bluuebunny/crossref_metadata_2025 huggingface dataset:
The potential role of fatty acid initiation in the biosynthesis of the fungal aromatic polyketide aflatoxin B1
Authors: Brobst Susan W. , Townsend Craig A.
Published: April 2011

Earlier work in this laboratory has shown the intact incorporation of [1-13C]hexanoate into averufin (1), a key intermediate in aflatoxin B1 biosynthesis. Parallel experiments with equimolar amounts of [1-13C]butyrate, [1-13C]-3-oxo-octanoate, and [1-13C]-5-oxo-hexanoate gave no detectable specific incorporation of heavy isotope but low and equivalent background incorporation comparable to [1-13C]acetate. Three of these potential intermediates in polyketide formation were reexamined as their corresponding N-acetylcysteamine (NAC) thioesters. The NAC thioester of [1-13C]hexanoic acid gave a remarkably high 22% intact incorporation while the NAC thioester of [1-13C]-3-oxo-octanoic acid afforded nearly 5% when an equimolar amount was administered to the producing organism Aspergillus parasiticus (ATCC 24551). In contrast, the NAC thioester of [1-13C]butyric acid showed no selective enrichment of averufin. This negative result was tested further in a more sensitive experiment with the NAC thioester of [2,3-13C2]butyric acid. No 1JCC coupling was detectable, indicating an incorporation efficiency of <0.1%. [1-13C,18O2]Hexanoate was prepared and gave a 53% retention of 18O relative to the I3C internal standard in keeping with previous experiments with [1-13C,18O2]acetate. It is concluded from these data that the initial C6 segment of polyketide biosynthesis is unlikely to arise by β-oxidation of a higher fatty acid but more probably is generated by a specialized fatty acid synthase (FAS) that provides this unit either separately to the polyketide synthase (PKS) or as part of a larger FAS/PKS fusion. While these two physical arrangements cannot be distinguished by these experiments, both must accommodate comparatively efficient exchange of the NAC thioesters of both hexanoic and 3-oxo-octanoic acid, but not the NAC thioester of butyric acid.
Absolute stereochemistry of (−)-camoensine and (−)-camoensidine in Maackia species
Authors: Kubo Hajime , Ohmiya Shigeru , Murakoshi Isamu
Published: April 2011

It was demonstrated that the attack of Grignard reagents such as methyl-, allyl-, and 3,3-dimethoxypropyl magnesium bromides on 11,12-dehydrocytisine occurs on the α-face, to give the corresponding 11α-alkylcytisine. (−)-Camoensine and (−)-camoensidine were synthesized from (−)-cytisine via the Grignard reaction. The absolute stereochemistry of the above alkaloids was confirmed to be 7R, 9R, 11R and 6S, 7R, 9R, 11R, respectively.
Allyldimethylsilyl triflate: a self-catalyzed silyl nucleophile
Authors: Brook Michael A. , Crowe Grant D. , Hiemstra Henk
Published: April 2011

Allyldimethylsilyl triflate 2 may be prepared by a protiodesilylation reaction between diallyldimethylsilane and triflic acid. This compound possesses both a silyl-substituted carbon nucleophile and the Lewis acid necessary for activation of an electrophile. Upon exposure to an aromatic aldehyde (e.g., p-MeOC6H4CHO), the homoallylic alcohol 4 is formed in good yield. The synthetic advantages of the intramolecular Cope-type cyclization reaction are discussed.
Quinones and hydroxyquinones as generators and quenchers of singlet molecular oxygen
Authors: Gutiérrez Isela , Bertolotti Sonia G. , Biasutti M.A. , Soltermann Arnaldo T. , García Norman A.
Published: April 2011

The role of quinones and hydroxyquinones as sensitizers and as quenchers in Type II photooxygenations has been examined. The second aspect is discussed here, through a systematic study, for the first time in the open literature. Quinonic compounds are excellent generators of O2(1Δg) in aprotic solvents (excluding those quinones possessing substituents in positions adjacent to the carbonyl groups, in the case of anthraquinone derivatives). Benzoquinones, anthraquinones, and hydroxy derivatives are good O2(1Δg) quenchers upon dye-sensitized photoirradiation. The excited oxygen species is deactivated with rate constants in the range 106–107 M−1 s−1 depending on the solvent employed. The quenching process deactivates O2(1Δg) without further destruction of the quinone. The main interaction with O2(1Δg) is driven by the quinone moiety, in spite of the presence of potentially active nuclear substituents. The quenching mechanism could involve a reversible charge transfer intermediate, with the quinonic compound acting as an electron donor. Keywords: photooxidation, quenching, quinones, rose bengal, singlet oxygen.
Isotope effects on the reaction of proton transfer to benzene anions in ethanol
Authors: Gelabert Ricard , Moreno Miquel , Lluch José M.
Published: April 2011

A theoretical study on the reaction of proton transfer from ethanol to benzene anions was performed at the ab initio level. It was found that in the gas phase the reaction proceeds without potential energy barrier with respect to reactants held at infinite distance, even though a potential energy barrier exists when measured from the prebarrier complex formed by the reactant molecules. When restrictions on orientation of the reactant molecules in the solid phase are taken into account, the potential energy barrier found exceeds 40 kcal/mol. A theoretical estimate of the kinetic isotope effect due to the tunneling effect, κH/κD, of 9.0 × 103 is found, in good agreement with available experimental data. Keywords: isotope effects, proton transfer, tunneling.
Stereoselective hydrogenation of lignin degradation model compounds
Authors: Hu Thomas Q. , Lee Chung-Li , James Brian R. , Rettig Steven J.
Published: April 2011

