bluuebunny/crossref_metadata_2025 huggingface dataset:
A theoretical study on the activation of hydrogen–hydrogen and hydrogen–alkyl bonds by electron-poor early transition metals
Authors: Folga E. , Ziegler T.
Published: May 2006

A theoretical study has been carried out on four σ-bond metathesis reactions mediated by the electron-poor lutetium metal centre. The four processes include hydrogen exchange, X2Lu-H + D2 → X2Lu-D + HD (1), and hydrogenolysis X2Lu-CH3 + H2 → Cl2Lu-H + CH4 (2), in which a H—H bond is activated, as well as methane exchange, X2Lu-CH3 + CH4 → X2Lu-CH3 + CH4 (3), and methylation, Cl2Lu-H + CH4 → X2Lu-CH3 + H2 (4), in which a C—H bond is activated. The [Formula: see text] fragment employed in a number of experimental studies was modelled by Cl2Lu and all calculations were based on approximate Density Functional Theory (DFT). The study combined methods from quantum mechanics and statistical mechanics to obtain enthalpies and entropies of activation as well as transition state structures. All four processes were found to have an ordered four-centre transition state with negative entropies of activation given by ΔS≠ = −109(1), −124(2), −131 (3), and −134(4) J mol−1 K−1 at T = 298.15 K. The Gibb's free energies of activation, ΔG≠(= ΔH≠ − TΔS≠) were calculated as ΔG≠ = 81.6 (1), 126.0 (2), 136.7 (3), and 130.6 (4) kJ mol−1 at T = 298.15 K. The calculated trends in ΔG≠ are consistent with the observed order of reactivity for σ-bond metathesis reactions between R—H and M—R′ bonds: R = R′ = H >> R = H, R′ = CH3 > R = R′ = CH3. The decrease in the reaction rate is related to the different abilities of the 1s hydrogen orbital and the [Formula: see text] methyl orbital to stabilize the four-centre transition state. Thus, the spherical 1s hydrogen orbital is better able to overlap fully with orbitals on adjacent centres than the directional [Formula: see text] orbital. As a consequence, the electronic barrier is seen to increase from the hydrogen exchange reaction towards the hydrogenolysis and methane exchange processes as one or two hydrogens, respectively, are replaced by methyl groups in the four-centre transition state. Keywords: Density Functional Theory, C—H activation, metathesis, hydrogenolysis, H—H activation.
Relativistic many-body perturbation theory using discrete basis expansion method: accurate representation of second-order energy of Xe atom with contracted well-tempered Gaussian basis set
Authors: Ishikawa Yasuyuki
Published: May 2006

Relativistic many-body perturbation theory calculations on Xe have been performed with basis sets of well-tempered Gaussian-type functions of Huzinaga and Klobukowski. The well-tempered Gaussian-type functions were used in both contracted and uncontracted form. The contracted Gaussian basis sets used in the relativistic many-body study are designed to retain flexibility for correlated calculations both in the core and the valence region. They reproduce second-order energy corrections computed with uncontracted Gaussian basis sets to an accuracy of greater than 99%. A compact representation of the relativistic wave functions in terms of contracted well-tempered Gaussian functions used in the present study provides a useful means of curtailing integral storage requirements and the time needed for correlated calculations, while still retaining high accuracy in relativistic electron correlation effects. Keywords: Dirac–Fock, relativistic many-body perturbation theory, well-tempered Gaussian basis, xenon.
Dipped adcluster model study for the end-on chemisorption of O2 on an Ag surface
Authors: Nakatsuji Hiroshi , Nakai Hiromi
Published: May 2006

Theoretical study for the end-on adsorption of an O2 molecule on an Ag surface is carried out with the use of the dipped adcluster model (DAM). The adcluster of AgO2 is taken and the highest spin coupling model is employed. Electron transfer from the bulk metal to the adcluster considered by DAM is important for the occurrence of the chemisorption, and this cannot be described by the small-size cluster model. Electron correlations are also quite important and are described by the SD-CI method based on the corresponding parent configurations. In the end-on geometry, the superoxide species is the ground state, and there are no peroxide species in the lower energy region. The calculated adsorption energy compares reasonably with the experimental value. The O—O axis of the superoxide is inclined by 70°–80° from the surface normal. The outside oxygen atom of the adsorbed species seems to be more reactive than the inside one, while the net charge on the former is smaller than that on the latter. Keywords: DAM (dipped adcluster model), silver, superoxide, chemisorption.
Chemical interpretation: basis set use and the Periodic Table
Authors: Allen Leland C.
Published: May 2006

Examination of contemporary quantum chemistry literature and texts strongly indicates the need for new interpretative measures and indices that have heuristic chemical appeal, but are rigorously defined by physics. A closer look at the Periodic Table in search of such measures leads to configurationenergy (CE), the average energy of a valence electron, which adds a third intrinsic dimension to the Table itself, and is also strongly correlated with atomic energy level spacings. CE can be translated into a quantum mechanical operator whose expectation value in molecules and solids is the Energy Index, EIA. Many chemical effects: rotation and inversion barriers, the anomeric and α-effects, hyperconjugation, etc., appear capable of being quantified, unified, and simplified by EIA, but because it is a one-electron entity, it does not correlate with binding energies or heats of formation. EIA is insitu electronegativity, and systematic release of basis set contraction constraints permits a controlled analysis of bond polarity in terms of radial and angular hybridization changes.
Properties of atoms in molecules: structures and reactivities of boranes and carboranes
Authors: Bader R. F. W. , Legare D. A.
Published: May 2006

