conformational isomerization of the aryldiazenido ligand are occurring, and ΔG≠270 for the ethylene rotation barrier is estimated at ≤ 51.5 ± 0.4 kJ mol−1. This is lower than the barrier of ΔG≠353 = 68.7 ± 0.2 kJ mol−1 determined previously for the analogous nitrosyl complex [Cp*Ir(C2H4)(NO)] [BF4] (2BF4) and it is suggested that in these half-sandwich complexes both NO and ArN2 function as single-faced π-acceptors, and in these circumstances ArN2 is the better π-acceptor. The ethylene in 1BF4 is readily displaced by PPh3 to give [Cp*Ir(PPh3)(N2Ar)][BF4] (3BF4). This reacts with NaBH4 to yield Cp*IrH(PPh3)(N2Ar) (4) in which the ArN2 ligand has switched to the doubly bent geometry, on the basis of the 15Nα NMR chemical shift data. Attempts to synthesize the corresponding chloro analogue 5 resulted in only the chloride salt of the singly bent ArN2 cation 3. For example, reaction of 3BF4 with HCl yields the aryldiazene complex [Cp*IrCl(PPh3)(NHNAr)] [BF4] (6), but deprotonation of this with Et3N yields 3Cl, not 5. Compound 1BF4 crystallized in the space group P21/n with a = 8.5780(10) Å, b = 20.5310(23) Å, c = 12.0310(15) Å, β = 93.500(10)°, and Z = 4. The structure was refined to Rf = 0.0281 on the basis of 2611 observed reflections with I°> 2.5σ(Io) in the range 2θ = 0–50° (Mo-Kα). Keywords: iridium, cyclopentadienyl, aryldiazenido, nitrosyl, ethylene.
On the use of diamond as a pressure calibrant for near-infrared FT–Raman microspectroscopy at high pressures
Authors: Markwell Ross D. , Butler Ian S.
Published: May 2006

The pressure-induced shift of the Raman-active t2g phonon mode of the diamond windows in a commercial diamond-anvil cell, located at l332.5 cm−1 at ambient pressure, can be used as an in situ calibrant for near-IR FT–Raman microspectroscopy at high pressures. The measured pressures are considered to be accurate to within ±2 kbar throughout the 4–45 kbar range for which the associated pressure (P, kbar) vs. wavenumber (v, cm−1) relationship is P = 6.66v − 1335.9. Keywords: FT–Raman spectroscopy, diamond, high pressures.
Novel binuclear doubly vinyl-bridged iridium complexes formed by olefinic C—H activation
Authors: Werner Helmut , Wolf Justin , Nessel Angelika , Fries Arno , Stempfle Bernd , Nürnberg Oliver
Published: May 2006

Treatment of [C5H5IrX2]n(2: X = Cl; 3: X = Br) with CH2=CHtBu in the presence of Na2CO3–iPrOH gives, besides the bis(olefin) complex [C5H5Ir(CH2=CHtBu)2] (4), the doubly vinyl-bridged iridium(II) compound {[C5H5Ir(μ-η1,η2-CH=CHtBu)]2} (5). The related binuclear complex {[CH2(C5H4)2][Ir2(μ-η1,η2-CH=CHtBu)2]} (9) was obtained from {[CH2(C5H4)2][IrBr2]2}n(8) and CH2=CHtBu–Na2CO3in ethanol and characterized by an X-ray structure analysis (monoclinic, C2/c (No. 15), a = 20.050(8), b = 8.843(1), c = 24.25(1) Å, β = 99.53(2)°, V = 4241 Å3, Z = 8, R = 0.032, Rw = 0.041). Protonation of both 5 and 9 with HBF4•OEt2yields the BF4salts of the unsymmetrical hydridometal cations 10 and 11 in which the vinyl bridges are still intact. The reaction of the binuclear bis(cyclooctene)metal compounds {[CH2(C5H4)2][Rh(C8H14)2]2} (14) and {[CH2(C5H4)2][Rh(C8H14)2][Ir(C8H14)2]} (17) with bromine yields almost quantitatively the polymeric (or oligomeric) dibromo derivatives 16 and 18, which could, however, not be converted to vinyl bridged complexes by treatment with CH2=CHtBu–Na2CO3–ROH. Keywords: iridium complexes, C-H activation, bridging vinyl ligands, cationic hydridometal complexes, heterometallic complex.
Ruthenium(II) complexes of the thiacyclophane ligands 2,5,8-trithia[9]-o-cyclophane (TT[9]OC) and 5-oxa-2,8-dithia[9]-o-cyclophane (ODT[9]OC). Structures of RuCl2(DMSO)(TT[9]OC) and RuCl2(PPh3)(ODT[9]OC)
Authors: Groot Broer de , Jenkins Hilary A. , Loeb Stephen J. , Murphy Shannon L.
Published: May 2006

Ruthenium(II) complexes of the thiacyclophane ligands 2,5,8-trithia[9]-o-cyclophane (TT[9]OC) and 5-oxa-2,8-dithia[9]-o-cyclophane (ODT[9]OC) were synthesized by ligand displacement reactions employing RuCl2(DMSO)4, RuCl2(PPh3)3, and RuHCl(PPh3)3 as starting materials. X-ray crystal structures of two of these complexes, RuCl2(DMSO)(TT[9]OC) and RuCl2(PPh3)(ODT[9]OC), demonstrate how TT[9]OC and ODT[9]OC bind to Ru(II). RuCl2(DMSO)(TT[9]OC) crystallized as the DMSO solvate in the orthorhombic space group Pbca with a = 19.590(5), b = 16.849(4), c = 13.149(4) Å, V = 4340(3) Å3, and Z = 8. The structure refined to R = 5.27% and Rw = 6.27% for 2472 reflections with Fo2 > 3σ(Fo2). RuCl2(PPh3)(ODT[9]OC) crystallized as a ClCH2CH2Cl solvate in the monoclinic space group P21/c with a = 7.912(1), b = 22.419(5), c = 18.794(3) Å, β = 101.12(1),° V = 3271.2(9) Å3, and Z = 4. The structure refined to R = 4.96% and Rw = 5.14% for 1800 reflections with Fo2 > 3σ(Fo2). Both compounds are octahedral with the thiacyclophane ligand bound through three donor atoms in a facial coordination mode, cis chlorine atoms, and the unique ancillary ligand, DMSO or PPh3, bound trans to the central S or O donor of the macrocycle. The X-ray structures support 1H NMR spectral evidence which shows that the ligands are bound in an "endo" mode for L = DMSO and in an "exo" mode when L = PPh3. The reaction of RuHCl(PPh3)3 with TT[9]OC yields the ruthenium hydride complex RuHCl(PPh3)2(TT[9]OC). 1H and 31P NMR spectroscopy are consistent with an octahedral species for which the macrocycle occupies only two coordination sites acting as a bidentate η2-chelating ligand. Keywords: thioether, macrocycle, hydride, crystal structure.