Di-µ-chloro-bis(η4-1,5-hexadiene)dirhodium(I) in a two-phase hexane–aqueous medium catalyzes the diastereoselective H2-hydrogenation of lignin degradation model compounds 4-propylphenol, 2-methoxy-4-propylphenol, and 2,6-dimethoxy-4-propylphenol. The all-cis diastereomer is obtained selectively when the phenolic hydroxy group is protected as a methyl ether or when a model compound possessing two methoxy substituents adjacent to the phenolic hydroxy group is used. The relative stereochemistries of the hydrogenated products are established by X-ray crystal structure analysis and (or) 1H NMR. Keywords: catalytic hydrogenation, diastereomer, lignin degradation model compounds, stereoselective, X-ray crystal structure.
A kinetic study of the reactions of dimethyl sulfide bridged tetramethylplatinum(II) and octamethylplatinum(IV) complexes with dimethyl sulfide and bidentate ligands
Authors: Nakayama Katsuyuki , Kondo Yuichi , Ishihara Koji
Published: April 2011

Kinetics for the reactions of [1] [PtIV2Me8 (μ - SMe2 )2] + 2 Me 2S kf<-->kd 2[PtIVMe4 (SMe2 ) 2] [2] [PtII2Me4 (μ - SMe2 )2] + 2 Me 2S kf--> 2[PtIIMe2(SMe2 ) 2 ] and [3] [PtIV2Me8 (μ - SMe2 ) 2 ] + 2NN --> 2 [PtIVMe4 (NN)]+2Me2S where NN = bipy or 4,4'-Me2-bipy, have been studied at various temperatures and pressures. Reaction [3] was shown to consist of the rapid conversion of the dimer to a monomer and the much slower subsequent substitution of the dimethyl sulfide with bidentate ligand NN: [PtIV2Me8 (μ - SMe2 )2] + 2 Me 2S <--> 2 [PtIVMe4 (SMe2 ) 2 ] and [PtIVMe4(SMe2)2] + NN --> [PtIVMe4(NN)] + 2 Me2S The rate constants and activation parameters for the reactions are as follows: kf = 3.16 ± 0.06 M-1 s-1 (25°C), Δ H doubledaggerf= 51.8 ± 1.7 kJ mol-1, Δ S doubledaggerf= -61.0 ± 5.8 J mol-1 K-1, kd = 1.18 ± 0.22 M-1 s-1 (25°C), Δ H doubledaggerd= 65 ± 22 kJ mol-1, Δ S doubledaggerd= -26 ± 73 J mol-1 K-1 for reaction [1] in n-hexane; kf = 6.68 ± 0.06 M-1 s-1 (25°C), Δ H doubledaggerf= 58.0 ± 3.1 kJ mol-1, Δ S doubledaggerf= -34.5 ± 10.5 J mol-1 K-1, Δ V doubledaggerf= -10.7 ± 1.3 cm3 mol-1 for reaction [2] in dichloromethane; k2 = (7.09 ± 1.89) x 10-4 M-1 s-1, Δ H doubledagger2= 95 ± 21 kJ mol-1, and Δ S doubledagger2= 18 ± 70 J mol-1 K-1, Δ V doubledagger2= 9 ± 9 cm3 mol-1 for reaction [3] with bipy, and k1 = (1.10 ± 0.10) x 10-2 s-1, k3/k-1 = (4.33 ± 0.30) x 10-2, and k2 = (6.09 ± 1.35) x 10-4 M-1 s-1 for reaction [3] with 4,4'-Me2-bipy. It was shown that reactions [1] and [2] proceed nucleophilically without any intermediates, and reaction [3] proceeds through a mainly k2 path for NN = bipy and through both k1 and k2 paths for NN = 4,4'-Me2-bipy, without appreciable participation of the direct reaction between the dimer and NN as shown by the following reaction scheme.Key words: octamethylplatinum(IV) dimer, tetramethylplatinum(II) dimer, reaction mechanism, high-pressure kinetics.
Energétique des liaisons inter- et intramoléculaires dans les trois isomères de l'aminopyridine
Authors: Sabbah Raphaël , da Silva Eusébio Maria Ermelinda
Published: April 2011

The present work is concerned with a thermodynamic study of the three aminopyridine (AP) isomers (general formula: C5H6N2). It was achieved using three techniques: combustion calorimetry of small amounts of substance (a few milligrams), sublimation calorimetry, and differential thermal analysis. From this study, it was possible to determine the enthalpies of combustion, sublimation, and fusion of these compounds and their triple point temperatures; to discuss the relative stability of the three molecules; to determine the experimental resonance energies and compare them with the theoretical values; to determine the atomization enthalpies and compare them with the values calculated from the energetical contributions previously determined in our laboratory; to determine the enthalpy of the intermolecular bonds.Key words: aminopyridine isomers; enthalpy of combustion, of sublimation, of fusion, of inter- and intramolecular bonds.
Coriolis coupling constants and inertial defect of B2F4 by ab initio calculations and the iterative method
Authors: Loroño Marcos , Anacona J R , Sierralta Anibal
Published: April 2011

Structural parameters, harmonic frequencies, and force constants of B2F4 were obtained in internal coordinates from ab initio RHF calculations with the 6-311G* basis set. The results obtained are used to calculate the Coriolis coupling constants and the inertial defect for 10B2F4 and 11B2F4 and to compare these values with those obtained by using the autoconsistency method. Both the ab initio and the iterative force fields, despite some significant differences, reproduce the vibrational wave numbers equally well and yield nearly the same Coriolis coupling constants and inertial defects. The calculated inertial defects for 10B2F4 and 11B2F4 are -0.942 and -0.919 amu Å2, respectively.Key words: Coriolis coupling constants, inertial defect, B2F4, harmonic frequencies, force constants.
Polyacrylic acid pore-filled microporous membranes and their use in membrane-mediated synthesis of nanocrystalline ferrihydrite
Authors: Winnik Françoise M , Morneau André , Mika Alicja M , Childs Ronald F , Roig Anna , Molins Elies , Ziolo Ronald F
Published: April 2011