The energies, geometries, and charge distributions of the closo-, nido-, and arachno-boranes and closo-carboranes are determined in SCF calculations using a contracted (9s,5p) basis set of Huzinaga, supplemented with polarization functions. Good agreement is obtained between the experimental and calculated geometries. Molecular structures are assigned on the basis of the bond paths defined by the topology of the charge density using the theory of atoms in molecules. This theory is used to characterize the atomic interactions and to account for the stabilities of these electron-deficient systems. It is shown that their stability is a consequence of the delocalization of charge over the surfaces of the three- and four-membered rings of atoms that result from the formation of bonds of reduced order, a delocalization that is itself essential to the formation of the ring bonds. The Laplacian of the charge density is used to predict the sites of nucleophilic and electrophilic attack in these molecules, predictions that are in agreement with known experimental findings. Keywords: boranes, carboranes, charge density, Laplacian of ρ.
Mono- and dinuclear palladium complexes containing 2-pyridylphosphine ligands, including X-ray characterization of Pd2I2(µ-PPh2py)2 and a dimethylacetylenedicarboxylate A-frame complex Pd2Cl2(µ-Ppy3)2(µ-MeO2C•C=C•CO2Me); py = 2-pyridyl
Authors: Xie Yun , Lee Chung-Li , Yang Yeping , Rettig Steven J. , James Brian R.
Published: May 2006

Dibromo- and diiodo[(2-pyridyl)phosphine]palladium(II) complexes are prepared by metathesis of cis-PdCl2(PPh3−npyn)2 species (n = 1–3) using the appropriate sodium halide; py = 2-pyridyl. NMR spectroscopy, particularly,13C{1H}, is used to distinguish cis and trans isomers. The dinuclear complexes Pd2X2(μ-PPh3−npyn)2, X = halide, are synthesized via a conproportionation reaction using PdX2(PPh3−npyn)2 and Pd2(dba)3; dba = dibenzylideneacetone. Both Pd2l2(μ-PPh2py)2 and a dimethylacetylenedicarboxylate A-frame complex Pd2Cl2(μ-Ppy3)2(μ-MeO2C•C=C•CO2Me) are characterized crystallographically as head-to-tail isomers. The former crystallizes in the monoclinic space group C2/c with a = 30.992(3), b = 18.764(1), c = 13.100(1) Å, β = 100.676(5)°, and Z = 8; the data were refined to R = 0.035 for 5874 reflections with I ≥ 3σ(I). The A-frame compound is triclinic of space group [Formula: see text] with a = 13.545(2), b = 15.064(2), c = 11.991(2) Å, α = 111.56(1), β = 95.36(1), γ = 97.63(1)°, and Z = 2; R = 0.033 from 7128 reflections with I ≥ 3σ(I). The Pd2X2(μ-PPhpy2)2 complexes exist as a mixture of diastereomers because of chirality induced at the phosphorus atoms. The Pd2X2(μ-Ppy3)2 complexes in water generate the [Pd2(H2O)2(μ-Ppy3)2]2+ dication, which is isolated as various salts. The mononuclear complexes in water generate aquo and hydroxo species. Keywords: dimethylacetylenedicarboxylate adducts, palladium complexes (dinuclear), pyridylphosphines.
Structural study in the oligomethylene dithiobenzoate series, nDBS, where n, the number of methylene groups, is odd
Authors: Leblanc Claude , Brisse François
Published: May 2006

Model compounds of the poly(oligomethylene thioterephthalates) have been synthesized and characterized by X-ray diffraction analyses. The pink n-methylenedithiobenzoates, obtained for n = 3, 5, 7, and 9, all have a low melting point of about 320–330 K. They all crystallize in the monoclinic system, space group P21/c. The a and b unit cell dimensions vary little while the c dimension increases monotonically as n changes from 3 to 9. The crystal structure of the four compounds were established from low-temperature X-ray intensities. Except for the lengthening of the methylenic sequence, the four molecules studied here are isostructural. One of the thiobenzoate groups of the molecules is coplanar with all the methylenic carbons, while the other thiobenzoate group is at 90° from the above plane of atoms. The methylenic sequence has the same conformation. It may be described by g+(t)n, where n is the number of methylene groups. Thus, the four compounds belong to a crystallographically homologous series. The molecules form centrosymmetric "dimers," which, in turn, are herring-bone packed in their respective unit cells. In each "dimmer" two aromatic rings are across from one another while being mutually perpendicular. On the basis of the above results, one proposes three potentially acceptable structures for the parent poly(oligomethylene thioterephthalates). Keywords: dithiobenzoate, methylene sequence, crystal structure, poly(oligomethylene thioterephthalate).
Electron thermalization distance distribution and ion mobility in n-C6F14
Authors: Yoshinari Takehisa , Freeman Gordon R.
Published: May 2006

Electron thermalization distances in liquid n-C6F14 were estimated from the electric field dependence of the free-ion yields at 0.3 ≤ E (MV/m) ≤ 3.7 and densities 1564 ≤ d (kg/m3) ≤ 1951; the latter correspond to 335.4 ≥ T (K) ≥ 191.3. The distance distribution function that best fits the data has a Gaussian body and power tail (YGP), the same as in n-alkanes and in C6F6 and SF6, but different from those in the liquids N2, CO, and CS2. The latter apparently capture electrons at higher energies than do the fluorine compounds. The static relative permittivity of liquid n-C6F14 was measured as a function of temperature and compared with the square of the refractive index and the Debye and Clausius–Mosotti equations. Atomic polarization appears to contribute 6% to the refractive index in the far infrared. Cation mobilities at temperatures 335.4 ≥ T (K) ≥ 191.2 are fitted by the free volume model, with v0 = 1.71 × 10−4 m3/mol and Ev = 3.24 kJ/mol. Keywords: electron, ion, liquid density, permittivity, mobility, n-C6F14, thermalization distance.
Comportement photoélectrochimique de ZnIn2S4(n) dans l'obscurité et sous éclairement en solution électrolytique aqueuse
Authors: Savadogo O.
Published: May 2006