A series of cation-exchange membranes were obtained by photoinitiated grafting of acrylic acid onto a polypropylene microporous support having 0.2 µm diameter pores. The poly(acrylic acid) was shown to be contained within the pores of the membrane. The ion-exchange capacities of these "pore-filled" membranes ranged from 65 to 80% of the theoretical values calculated on the basis of their measured graft yields, with water contents ranging from 72 to 77%. The membranes exhibited a chemical valve effect of flux as a function of pH. Treatment of a poly(acrylic acid) grafted membrane with a solution containing ferrous ions resulted in a large uptake of iron (>24% by weight of iron with a membrane containing 152% graft yield of poly(acrylic acid)). Subsequent alkaline (pH 14) oxidation at 70°C with either oxygen or hydrogen peroxide led to the formation of a superparamagnetic nanocrystalline form of ferrihydrite, 5Fe2O3 ·9H2O. The oxide was identified by temperature-dependent Mössbauer spectroscopy recorded from 300 K to 4.2 K. TEM micrographs of sectioned membranes that were treated with H2O2 revealed the presence of two iron oxide morphologies, disk-shaped particles 3-4 nm in diameter and needle-like particles about 4 nm wide and 50 nm long. Only disk-shaped particles were observed in membranes treated with O2. Magnetization measurements indicate that the membranes are superparamagnetic above 50 K with a room temperature magnetization of 3.3 emu g-1 at 20 kOe.Key words: membrane, poly(acrylic acid); iron oxide, magnetic, Mössbauer spectroscopy; magnetization, photografting.
Molecular modelling and NMR studies of the caffeine dimer
Authors: Falk Michael , Chew Warren , Walter John A , Kwiatkowski Witold , Barclay Katherine D , Klassen Gerald A
Published: April 2011

A survey of crystal structures containing the xanthine ring shows that caffeine-like molecules tend to stack in parallel in a variety of stacking geometries, suggesting that a stacked pair of caffeine molecules has many conformations of comparable energy. Molecular modelling suggests that the caffeine dimer, the major associated species in aqueous solution, may occur in nine distinct conformations. In agreement with the crystal survey and the modelling results, NMR spectra of caffeine solutions exhibit intermolecular nuclear Overhauser enhancements (NOEs), consistent with a dynamic equilibrium of several conformations of the dimer of comparable energies.Key words: caffeine, dimer, self-association, molecular modelling, intermolecular nuclear Overhauser enhancements (NOEs).
Article
Authors: Xi Fengde , Barclay L Ross
Published: April 2011

The antioxidant activities, kinh, of catechol, 1, 4-tert-butylcatechol, 2, and 3,5-di-tert-butylcatechol (DTBC), 3, determined by the inhibited oxygen-uptake method during peroxidation of styrene initiated by AIBN are 55.0 x 104, 88.4 x 104, and 149 x 104M-1s-1, respectively, and the stoichiometric factors (n) were 2.1-2.3. A decrease by 50-fold in kinhfor 3 and a drop of 1.1-1.4 in n observed during inhibited peroxidation of methyl linoleate in aqueous sodium dodecyl sulfate (SDS) initiated by di-tert-butylhyponitrile (DBHN) is attributed to hydrogen bonding by water on the antioxidant and on the intermediate di-tert-butylsemiquinone radical, 5, formed in the inhibition step. Combinations of ascorbyl palmitate with 3 exhibited cooperative (not synergistic) antioxidant effects during inhibited peroxidation of styrene in solution. Combinations of ascorbic acid with 3 exhibited synergistic effects during inhibited peroxidation of methyl linoleate initiated by DBHN in SDS micelles. A profile of the effect of concentration of ascorbate on this synergism indicates a mole of 3 is regenerated per mole of ascorbate. The thiols, homocysteine, and polyethylene glycol thiol (polythiol) also exhibited synergistic effects with DTBC in this inhibition. Either ascorbate or polythiol rapidly reduces di-tert-butyl-ortho-quinone (DTBQ), 4, to 3 in methanol or in SDS micelles, and the combination of 3 + ascorbate acted as an efficient inhibitor in this medium. The esr studies indicate the semiquinone radical, 5, produced photochemically from 3 or spontaneously from 3 + 4 in solution, to be very persistent at room temperature. A pathway, mediated by 5, is proposed to account for the cooperative and synergistic effects observed and for the additional combination effect discovered when the three inhibitors: 3, ascorbate, and a thiol are used in the SDS medium. Combinations of such antioxidants are expected to be useful for inhibition of yellowing of pulps and paper with high lignin content, and to be significant in the in vivo reductions of ortho-quinones and semiquinone radicals formed during oxidations of various biomolecules.Key words: catechols, ascorbate, thiols, radicals, antioxidants.
Article
Authors: McIntosh John M , Kiser E Jay , Tian Zhigang
Published: April 2011

Alkylation of the hippurate esters of trans 2-(p-substituted phenyl)cyclohexanol with benzyl bromide affords phenylalanine derivatives in high chemical yield. The reaction stereoselectivity varies from 20 to >98% depending on the aromatic group in the cyclohexyl auxiliary. A model that correctly predicts the sense of the induction is proposed.Key words: enolate conformation, pi-stacking, 2-phenylcyclohexanol, chiral auxiliary.
Kinetic studies of bold σ -adduct formation and nucleophilic substitution in the reactions of ethyl 2,4,6-trinitrophenyl ether, some phenyl 2,4,6-trinitrophenyl ethers, and phenyl 2,4-dinitronaphthyl ether with aniline in dimethyl sulfoxide
Authors: Crampton Michael R , Robotham Ian A
Published: April 2011

The reaction of ethyl 2,4,6-trinitrophenyl ether with aniline in dimethyl sulfoxide containing Dabco occurs in two stages. The first gives 5, the σ -adduct intermediate on the substitution pathway, which has been identified spectroscopically. The second yields 2,4,6-trinitrodiphenylamine, the substitution product. Kinetic studies show that proton transfer is rate limiting both in the formation of the intermediate and in its subsequent acid-catalysed decomposition. Phenoxide is a considerably better leaving group than ethoxide and the substitution reactions of phenyl 2,4,6-trinitrophenyl ethers and phenyl 2,4-dinitronaphthyl ether with aniline in DMSO occur without the accumulation of intermediates. The kinetics indicate both uncatalysed and base-catalysed pathways. The kinetic and equilibrium data for reaction of the ethyl and phenyl ethers are compared with data for σ -adduct formation from 1,3,5-trinitrobenzene and aniline.Key words: nucleophilic substitution, proton transfer, base catalysis, σ -adducts
Thermolysis of 2,2-dimethoxy-5,5-dimethyl- Δ3- 1,3,4- oxadiazoline studied with photoelectron spectroscopy. He(I) photoelectron spectrum of dimethoxycarbene
Authors: Muchall H M , Werstiuk N H , Choudhury B , Ma J , Warkentin J , Pezacki J P
Published: April 2011

Gas phase thermolysis of 2,2-dimethoxy-5,5-dimethyl- Δ3-1,3,4-oxadiazoline (1) in an ultraviolet photoelectron spectrometer by means of a CW CO2 laser as directed heat source at 26 W gave a complex PE spectrum that included ionization bands belonging to acetone, tetramethoxyethylene (3), and dimethyl oxalate (4). Subtraction of the spectra of acetone, 3, and 4 from the pyrolysis spectrum of 1 left a simple PE spectrum that is attributed to dimethoxycarbene (2) along with some ethane. ecke3LYP/6-31+G* calculations gave first adiabatic and vertical ionization potentials of 2 as well as orbital energies that are in perfect agreement with experimental values. From the available experimental and calculational data, 2 is assumed to adopt a w conformation.Key words: dimethoxycarbene, 2,2-dimethoxy- Δ3-1,3,4-oxadiazoline, tetramethoxyethylene, He(I) photoelectron spectroscopy, thermolysis.
Synthesis, spectroscopy, and structures of the seven-coordinate complexes (CH3)2AsC(CF3)== C(CF3)As(CH3)2W(CO)2I2P(OC6H5)3 and [(CH3)2AsC(CF3)== C(CF3)As(CH3)2]2W(CO)Br2 and spectroscopy of related seven-coordinate complexes
Authors: Barton Richard J , Manocha Sushil K , Robertson Beverly E , Mihichuk Lynn M
Published: April 2011

(L-L)W(CO)3I2 (L-L = (CH3)2AsC(CF3)== C(CF3)As(CH3)2) reacts with the monodentate phosphite P(OC6H5)3 and (L-L)W(CO)3Br2 reacts with L-L to form new seven-coordinate complexes (L-L)W(CO)2I2P(OC6H5)3 and (L-L)2W(CO)Br2. Low-temperature X-ray diffraction analyses show the tungsten atom to be seven coordinate in both complexes, with the geometry most closely approximated by a monocapped octahedral environment, the capping group being a carbonyl in the dicarbonyl complex; the geometry is most closely approximated by a pentagonal bipyramidal environment in the monocarbonyl complex. The 1H, 13C, and 19F NMR data indicate that the dicarbonyl complex is stereochemically nonrigid at 298 K and rigid at lower temperatures, while the monocarbonyl is nonrigid both at 298 K and at lower temperatures. ΔG not equal values calculated at coalescence temperatures are consistent with an intramolecular rearrangement process for both complexes. The 13C chemical shifts and 2J(13C-31P) values provide important structural considerations in the assignment of a seven-coordinate geometry. Spectroscopic properties for the related seven-coordinate dicarbonyl complexes (L-L)W(CO)2PX2 (P = P(OC6H5)3; X = Br; P = P(OCH3)3, P(C6H5)3; X = Br, I) and monocarbonyl complexes (L-L)2W(CO)I2 and (L-L)W(CO)X2[P(OCH3)3]2 (X = Br, I) are presented and compared to those of the two title complexes.Key words: seven-coordination, X-ray, NMR analysis.
Primary and secondary 5-(alkyloxy)thianthrenium perchlorates. Characterization with1H NMR spectroscopy, reactions with iodide and bromide ion, and thermal decomposition in solution
Authors: Zhao Wenyi , Shine Henry J
Published: April 2011

A series of 5-(alkyloxy)thianthrenium perchlorates has been made in which the alkyl group is primary (1a-1p) and secondary (2a-2g). Preparations were carried out by reaction of the corresponding alkanol with thianthrene cation radical perchlorate in CH2Cl2solution followed by precipitation of the perchlorate salt with dry ether.1H NMR spectroscopy reveals that the presence of a stereogenic center in the alkyl group causes inequivalence in the ordinarily paired protons (e.g., H-4, H-6) of the thianthrenium ring. Reaction of iodide and bromide ion with primary alkyl-group compounds (e.g., methyl, ethyl, propyl, butyl) gave the alkyl halide in very good yield and by a second-order kinetic displacement. The second product was thianthrene 5-oxide (ThO). Rate constants for some of these reactions are reported. Reaction of secondary alkyl group compounds (e.g., 2-propyl, 2-pentyl, 2-hexyl, and 3-hexyl) with iodide ion gave good yields of alkyl iodide but also increasing evidence for a side reaction at the sulfonium sulfur, leading to I2, thianthrene, and secondary alkanol. Decomposition of some compounds at 100°C in solution (acetonitrile or 1,2-dichloroethane) was studied and gave alkene(s) and ThO.Key words: thianthrene cation radical, 5-(alkyloxy)thianthrenium perchlorates.
Article
Authors: Marshall Heather , Devine Marisa A
Published: April 2011