Photoelectrochemical characteristics of ZnIn2S4(n) have been studied by potentiodynamic methods and capacity measurements. The variation of the anodic current density (ia) as a function of [OH−] and [H3O+] has been studied under darkness. This darkness current is small (~1 μA cm−2) and it has been attributed to the anodic dissolution of the material. This process is not diffusion controlled. This behaviour was attributed to electron transfer by tunnel effect via localized interface states. The dissolution of the material under illumination has been determined. Its dissolution requires eight holes. The variation of the stabilisation coefficient S (iox/it) with the redox potential (Vredox) and the S2− species concentration (Cr) has been studied. The effects of [S2−] and illumination on the flat band potential of the electrode have been determined.
Flame photometric detection of some transition metals. I. Calibrations and spectra
Authors: Sun Xun-Yun , Millier Brian , Aue Walter A.
Published: May 2006

After gas-chromatographic separation as volatile organometallics, some transition elements were found to respond with analytically relevant sensitivities in the flame photometric detector. Their minimum detectable amounts, in mole of metal per second at an S/Np-t-p ratio of 2, were 1 × 10−14 for nickel and 6 × 10−13 for rhenium, as well as a less sensitive 2 × 10−12 for molybdenum and 3 × 10−12 for cobalt. (When divided by 3.7, these values yield the S/σ = 3 limit of detection as recommended by IUPAC.) Calibration curves were established for these elements in comparison with transition metals already known to respond in the flame photometric detector. Their chemiluminescent spectra, together with that of chromium, were measured in the detector at analytically (as opposed to spectroscopically) optimized conditions. Atomic lines, molecular bands, and continua were all present. Also, the spectra produced by several types of carbon compounds were recorded for a definition of potential spectral interferences from hydrocarbonaceous sample matrices. The atomic lines and the massive continua displayed by certain metals are discussed in some detail. Atomic lines appear up to a limiting energy level of 3.6 eV above ground state. It is suggested that continua could arise from small, perhaps catalytically active particles. Also discussed, in accordance with a reviewer's request, are the analytical performance of the FPD compared to ICP and MIP sources, and the definition and influence of noise on measured performance criteria.
NMR study of the protonation of domoic acid
Authors: Walter John A. , Leek Donald M. , Falk Michael
Published: May 2006

The 1H and 13C NMR spectra of the amnesic shellfish poison, domoic acid, in H2O and D2O solutions have been studied as a function of pH or pD. The results yielded the NMR spectral parameters for each of the five protonation stages of domoic acid, provided accurate pKa values, and enabled each pKa to be associated with a particular protonation site.
Synthesis and tritium-induced F NMR shifts of 1,1,1,4,4,4-hexafluoro-2,3 ditritio-2-butene
Authors: Angelini G. , Lilla G. , Sparapani C. , Ursini O. , Rossi E. , Segre A. L. , McAllister M. A. , Tidwell T. T.
Published: May 2006

The title compound 3 has been prepared by reaction of CF3C≡CCF3 with tritium gas on Lindlar catalyst poisoned with quinoline. The qualitative and quantitative analyses of the products (CF3CH=CHCF3, CF3CH=C3HCF3, CF3C3H=C3HCF3) have been carried out using GC/MS and 1H, 3H, and 19F NMR techniques. A strong isotope-induced shift has been detected in the 19F spectra of 3.
β-Enolization in α-phenyl-α,α′,α′-trimethylcycloalkanones
Authors: Sifton Wendy , Stothers J. B. , Thomas Shirley E.
Published: May 2006

Ring expansion through β-proton abstraction from α-methyl groups has been observed for the α-phenyl-α,α′,α′,-tri-methyl derivatives of cyclopentanone and cyclohexanone upon treatment with t-BuO−/t-BuOH/185 °C. This contrasts with the lack of rearrangement found for the α,α,α′,α′-tetramethyl derivatives but is analogous to the behavior of some α-phenyl acyclic ketones. However, this process is reversible in the cyclic systems, but irreversible in the acyclic cases. For all of these α-phenyl substituted ketones, rearrangement is in competition with Haller–Bauer type cleavage. In the cyclic systems, a minor process was observed whereby some of the ring-expanded product is reduced to the corresponding secondary alcohol but there was no evidence of rearrangement through γ-enolate formation involving phenyl proton abstraction, which is a minor process in the α-phenyl acyclic systems.
2,2-Dimethylsuccinic acid derivatives in isoprenoid synthesis: a 2,2-dimethylvinyl anion equivalent in the synthesis of ar-atlantone, ar-turmerone, and prenylated aromatic compounds
Authors: Strunz George M. , Ya Li
Published: May 2006

The anion of methyl 2,2-dimethylsuccinate was alkylated with benzylic bromides to give the corresponding 3-substituted-2,2-dimethylsuccinates. Hydrolysis to the dicarboxylic acids, followed by bisdecarboxylation with lead tetraacetate, afforded 1-aryl-3-methyl-2-butenes, which are model prenylated aromatic compounds. Acylation of methyl 2,2-dimethylsuccinate with E-3-(4′-methylphenyl) crotonyl chloride gave the substituted succinate 9. Hydrolysis of the ester groups and acid-catalyzed decarboxylation of the resulting β-ketoacid produced the keto acid 10, which was decarboxylated by the Kochi method, furnishing ar-atlantone. Hydrogenation of 10 yielded 12, which on similar decarboxylation afforded ar-turmerone.
Biosynthesis and biogenetic interrelationships of metabolites of the fungus Arthropsistruncata
Authors: Ayer William A. , Craw Peter A.
Published: May 2006

The incorporation of sodium [1-13C]acetate and sodium [1,2-13C2]acetate into a number of polyketide metabolites produced by the fungus Arthropsistruncata has been examined. The location of sodium [1-13C]acetate incorporation into arthropsadiol A (1), cycloarthropsone (2), and arthropsatriol C (3) has been determined by 13C nuclear magnetic resonance spectra of the corresponding diacetate derivatives, and a biosynthetic pathway is proposed to account for the formation of 1–3. Multiple incorporation of sodium [1,2-13C2]acetate into arthropsadiol A (1) leads to a complicated pattern of satellite multiplets in the 13C nmr spectrum of the corresponding diacetate 6 that are interpreted with the aid of 2D-INADEQUATE spectra. A biosynthesis of arthropsolide A (5) is proposed and the absence of 5 and related metabolites in cultures of the Arthropsistruncata fed with 13C-labelled sodium acetate is discussed.
Diels–Alder approach to (±)-longifolene: A formal synthesis
Authors: Ho Tse-Lok , Yeh Wen-Lung , Yule John , Liu Hsing-Jang
Published: May 2006