The fluorescence signals from replicate samples of uranium taken into fused sodium fluoride beads, read in the prescribed fashion, are not distributed normally. The signal from a single bead shows marked sinusoidal variation as the bead is rotated, and it is the superposition of this sinusoidal variation on the normal variation between beads that gives rise to a widened or even bimodal distribution among replicates. If each bead is read a second time, following a single half rotation, the average of the two readings gives the true signal underlying the sinusoidal variation; the distribution of these true signals represents simply the normal variation between replicate samples, and the precision of results is much improved.Key words: uranium, fluorescence, fluorometry.
Article
Authors: Majumdar Krishna C , Jana Gour H
Published: April 2011

A number of allyl aryl propargyl amines 3a-3f were synthesized in 68-82% yield. These tertiary amines were then treated with one equivalent of m-chloroperoxybenzoic acid (m-CPBA) to give products 5a-5f in 79-90% yields arising out of the Meisenheimer rearrangement of the allyl aryl amine moiety accompanying the amine oxide rearrangement of the aryl propargyl amine moiety.Key words: Meisenheimer rearrangement, [2,3] sigmatropic shift, amine oxide, propargyl amine, allyl amine.
The acid dissociation constant of triphenylethenethiol, a simple thioenol, and that of its oxygen-enol analog
Authors: Chiang Yvonne , Kresge A Jerry , Schepp Norman P , Popik Vladimir V , Rappoport Zvi , Selzer Tzvia
Published: April 2011

The acidity constant, pQaE = 8.49, for the stable thioenol, triphenylethenethiol, was determined by spectrophotometric titration, and that, pQaE = 11.37, for its unstable oxygen analog, triphenylethenol, was determined by analysis of its ketonization rate using enol generated flash photolytically from triphenylvinyl bromide. (These acidity constants are concentration quotients applicable at 0.10 M ionic strength in 50 vol.% aqueous methanol.) This appears to be the first determination of the acid strength of a simple thioenol. Triphenylethenethiol was unreactive in dilute acid or base, but in concentrated perchloric acid solutions it was slowly transformed into diphenylacetophenone and 2,3-diphenylbenzo[b]thiophene; the mechanisms of these reactions are believed to involve rapid equilibrium protonation of the enol on the beta -carbon followed by rate-determining capture of the cation so formed, either externally by solvent or internally by one of the beta -phenyl groups of the ion.Key words: enol ketonization, thioenol, flash photolysis, vinyl halide.
The solubility of gases and vapours in ethanol - the connection between gaseous solubility and water-solvent partition
Authors: Abraham Michael H , Whiting Gary S , Shuely Wendel J , Doherty Ruth M
Published: April 2011

Ostwald solubility coefficients, as log L, for solutes in water and ethanol have been combined to give log PEtOH for partition between the two pure solvents. Sixty-four such values have been correlated through our solvation equation, the coefficients of which lead to the conclusion that ethanol and water solvents are equally strong hydrogen-bond bases, but that ethanol is much weaker as a hydrogen-bond acid. A slightly different solvation equation has been used to correlate 68 values of log LEtOH; the coefficients in this equation yield the same conclusions as to the hydrogen-bond acidity and basicity of bulk ethanol. In addition, an analysis of the various terms in the log LEtOH correlation equation allows the elucidation of the various chemical factors that govern the solubility of gaseous solutes in ethanol solvent at 298 K.Key words: solubility, partition, hydrogen-bonding, ethanol, water.
Structure and dynamics of azapropazone derivatives studied by crystallography and nuclear magnetic resonance
Authors: Fauconnier Theresa , Bain Alex D , Hazendonk Paul , Bell R A , Lock Colin JL
Published: April 2011

The X-ray crystal structure of a derivative of the nonsteroidal anti-inflammatory drug, azapropazone, has been determined using a data set obtained from a charge-coupled device (CCD) area detector. Azapropazone itself, 5-(dimethylamino)-9-methyl-2-propyl-1H-pyrazolo[1,2-a][1,2,4]benzotriazine-

1,3(2H)-dione, has previously been shown to be nonplanar, probably due to steric interactions between the pyrazolidine ring and the exocyclic dimethylamino group. The derivative studied in this work, 3-dimethylamino-7-methyl-1,2,4-benzotriazine, does not have the pyrazolidine ring and is shown here to be strictly planar. There is also a significant difference in the kinetics of restricted rotation around the bond joining the dimethylamino group to the triazine ring, as measured by NMR. Both the enthalpy and entropy of activation are quite different. For the chemical exchange of the two methyl groups in azapropazone, Δ Hdouble dagger = 36 ± 0.5 kJ mol-1, and Δ Sdouble dagger = -34 ± 2 J K-1. For the derivative, Δ Hdouble dagger = 48 ± 0.4 kJ mol-1, and Δ Sdouble dagger = -12 ± 2 J K-1. This is rationalized by assuming the equilibrium state of the derivative is planar, and this defines the potential energy curve. The ground state of azapropazone is relatively destabilized, and somewhat disordered, by the steric interaction. Therefore, azapropazone has a small barrier to surmount, and the entropy of activation should be negative.Key words: NMR, chemical exchange, entropy, barriers to rotation, crystal structure.
The reactivity of the high-energy intermediates formed in the reactions of Group 13 metal atoms and aromatic alkenes
Authors: Joly Helen A , Kepes Maria , Roy Natalie , Prpic Jason
Published: April 2011

Group 13 metal atoms were reacted with aromatic alkenes in a specialized metal atom reactor known as a "rotating cryostat." The nature of the intermediates formed was deduced from a GC-MS study of their hydrolysis and deuterolysis products. The product studies suggest that 2-phenylaluminacyclopropane, cis- and trans-3,4-diphenylaluminacyclopentane, and cis- and trans- 2,4-diphenylaluminacyclopentane are formed when Al atoms react with styrene, and 2-methyl-2-phenylaluminacyclopropane and 3,4-dimethyl-