A formal synthesis of longifolene in racemic form is concluded starting from the Diels–Alder reaction of 6,6-di-methylfulvene and maleic anhydride. Key steps are cyclodehydration, conjugate methylation, and ring expansion.
Diels–Alder reactions of bicyclic, heteroannular dienones
Authors: Minuti Lucio , Selvaggi Roberta , Taticchi Aldo , Guo Ming , Wenkert Ernest
Published: May 2006

Aluminum chloride catalyzed Diels–Alder reactions of bicyclic dienones of the hexalone type with 1,3-butadiene, isoprene, (E)-piperylene, and (E)-1-methoxy-1,3-butadiene are described. Structure analysis of the adducts by NMR spectroscopy is presented. The site-, endo–exo-, and face-selectivity of the reactions are discussed.
The synthesis of norbornanes with functionalized carbon substituents at a bridgehead. 1-(3-Oxonorborn-1-yl)ethanone and 1-(3-oxonorborn-1-yl)-2-propanone
Authors: Yates Peter , Kaldas Magdy
Published: May 2006

Treatment of 2-norobornene-1-carboxylic acid (7) with one equivalent of methyllithium in ether followed by a second molar equivalent after dilution with tetrahydrofuran gave 1-(norborn-2-en-lyl)ethanone (10) and only a trace of the tertiary alcohol 11. Reaction of 7 with formic acid followed by hydrolysis gave a 4:3 mixture of exo-3- and exo-2-hydroxynorbornane-1-carboxylic acid (16 and 17), whereas oxymercuration–demercuration gave only the exo-3-hydroxy isomer 16. Oxidation of 16 and 17 gave 3- and 2-oxonorbornane-1-carboxylic acid (27 and 29), respectively. Oxymercuration–demercuration of 10 gave exclusively 1-(exo-3-hydroxynorborn-1-yl)ethanone (30), which was also prepared by treatment of 16 with methyllithium in analogous fashion to that used for the conversion of 7 to 10. Oxidation of 30 gave 1-(3-oxonorborn-1-yl)ethanone (1). Dehydrobromination of exo-2-bromonorbornane-1-acetic acid and dehydration of 2-hydroxy-norbornane-2-acetic acid derivatives gave 1-(norborn-2-ylidene) acetic acid derivatives to the exclusion of norborn-2-ene-1 -acetic acid derivatives. Treatment of exo-5-acetyloxy-2-norobornanone (52) with ethyl bromoacetate and zinc gave ethyl exo-5-acetyloxy-2-hydroxynorbornane-(exo- and endo-2-acetate (53 and 54). Reaction of 53 with hydrogen bromide gave initially ethyl endo-3-acetyloxy-exo-6-bromonorbornane-1-acetate (59), which was subsequently converted to a mixture of 59 and its exo-3-acetyloxy epimer 61. Catalytic hydrogenation of this mixture gave a mixture of ethyl endo- and exo-3-acetyloxynorbornane-1 -acetate (62 and 63). Basic hydrolysis of this gave a mixture of the corresponding hydroxy acids, 70 and 71; the former was slowly converted to the latter at pH 5. Oxidation of the mixture of 70 and 71 gave 3-oxonorbornane-1-acetic acid (72). Treatment of the mixture with methyllithium as for 16 gave a mixture of 1-(endo- and exo-3-hydroxynorborn-1-yl)-2-propanone (73 and 74), which was oxidized to 1-(3-oxo-norborn-1-yl)-2-propanone (2). Reaction of exo-2-hydroxynorbornane-1-acetic acid lactone (75) with methyllithium in ether gave (1-(exo-2-hydroxynorborn-1-yl)-2-propanone (76), which on oxidation gave the 2-oxo isomer 78 of 2.
An enantiospecific route to C,D ring synthons for steroid synthesis
Authors: Clase J. Andrew , Money Thomas
Published: May 2006

A simple enantiospecific route to a hydrindenone intermediate for steroid synthesis has been accomplished. The introduction of a wide variety of side-chain units is made possible by stereoselective alkylation of a bicyclic ester 13 derived from (+)-camphor 4.
Synthesis of the thymidine building blocks for a nonhydrolyzable DNA analogue
Authors: Kawai Stephen H. , Wang Daguang , Just George
Published: May 2006

The 3′-deoxy-3′-C-(2″-hydroxyethyl)thymidine derivatives 4 and 13 were efficiently synthesized from either thymidine via a known 3′-radical allylation, or from the branched-chain furanose 5 involving the removal of the 2′-hydroxyl after stereospecific nucleosidation. The 5′-deoxy-5′-thiothymidine 20 was prepared via a regiospecific Mitsunobu coupling, providing the third unit required for the synthesis of sulfide-linked DNA analogues.
Micellar rate enhancement studies in mixed sodium fluorocarbon/hydrocarbon surfactant solutions
Authors: Connolly Terrence J. , Reinsborough Vincent C.
Published: May 2006

Stopped-flow kinetic studies were conducted in mixed micellar solutions of fluorocarbon and hydrocarbon anionic surfactants to determine the prevalent micellar form. The probe reaction was the Niaq2+/pyridine-2-azo-p-dimethylaniline (PADA) complexation, which is many times accelerated in the presence of anionic micelles. Binding constants for Niaq2+ and PADA partitioning between bulk solution and micelles were determined through the murexide technique and solubility measurements respectively and the molar reaction volume was obtained from the Robinson equation. The three binary surfactant systems investigated had sodium perfluoroheptanoate as the fluorocarbon surfactant while the hydrocarbon surfactants were sodium decylsulfate, sodium nonanesulfonate, and sodium octanesulfonate. The kinetic results were consistent with unimicellar composition in all three systems which was not the behaviour previously found with the sodium octane sulfonate/sodium perfluoroctanoate system. The difference was attributed to closer similarity in the surfactant pair hydrophobicities as revealed through their critical micelle concentrations. Another finding was that mixed micelles synergistically can lead to a much greater solubilization of PADA than is possible through either of the pure surfactants.
Studies of complex formation from permittivity measurements: a statistical reexamination
Authors: Exner Otto
Published: May 2006