3,4-diphenylaluminacyclopentane are formed when Al atoms react with α -methylstyrene. These findings are consistent with the radicals detected in the EPR spectroscopic studies of Al-alkene reaction mixtures prepared under similar conditions. Mechanisms for the formation of the organoaluminium intermediates are discussed. Analogous organogallium intermediates are formed when gallium atoms react with styrene. The reductive coupling of styrene did not occur when In and Tl atoms were used. Only trace quantities of phenylethane were detected in the hydrolyzed reaction mixture.Key words: Group 13 metal atoms, aluminium atoms, organoaluminium intermediates, metal atom reactions.
Article
Authors: Gregoire Sébastien , Mathurin Jean-Charles , March Raymond E , Tabet Jean-Claude
Published: April 2011

The quadrupole ion trap mass spectrometer is of great interest for chemical analysis. Nevertheless, negative ion studies using in situ ionization (negative ions created inside the ion trap) are difficult and limited. This paper describes the difficulties that occurred during negative ion analysis when using a commercial gas chromatography quadrupole ion trap mass spectrometer (GC/MS) system. Detection problems, due to simultaneous trapping of both positive and negative ions, are explained by the large kinetic energies of the ejected ions. Negative ion formation, produced by electron capture, is limited by the fundamental RF field, which imparts high kinetic energy to the electrons, precluding the possibility that the electrons will reach the thermal energies needed for electron capture processes. In addition, simultaneous confinement of negative and positive ions affects the recorded mass spectra. Space charge potentials that exist inside the ion trap (due to the ionization conditions necessary to detect negative ions) induce the destabilization of positive ions at higher m/z ratios, while negative ions in the same m/z range are stabilized. Moreover, loss of resolution and decalibration could occur for negative ions when they are ejected in the presence of positive ions with higher m/z ratios. At the very least, ion-ion reactions could limit the observation of negative ions. The understanding of these phenomena, viz. detection, formation, and simultaneous confinement, will permit the proposal of solutions for negative ion analysis with quadrupole ion trap mass spectrometers using in situ ionization.Key words: mass spectrometry, quadrupole ion trap, negative ion, perfluorotributylamine, space charge.
Thermodynamics of dissolving gaseous argon in different solvents
Authors: Graziano Giuseppe
Published: April 2011

The solvation of argon in 10 different solvents at room temperature is analysed in terms of the theoretical framework developed by Lee. In order to perform calculations, we used the approach devised by Pierotti and firmly validated by Lee's theory. The fair agreement between the experimental and the calculated values of Δ G.Δ H., and Δ S.is carefully analysed. It proves that the excluded volume effect, due to cavity creation in the solvent, opposes the solubility process. This effect in water is exaggerated by the small size of the water molecules and is the cause of hydrophobicity. The qualitative difference between water and hydrazine with regard to the solvation enthalpy and entropy changes is rationalized on the basis of the contributions arising from the structural reorganization in the two solvents on solute insertion.Key words: hydrophobic hydration, cavity creation, structural solvent reorganization.
Effects of superoxide anion generated from aromatic radical anions produced in nucleophilic aromatic photosubstitution reactions
Authors: Cervera Maria , Marquet Jordi
Published: April 2011

Superoxide anion is generated from aromatic radical anions produced in nucleophilic aromatic photosubstitutions when the reactions are carried out in non-deoxygenated solutions of polar aprotic solvents. Superoxide anion thus generated displaces cyanide anion from acetonitrile and benzyl cyanide, ethoxide anion from ethyl acetate, and methanesulfenate anion from dimethyl sulfoxide. Hence, non-deoxygenated polar aprotic solvents should be avoided in nucleophilic aromatic photosubstitution reactions.Key words: superoxide, fluoride, nucleophilic aromatic photosubstitution.
Article
Authors: Saied Okba , Bachand Benoit , Wuest James D
Published: April 2011

Carbonyl oxygen atoms have two formal lone pairs of electrons. In principle, both can be used simultaneously to form complexes with two or more Lewis acids. This multiple coordination promises to have a variety of interesting consequences; unfortunately, however, complexes of carbonyl compounds with multiple Lewis acids are extremely rare. To promote multiple coordination, we have made a series of symmetric ketodiesters and related compounds in which the carbonyl group of a ketone is flanked by two additional sites of Lewis basicity. In such compounds, the flanking bases and both lone pairs of the central ketone are available for binding two equivalents of suitable Lewis acids, thereby producing symmetric double chelates in which the central ketone interacts with two Lewis acids at the same time. As expected, treatment of 3-oxoglutarates and 4-oxopimelates with TiCl4in a 1:1 ratio yielded unsymmetric single chelates in which the carbonyl groups of the ketone and one ester bind TiCl4, while the other ester remains free. Unfortunately, treatment of the same ketodiesters with TiCl4in a 1:2 ratio did not produce the desired symmetric double chelates. Instead, 2:4 complexes were formed in which the free esters of the unsymmetric single chelates bind TiCl4in the normal way, without assistance from the keto group. We attribute this observation to the inherent reluctance of ketones to bind multiple Lewis acids, as well as to unfavorable Cl · · ·Cl interactions created in the hypothetical double chelates by the simultaneous attachment of two octahedrally coordinated atoms of titanium to a single carbonyl oxygen atom.Key words: Lewis acids, chelation, ketodiesters, TiCl4.
Article
Authors: McDowell Sean AC , Meath W J
Published: April 2011

Average energy approximations for the anisotropic triple-dipole dispersion energy coefficients are tested using reliable results for these coefficients, which are available for all interactions involving the rare gases, H2, N2, CO, O2, and NO. The original average energy approximation does not reproduce any of the anisotropic coefficients to within their estimated uncertainties. More recently derived average energy approximation formulae, requiring the isotropic and anisotropic polarizabilities and average energies for the interacting species as input, reproduce all but 69 of the 680 isotropic and anisotropic coefficients considered to within their estimated uncertainties.Key words: nonadditive, three-body interactions, dispersion energies.
Escherichia coli β-galactosidase is heterogeneous with respect to the activity of individual molecules
Authors: Craig Douglas B , Dovichi Norman J
Published: April 2011