Permittivity measurements on solutions of two compounds present in a variable ratio may reveal complex formation: both the equilibrium constant and the dipole moment of the complex can be in principle estimated. However, an incorrect statistical treatment involving transformation of variables is used quite often, for example, in the classical method of Few and Smith. In this case the estimates obtained are usually biased, or loaded with such a great uncertainty that they are practically worthless. The results of numerous papers, even some recent ones, are thus completely at variance with facts. This is shown in this paper by recalculating equilibrium constants and dipole moments of the complex in 17 examples from the literature. Simultaneously, a correct and simple statistical procedure is suggested. The danger of an incorrect and (or) incomplete statistical treatment may be encountered, for the same reason, even in other, quite different, areas.
Reduction of two-component CuO–Cr2O3 mixed oxides and their catalytic activity
Authors: Múčka V.
Published: May 2006

The reduction with hydrogen of the two-component mixed oxides CuO–Cr2O3 containing various amounts of both components in the whole range from 0 to 100% was studied, as well as some physico-chemical and catalytic properties. The hydrogen peroxide decomposition served as a test catalytic reaction. A significant effect of the reduction on the rate constant of the test reaction was found, as well as on the value of its apparent activation energy. Pronounced changes were observed, especially in the border ranges of the composition. This fact may be connected with the mutual influence of both components in the catalyst.
Combined use of NMR, distance geometry, and restrained energy minimization for the conformational analysis of 8-lysine–vasopressin
Authors: Yu Chin , Yang Ta-Horng , Yeh Chao-Jung , Chuang Li-Chin
Published: May 2006

The conformation of the neurohypophyseal hormone, 8-lysine–vasopressin, in DMSO solution has been determined from 2D-NMR and distance geometry followed by the restrained energy minimization calculation. The conformation of 8-lysin–vasopressin in the 20-membered ring region is well defined but the acyclic tripeptide tail region is disordered.
Diurnal and seasonal modelling of the tropospheric half-lives of polycyclic aromatic hydrocarbons
Authors: Bunce Nigel J. , Dryfhout Helena G.
Published: May 2006

The half-lives of polycyclic aromatic hydrocarbons (PAH) were calculated under different conditions of location, season, and time of day, based on the reaction of PAH with the hydroxyl radical OH as the major sink. The calculations required a method of estimating the diurnal variation in the concentration of OH, which is not normally available experimentally. For naphthalene as a prototype PAH, the half-lives were of the order of several hours, but varied by more than an order of magnitude with season and geographical location. The previously developed model applicable to rural atmospheres was modified to estimate the half-lives of PAH in the urban troposphere by including of the reaction of OH with NO2 as a major urban sink for OH. The half-lives of several PAH were estimated for North York, Ontario, under different conditions of NO2 and ozone concentrations during a period of high tropospheric air pollution in July 1988.
Mathematical modelling of atom loss in delivery tubing
Authors: Daniels R. Scott , Wigfield Donald C. , Williams Kenneth S.
Published: May 2006

A mathematical model is presented to establish a relationship between the quantity of trace analyte vapor lost to the surface of delivery tubing and the tubing diameter, while in transit between stages of an instrument. Cold-vapor atomic absorption spectrometry for mercury, hydride generation techniques, and interfaces like the interface between electrothermal vaporization and inductively coupled plasma mass spectrometry all risk significant analyte loss before measurement. The results of this modelling substantiate the results of limited experimental work published elsewhere suggesting the use of the smallest possible tubing diameter for the delivery of atomic vapor through a tube. This diameter is calculable using Poiseuille's formula. Using this model, kinetic theory, and experimental data, the sticking probability for mercury on latex tubing is calculated to be approximately 1.6 × 10−6.
Étude des conditions de la réaction de condensation de Claisen–Schmidt de la furylméthylcétone sur le 2,5-furanedicarboxaldehyde
Authors: Dominguez Carmen , Plumet Joaquín , Gaset Antoine , Rigal Luc
Published: May 2006

The Claisen–Schmidt reaction of 2,5-furandicarboxaldehyde (FDC) with 2-acetylfuran allows for the synthesis of three products as a function of the reaction conditions. The main effects of five factors and their interaction effects on seven responses have been quantified by means of a fractional factorial design.
Nonbonded and interactions in 2-(4-chlorophenylthio) benzaldehyde in solution. An average skew conformation
Authors: Schaefer Ted , Lee Lina B.-L. , Sebastian Rudy
Published: May 2006

The 1H nuclear magnetic resonance spectral parameters are reported for 4.0 mol% solutions of 2-(4-chlorophenylthio) benzaldehyde in CS2/C6D12 and acetone-d6 at 300 K. In CS2 the O-syn conformer is 36% abundant, rising to 50% in acetone-d6. These abundances are compared to those of the O-syn and O-trans conformers of 2-(alkylthio) benzaldehydes in CCl4, solution. On the basis of coupling constants and chemical shifts it is concluded that the skew conformer of the title compound is very likely the one of minimum energy in both solutions. In the skew conformer the plane of the 4-chlorophenyl group lies perpendicular to the CSC plane and also to that of the other aromatic moiety. It is suggested that the [Formula: see text] interaction is rather weak and that the population of the O-syn conformer is controlled by the orientation of the mainly 3p lone-pair orbital on sulfur. At best, the [Formula: see text] interaction is attractive only when the 3p orbital lies perpendicular to the plane of the formyl group. The skew conformation of the title compound is contrasted to the skew conformation of 2-hydroxyphenyl phenyl sulfide in which, however, the role of the two aromatic planes is reversed; the 3p orbital now lies in or near the plane of the phenyl group COH due to an attractive [Formula: see text] interaction.
Ionic dissociation in complexes of iodine with triphenylphosphine and triphenylamine
Authors: Zhang Ying Ru , Aronson Seymour , Mennitt P. Gary
Published: May 2006

An electrochemical technique has been employed to study the ionization of the iodine complexes of (C6H5)3N and (C6H5)3P. Comparison of the results with a previous study indicates that, at high iodine concentrations, the phosphorus compound behaves like (C6H5)3As and (C6H5)3Sb with the formation of a doubly charged ionic species, (C6H5)3PI22+. The nitrogen compound resembles pyridine in its behavior. Chemical shift data on the (C6H5)3P–I2 system using 31P NMR indicates the presence of additional equilibrium processes.
Differential reactivity of carbohydrate hydroxyls in glycosylations. I. Intramolecular interaction of the 5′-hydroxyl group with the heteroaromatic base in a model compound of 2′-deoxycytidine
Authors: Tang Ting-Hua , Whitfield Dennis M. , Douglas Stephen P. , Krepinsky Jiri J. , Csizmadia Imre G.
Published: May 2006

It is a well-recognized conjecture that the unusual reactivity of certain carbohydrate hydroxyls in glycosylation reactions is due to non-covalent intramolecular bonding interactions involving that hydroxyl. A model compound 1-[β-D-2′,3′-dideoxyribofuranosyl]-2-(1H)-pyrimidinone, which is related to the poor glycosyl acceptor 2′-deoxy-3′-O,4-N-diacetylcytidine (1), has been studied in order to assess the effects of hydrogen bonding involving 05′—H and the heteroaromatic system present in the molecule. The conformational potential energy surfaces of the model compound (lacking only the acetoxy at C3′ and the acetamido at C4) were calculated, using semiempirical (PM3) and abinitio (STO-3G) methods. The [Formula: see text] intramolecular hydrogen-bonded syn conformation of the model compound is the global minimum at the abinitio level of theory. The existence of this intramolecular hydrogen bonding was confirmed, theoretically, by Bader-type topological analysis of charge distribution at the 3-21G**//STO-3G level of theory. Such a conformation of the model compound strongly resembles that found for 1 by NMR in CD2Cl2 solution. The complex formation between this model compound and BF3 was also studied at the STO-3G, 3-21G**//STO-3G, and 6-31G**//STO-3G levels of theory. The results explain why glycosylation of hydrogen-bonded substrates succeeds when promoted by Lewis acids.
Photodissolution of iron oxides. IV. A comparative study on the photodissolution of hematite, magnetite, and maghemite in EDTA media
Authors: Litter Marta I. , Blesa Miguel A.
Published: May 2006

The thermal and 254-nm photochemical dissolution reactions of magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) suspended in EDTA aqueous solutions were compared. γ-Fe2O3 and Fe3O4 are thermally and photochemically more reactive than α-Fe2O3. Both thermal and photochemical dissolution reactions are governed by an initiation step, which involves the production of FeIIaq, and a subsequent thermal reaction of these ions with the solid, to produce FeIIIaq. The initiation step under UV irradiation involves the photoreduction of surface >FeIII – EDTA complexes to yield FeIIaq and the photooxidation of adsorbed EDTA to yield CH2O and other oxidation products. After FeIII – EDTA complexes build up in solution through the following step, homogeneous photolysis is the main source of FeII and CH2O. Oxides with spinel type structure are characterized by faster rates in the two processes, and O2 may inhibit the dissolution processes by changing the stoichiometry of the initiation step to that of the autooxidation of EDTA. The relative importance of autooxidation and photodissolution depends on the nature of the oxide and the experimental conditions. Photooxidation reaction rates parallel those of the photodissolution initiation steps, and long-term stability towards photocorrosion (dissolution) implies low photocatalytic activity for the oxidation of EDTA. The set of differential equations describing all the reaction rates is discussed and applied to the different cases.
Solvent effects in the metal interchange of crown ether – alkali metal cation complexes. Transition from an associative exchange in nitromethane to a dissociative exchange in acetonitrile studied by 23Na nuclear magnetic resonance
Authors: Brière Kathleen M. , Detellier Christian
Published: May 2006

The role of the solvent in the dissociation kinetics and cation exchange mechanisms of the complex sodium–monobenzo-15-crown-5 (Na: B15C5)+ was examined by 23Na NMR. In nitromethane (NM), the cationic exchange between the complexes takes place via an associative metal interchange mechanism, 1M. In acetonitrile (AN), it takes place via a dissociative (dissociation/recombination) mechanism. In AN–NM binary mixtures, the rate constant of the dissociative exchange (k−1) increases with the AN molar fraction, XAN, from 1.1 × 10−2 s−1 for XAN = 0 to 89 × 10−2 s−1 for XAN = 0.400 (corresponding to a decrease of the free energy of activation, ΔGdi≠ from 62.1 to 51.1 kJ mol−1 respectively, at 301.5 K). The activation parameters were ΔHdi≠ = 48 kJ mol−1 and ΔSdi≠ = −20 J K−1, mol−1 for XAN = 0.200. This rate increase was related to the concentration increase in solution of the AN monosolvated complex (AN: Na: B15C5)+. In the whole range of AN mole fractions studied, the rate constant of the associative exchange, k2, was not dependent upon XAN in the error limits: k2 ≈ 9 × 104 M−1 s−1 at 301.5 K (ΔGas≠ = 45 kJ mol−1). The activation parameters were determined to be ΔHas≠ = 23 kJ mol−1 and ΔHas≠ = −75 J K−1 mol−1. These findings are in good agreement with an associative exchange controlled mainly by the conformational changes of the ligand during the concerted partial decomplexation of a sodium cation and partial complexation of a second one, while solvation of the complexed cation plays a major role in the dissociative mechanism.
Preparation and 31P NMR characterization of N-bonded complexes of platinum(II) with a phosphadithiatriazine: X-ray structure of trans-PtCl2(Pet3)(η1-N-Ph2PS2N3)
Authors: Chivers Tristram , Hilts Robert W. , Krouse Ian H. , Cordes A. Wallace , Hallford Randal , Scott Syrona R.
Published: May 2006