Escherichia coli β-galactosidase molecules were incubated with fluorogenic substrate in a capillary. Upon flushing the reactor contents past the detector, individual randomly distributed peaks of product were observed, each representing the activity of an individual enzyme molecule. Individual molecules were found to differ with respect to their activities. Molecules showed a 23-fold distribution of activities with the majority of molecules within a 4-fold distribution.Key words: single-molecule chemistry, β-galactosidase, capillary electrophoresis, laser-induced fluorescence, enzyme assay.
Concerted motion in bicyclobutane
Authors: Sella Ariel , Basch Harold , Hoz Shmaryahu
Published: April 2011

In bicyclobutane, the bridgehead substituents move inward as the bridge bond lengthens, and vice versa. The effect of BH2, H, F, and OH as bridgehead substituents was investigated using quantum-mechanical calculations. The degree of invertedness of the bridgehead carbon was found to depend on the electronegativity of the substituent. It becomes more pronounced with electropositive substituents which, at the same time, also cause lengthening of the central bond (1.43 Å for F to 1.55 Å for BH2). The oscillation phenomenon was found to be common to all the substituents regardless of their electronegativity. Substituent effects on the structure, energy, and electronics of the system are discussed.Key words: bicyclobutanes, oscillation, geometry distortion.
Correlation of the rates of solvolysis of arylmethylp-toluenesulfonates: application of the aromatic ring parameter and a discussion of similarity models
Authors: Kevill Dennis N , D’Souza Malcolm J , Ren Haobo
Published: April 2011

The specific rates of solvolysis of benzyl p-toluenesulfonate and nine benzylic-ring-substituted derivatives are satisfactorily correlated using NTand YOTsscales within the extended Grunwald-Winstein equation. Addition of a third term, governed by the aromatic ring parameter, shows the additional term to be statistically significant. As one incorporates increasingly more electron-withdrawing substituents, the sensitivities towards changes in solvent nucleophilicity (l) increase, and those towards changes in ionizing power and changes in the aromatic ring parameter decrease. The trend of l values can be very nicely shown, with very precise correlations, using an internally generated scale of Y values. However, it is demonstrated that there is no overall advantage to use of an internally generated scale if one ultimately wishes to make a comparison with sensitivity values generated using an external scale of Y values.Key words: Grunwald-Winstein equation, aromatic ring parameter, solvent nucleophilicity, solvolysis, arylmethyl p-toluenesulfonates.
Article
Authors: Ramek Michael , Yu Ching-Hsing , Schäfer Lothar
Published: April 2011

The ab initio conformational energy minima of the model tripeptide N-formyl-L-alanyl-L-alanine amide (ALA-ALA) were determined by ab initio RHF/4-21G and RHF/6-31G* gradient geometry refinement. For the current investigation, 11 664 RHF/4-21G structures were optimized, representing grid points in the four-dimensional (phi1, psi1, phi2, psi2) conformational space, which were constructed in 40° increments along the outer torsions phi1 and psi2 and in 30° increments along the inner torsions psi1 and phi2 of ALA-ALA. Two new energy minima, previously not reported, are described. The positions of the RHF/6-31G* energy minima in phi,psi-space can differ significantly from the corresponding RHF/4-21G locations, and both sets are not clustered in the centers but on the fringes of the most populated regions of phi, psi-space in protein crystal structures. Thus, the torsion angles of the ab initio energy minima are not those of the typical substructures of proteins: the most stable helices are not alphaR, and the torsion angles of the most stable bend forms are not those most frequently encountered in protein bends. Limitations of the dipeptide approximation are explored, illustrating how the conformational energies of an amino acid residue depend on the state of its neighbor.Key words: alanyl alanine amide, dipeptide approximation, model tripeptide, peptide models, structure of peptides.
Article
Authors: Auwer C Den , Andrews M P , Briois V , dit Moulin C Cartier , Dexpert H
Published: April 2011

A new method of preparing solid state chlorometallates by reductive dechlorination of solvated niobium pentachloride with the vapour of nominally free atoms of erbium and samarium is described. Products were prepared in a model of a rotatable Pyrex cryostat by evaporating Sm or Er into a turbulently mixed solution of NbCl5in toluene at 130 K. Since no single crystal had been isolated, local order around the Nb atom was probed by far-IR, XPS, and EXAFS spectroscopy. Depending on the vaporized rare earth, two structurally distinct products were found. Combining information gathered by EXAFS results and ab initio EXAFS calculations on clusters to model the growing solid state compound using the FEFF6 code, two different condensation patterns were proposed: chlorine edge-shared aggregation was preferred for Er vaporization, face-shared condensation patterns emerged from Sm vaporization.Key words: X-ray absorption spectroscopy, niobium, metal vapour synthesis, XPS.
Extended lithium ion pair indicator scale in tetrahydrofuran
Authors: Streitwieser Andrew , Wang Daniel Zerong , Stratakis Manolis , Facchetti Antonio , Gareyev Roustam , Abbotto Alessandro , Krom James A , Kilway Kathleen V
Published: April 2011

The lithium pK scale has been extended to 25 indicators with a pK range of 9.7-24.4. The resulting scale is compared with the cesium ion pair acidities and to ionic pK's in DMSO and aqueous DMSO.Key words: ion pair acidity, organolithium compounds, indicator, acidity scale.
Article
Authors: Gedye R N , Wei J B
Published: April 2011

Several different reactions have been studied to determine whether they occur more rapidly than conventionally heated reactions at atmospheric pressure. Small rate enhancements have been observed for some reactions carried out under microwave reflux in a modified domestic microwave oven. The Knoevenagel reaction of acetophenone with ethyl cyanoacetate was shown to have a rate enhancement of 2.5 times. However this reaction showed no rate increase over conventional heating, at the same temperature, in a variable-frequency microwave oven. It is therefore probable that the small rate enhancements observed in these experiments, using microwave heating, were due to hot spots or superheating of the solvent rather than to nonthermal effects.Key words: microwave, nonthermal effects, superheating, hot spots.
Crystal chemistry of tetraradial species. Part 9. The versatile BPh4- anion, or how organoammonium H(N) atoms compete for hydrogen bonding
Authors: Robertson Katherine N , Bakshi Pradip K , Lantos Susanne D , Cameron T Stanley , Knop Osvald
Published: April 2011