The reaction of Ph2PS2N3 with [Pt2(μ-Cl)2(PEt3)4][BF4]2 or [PtCl2(PEt3)]2, in dichloromethane at 23° C produces the 1:1 adducts cis-[PtCl(PEt3)2(Ph2PS2N3)][BF4], 3, and trans-[PtCl2(PEt3)(Ph2PS3N2)], 4, respectively, in good yields. The 31P NMR data for 3 and 4 indicate that (i) the platinum is attached to a nitrogen atom adjacent to phosphorus in both these adducts, (ii) the PEt3 ligands in 3 are in mutually cis positions, and (iii) the PEt3 ligand in 4 is trans to the heterocyclic nitrogen. These structural features were confirmed by an X-ray analysis of 4. Crystals of 4 are monoclinic, space group P21/c, with a = 14.920(3) Å, b = 8.966(5) Å, c = 19.103(5) Å, β = 109.32(2)°, V = 2411.6(16) Å3, and Z = 4. The least-squares refinement with anisotropic thermal parameters for all non-hydrogen atoms converged at R = 0.050 and Rw = 0.053. The Pt—N bond length is 2.122(15) Å and the coordinated nitrogen atom is lifted ca. 0.63(2) Å out of the plane containing the other heterocyclic ring atoms. The attachment of a platinum(II) centre to the PN3S2 ring perturbs the S—N bond lengths significantly. The S—N distance involving the coordinated nitrogen is 1.672(16) Å, while the other S—N distances are 1.631(19), 1.555(19), and 1.562(19) Å, indicative of a localized sulfur diimide (-N=S=N-) structure. The UV–visible spectra of 3 and 4 in CH2Cl2 exhibit absorption bands at 514 and 528 nm, respectively, but dissociation of these adducts to give the free ligand Ph2PS2N3 occurs readily in dilute solution.
Thermal expansivities of aqueous solutions of 2-butoxyethanol in the water-rich region: transition of mixing scheme
Authors: Davies James V. , Lau Frankie W. , Le Loanne T. N. , Lai John T. W. , Koga Yoshikata
Published: May 2006

Thermal expansivities of aqueous solutions of 2-butoxyethanol (BE) were measured at concentrations of xBE < 0.04, where xBE is the mole fraction of BE. Thermal expansivity is a second derivative of the Gibbs free energy. The composition derivatives of thermal expansivities, the third derivatives, show peak anomalies at the same loci as the other third derivatives of the Gibbs free energy reported earlier from this laboratory (Can. J. Chem. 67, 671 (1989); J. Phys. Chem. 94, 3879 (1990); J. Phys. Chem. 95, 4119 (1991)). The loci of such anomalies form a boundary that separates two regions of totally different mixing schemes. The mixing scheme in the water-rich region seems to be consistent with the "iceberg formation," the "structure enhancement of H2O by hydrophobic solute," and the "hydrophobic attraction." In the intermediate composition region, the hydrogen bond network of H2O collapses due to the presence of too many molecules of BE, and H2O and BE molecules interact with each other as normal liquid molecules.
Isomerism in the diphosphirane series
Authors: Fayet Jean-Pierre , Mermillod-Blardet Danièle , Tachon Christine , Etemad-Moghadam Guita , Koenig Max
Published: May 2006

The cis/trans stereochemistry of diphosphiranes 1a–g, 2a, and 2h is investigated by MNDO calculations in terms of substituent effects on intracyclic phosphorus and carbon atoms. This approach is completed by calculations of dipole moments. These results are compared to experimental dipole moment measurements of the synthesized diphosphiranes 3a–f.
Solubility measurements and determination of Setschenow constants for the pesticide carbaryl in seawater and other electrolyte solutions
Authors: Huerta-Diaz Miguel Angel , Rodriguez Silvio
Published: May 2006

Accurate solubility measurements covering the range 278–318 K were obtained for the carbamate pesticide carbaryl (1-naphthyl-N-methylcarbamate) in water, natural and artificial seawater, and NaCl, Na2SO4, CaCl2, and (CH3)4NBr electrolyte solutions. Carbaryl solubilities were determined using a combination of a generator column method and spectrophotometric techniques. In all cases the solubility of carbaryl in water and in the electrolyte solutions increased with an increase in temperature. The salt effect on the pesticide solubility as a function of temperature was obtained by calculating the Setschenow constants for each one of the electrolyte solutions. Salting-out was observed in solutions prepared with NaCl, Na2SO4, and CaCl2 as well as in natural and artificial seawater, whereas salting-in was observed with (CH3)4NBr solutions.
Cycloaddition reactions of SNSAsF6 with aryl nitriles and diphenylacetylene; the preparation and characterization of aryl 1,3,2,4- and 1,2,3,5-dithiadiazole radicals
Authors: Passmore Jack , Sun Xiaoping , Parsons Simon
Published: May 2006

The SNS+ cation (in SNSAsF6) underwent symmetry-allowed concerted cycloadditions with the aryl nitriles RCN (R = 2,5-Me2C6H3, Ph, p-O2NC6H4, and 3,5-(O2N)2C6H3) and diphenylacetylene. The cycloadducts [Formula: see text] (R = 2,5-Me2C6H3, Ph, p-O2NC6H4, and 3,5-(O2N)2C6H3) and Ph [Formula: see text] were characterized by elemental analyses and IR and NMR (1H, 13C, 14N) spectroscopies. The reduction of [Formula: see text] gave the corresponding 1,3,2,4-dithiadiazoles [Formula: see text], which in turn rearranged to the corresponding 1,2,3,5-dithiadiazoles [Formula: see text]. The rate of this rearrangement was correlated with the ionization potentials of the corresponding nitriles. The radicals [Formula: see text] (R = Ph, p-O2NC6H4) were isolated as pure compounds, and characterized by mass spectrometry, IR, FT-Raman, and ESR spectroscopies. The reduction of [Formula: see text] led to neutral radical [Formula: see text], which is stable in solution up to ca. 0.5 M, but decomposes at higher concentrations.
The boron trifluoride – base system: application of the coordination model of reaction mechanisms
Authors: Ou Xiaobo , Wallace Robert , Janzen Alexander F.
Published: May 2006

The mechanism of reaction of boron trifluoride with amines, dialkyl ethers, and pyridine has been analyzed on the basis of the coordination model of reaction mechanisms. This model is tested mathematically by carrying out kinetic simulations of pathways P(X,C), accompanied by the calculation of structures of postulated intermediates by GAUSSIAN86 methods. Analyses of reaction mechanisms, including calculation of concentration vs. time curves, are reported for the following systems: NH3–BF3, MeNH2–BF3, Me2O–Et2O–BF3, Me2O–Et2O:BF3, pyridine–BF3, pyridine–Me3N–BF3, F3B:N–N:BF3, and (N–N)BF2+.
The structure and vertical excitation energies of the chlorothioformyl radical, ClCS: implications for the photofragmentation of thiophosgene
Authors: Hachey M. , Grein F. , Steer R. P.
Published: May 2006

Ab initio CI studies have been performed to determine the geometry of the ground and first electronically excited states of the chlorothioformyl radical, ClCS, and the vertical excitation energies of its ten lowest doublet states and two lowest quartet states. The results are used to construct a more complete energy level correlation diagram for the photofragmentation of Cl2CS. The lowest excited electronic state of ClCS lies only 0.79 eV (adiabatic) above the ground state. Its discovery indicates that the results of previous photofragmentation experiments may need to be reinterpreted.
Dioxirane formation in ozonolysis of E- and Z-1,2-dimethoxy-1,2-diphenylethene
Authors: Kopecky Karl R. , Xie Yu , Molina José
Published: May 2006

Addition of ozone to E- or Z-1,2-dimethoxy-1,2-diphenylethene, E- or Z-1, in inert solvents results in formation of ~1 mole of methyl benzoate, ~0.5 and ~0.1 mole of the corresponding oxiranes 2 and dioxetanes 3, respectively, and 0.03 mole of 3,6-dimethoxy-3,6-diphenyl-1,2,4,5-tetroxane 4 at −20 °C. Product distributions vary with starting material, initial concentration, extent of reaction, and temperature. At −70 °C ~0.3 mole of trans-2 and 0.2 mole of cis-and trans-4 are formed from E-1. Addition of E- or Z-1 to solutions of excess ozone results in formation of up to 0.7 mole of (methoxy)phenyldioxirane 5, which is a strong, stereospecific epoxidizing agent.
Use of a laboratory robot for repetitive solution NMR sample preparation
Authors: Jeannot Michael A. , Reid R. Stephen
Published: May 2006

Observing ligand 1H NMR signals while conditions (e.g., analytical concentrations, pH) are changed in an aqueous metal–proton–ligand system is one of the most potent methods of elucidating the precise details of speciation in such systems. Typically this is performed by changing the conditions in a solution reservoir, while withdrawing a sample for NMR analysis at each stage. However, such sample preparation is tedious and time-consuming. We report here studies, using a Mitsubishi RM-101 laboratory robot, of the feasibility of completely automating such a procedure. A simple acid–base study is used to evaluate performance.
Diastereoselective intramolecular Diels–Alder reactions of the furan diene: the synthesis of (±)-1,4-epoxycadinane
Authors: Rogers Christine , Keay Brian A.
Published: May 2006

The diastereoselective intramolecular Diels–Alder reactions of the furan diene (IMDAF), in which the side arm comprises four carbon atoms and is substituted by a methyl group adjacent to, or one carbon removed from, the furan ring, are described. Adducts are formed in moderate to excellent yields when treated with either Florisil in methylene chloride at room temperature or methylaluminum dichloride in methylene chloride at −78 °C. Florisil is most effective for substrates containing unsubstituted dienophiles. An equimolar quantity (1.1 equivalents) of methylaluminum dichloride is most effective for precursors having methacrolein-type dienophiles, while a catalytic quantity (0.1 equivalent) is effective for crotonaldehyde-type dienophiles. Only the adducts with the side arm orientated syn with respect to the oxygen bridge are detected and isolated. The major diastereomer is the one in which the methyl group (initially on the side arm) is situated equatorially on the newly formed six-membered ring. An application of the diastereoselective IMDAF reaction to the synthesis of (±)-1,4-epoxycadinane is described, beginning with 2-methylfuran.
Transannular Diels–Alder reactions on 14-membered macrocyclic trienes. Part I: stereoselective syntheses of the macrocyclic trienes precursors
Authors: Ndibwami Alexis , Lamothe Serge , Guay Daniel , Plante Raymond , Soucy Pierre , Goldstein Solo , Deslongchamps Pierre
Published: May 2006

Transannular Diels–Alder reactions on 14-membered macrocycles containing properly located diene and methyl-substituted dienophile units lead to A.B.C.[6.6.6] tricycles related to steroids. To study the influence of the olefin and diene geometry on the stereochemical outcome of the Diels–Alder reaction it was necessary to prepare macrocyclic trienes of well-defined stereochemistry. Eight different types of macrocyclic trienes might be obtained by the coupling of appropriate dienophiles and dienes, namely, TTC, TTT, TCC, TCT, CTC, CTT, CCC, and CCT. In this paper (first in a series of two), the synthesis of appropriately functionalized dienophile and diene synthons, as well as their coupling reaction affording macrocyclic triene precursors, is described.