In organoammonium cations containing two or more N atoms, the H(N) atom introduced by the protonation may engage in an N→H(N)...Y bond to an external acceptor (anion or solvent molecule); in an intra-cation (intra-annular) N→H(N)...N' bond; or in a branched N→H(N)...N',Y bond(s), simultaneously intra-cation and to one or more external acceptors. The outcome of the competition for these bond types is determined by various factors, e.g., the size of the ring to be closed by insertion of the H(N) atom, steric accessibility of H(N), and the H-bond-forming strength of Y. Some of these are discussed in considerable detail and are exemplified by the hitherto unreported crystal structures of the tetraphenylborates (solvated or unsolvated) of the 2,2':6',2"-terpyridinium (1), 2,3-bis(2-pyridyl)pyrazinium (2), 2,2'-dipyridylammonium (3), 6,7-dihydro[b,j][1,10]phenanthrolinium (9), 2,4,7-triamino-6-pteridinium (triamterenium, 10), proton-sponge (1-Me2N-8-Me2NH-naphthalene, 11, 12), and 9-amino-1,2,3,4-tetrahydroacridinium (tacrinium, 13) cations. The crystal structures of the comparison compounds 2,3-bis(2-pyridyl)pyrazine (4) and its mono- (2A) and diprotonated (3) chlorides, and of 2-phenylpyridinium (6) and 7,8-benzoquinolinium (7, 8) tetraphenylborates, have also been determined. The many interesting features of these crystal structures, such as the X→H(X)...phenyl bonding (X = N, O, C), are commented upon and crystallographic comparisons with numerous literature compounds are offered. Included in the discussion are the monoprotonated intrabridgehead cations of the bicyclic diamines of Alder et al., the N-H(N)-N' angle of which has been shown, analytically, to be dominated by the size of the smallest of the rings in these tricyclic systems.Key words: crystal structures, hydrogen bonding, intrabridgehead cations, proton sponges, tetraphenylborates.
Additions and corrections: Transverse compression and the secondary H/D isotope effects in intramolecular SN2 methyl-transfer reactions
Authors: Wolfe Saul , Kim Chan-Kyung , Yang Kiyull , Weinberg Noham , Shi Zheng
Published: April 2011

Using ab initio molecular orbital theory mainly at the 3-21+G level, intramolecular SN2 methyl transfer between two oxygens confined within a rigid template is found to proceed exclusively by a high energy retention mechanism when the oxygens are separated by three or four bonds, and by a high energy inversion mechanism when the oxygens are separated by six bonds. Both mechanisms exist when the oxygens are separated by five bonds. The CH3/CD3 kinetic isotope effects are normal (1.21-1.34) in the retention processes and inverse (0.66-0.81) in the inversion reactions. In the case of inversion, compression of C-H bonds of the transition state by structural effects in the plane perpendicular to the O-C-O plane increases the inverse isotope effect. The retention barriers are high because retention is inherently unfavorable, even when pericyclic stabilization of the transition state is possible. The inversion barriers are high because a rigid template cannot accommodate a linear O-CH3 -O structure, and the O-C-O bending vibration is stiff (the Eschenmoser effect). Using a novel design strategy, a nonrigid template has been found in which the barrier and the CH3/CD3 kinetic isotope effect are the same as in an intermolecular reaction.Key words: Eschenmoser effect, isotope effect, compression, SN2, sigmatropic rearrangement.
Studies with polyfunctionally substituted heteroaromatics: synthesis of new heterocyclic aromatic amines as potential intermediates for preparation of dyes for thermal diffusion printing
Authors: Erian Ayman Wahba , Hafez Ebtisam Abdul Aziz , Darwish Elham Sayed , Elnagdi Mohamed Hilmy
Published: April 2011

Several new dibenzopyrans as well as novel thiadiazaacenaphthenes are obtained via 4 + 2 addition of the thienocoumarins (1) and thienopyridazines (2) to electron-poor olefins.Key words : pyridazines, thienopyridazines, phthalazines, coumarines, thiadiazaacenaphthenes.
A new one-dimensional thiocyanato-bridged bimetallic compound [Cu(en)2Mn(NCS)4(H2O)2]n. Synthesis, crystal structure, and magnetic properties
Authors: Kou Hui-Zhong , Liao Dai-Zheng , Cheng Peng , Jiang Zong-Hui , Yan Shi-Ping , Wang Geng-Lin , Yao Xin-Kan , Wang Hong-Gen
Published: April 2011

A bimetallic compound [Cu(en)2Mn(NCS)4(H2O)2]n (en = ethylenediamine) has been prepared and its structure determined. It crystallizes in space group , with a = 7.432(1), b = 7.492(1), c = 9.751(2) Å, α = 91.00(3)°, ß = 99.14(3)°, γ = 112.51(3)°, and Z = 1. The structure consists of one-dimensional chains in which alternating Mn(II) and Cu(II) ions are bridged by NCS ligands. The coordination environment is octahedral for Mn(II) and elongated octahedral for Cu(II). From magnetic susceptibility measurements the compound was found to exhibit a ground state of low-spin multiplicity. Comparison between theory and experimental has been made using 1-D model and taking into account the zero-field splitting effect. The decrease in chimT observed at low temperature for the compound is attributed to a dominant zero-field splitting of the single Mn(II) ions with a weak antiferromagnetic interaction superimposed between the Mn(II) ions via the NCS-Cu-SCN pathway within each chain. Key words: crystal structure, copper(II) complexes, manganese(II) complexes, thiocyanate bridges, bimetallic complexes.