bluuebunny/crossref_metadata_2025 huggingface dataset:
General acid-catalyzed addition of methanol to (E)-N-benzylideneanilines
Authors: Bennour Sadjia , Toullec Jean
Published: April 2011

The reaction of equilibrium addition of methanol (α-amino ether formation) to benzylideneanilines (C6H5=NC6H4Y, with Y = H (1a), 3-Cl (1b), 3-NO2(1c), 4-CN (1d), and 4-NO2(1e)) in methanol is shown to be general acid-catalyzed in carboxylic acid buffers. The mechanism involves fast iminium ion formation followed by base-assisted addition of methanol. The α Brønsted exponents are in the 0.67-0.88 range, and α increases with the electron-withdrawing ability of Y. The same mechanism is valid for MeOH2+-catalysis, meaning that two solvent molecules are involved in the addition process, one of them playing the role of base. The equilibrium constant, K, is increased by electron-withdrawing substituents, log K depending linearly on the σ-substituent parameters. The substituent effects on the forward and reverse catalytic rate constants are analyzed by means of the log k = ρnσn+ ρr(σ-- σn) + constant (Young-Jencks) equation. For carboxylic acid catalysis, the ρnand ρrparameters are in keeping with ca. half CO bond forming or breaking at the transition state. The catalytic rate constants and α exponent for elimination of ClCH2CH2OH in methanol from the C6H5CH(OCH2CH2Cl)NH(4-CNC6H4) chloroethyl adduct are compared with those for the elimination of methanol from C6H5CH(OCH3)NH(4-CNC6H4). The chloromethyl group makes the reaction slower and α lower. This indicates that proton transfer is a little ahead of CO bond cleavage at the transition state. Y substituent effects, α values, and the effects of the CH2Cl group are interpreted on the basis of a More O'Ferrall - Jencks diagram.Key words: imine, free energy linear relationship, nucleophilic addition, More O'Ferrall - Jencks diagram, Schiff base
Isotope effects in nucleophilic substitution reactions XI. The effect of ion-pairing, substituents, and the solvent on SN2 transition states
Authors: Westaway Kenneth Charles , Jiang W
Published: April 2011

The secondary alpha deuterium and primary leaving group nitrogen KIEs and Hammett ρ values found for the free ion and ion-pair SN2 reactions between benzyldimethylphenylammonium ion and sodium para-substituted thiophenoxides in methanol at 20.000°C show how (i) ion-pairing of the nucleophile, (ii) a change in substituent in the nucleophile, and (iii) a change in solvent alters the structure of a Type II SN2 transition state. Ion-pairing shortens the weaker sulfur - alpha carbon (SCα) transition state bond significantly but does not alter the stronger alpha carbon - leaving group (CαN) transition state bond as the bond strength hypothesis predicts. However, the effect of ion pairing, i.e., the decrease in the SCα bond on ion-pairing, decreases as a more electron-withdrawing substituent is added to the nucleophile, and the SCα bond actually increases when the nucleophile is the p-chlorothiophenoxide ion. The identical Hammett ρ values of -0.85 and -0.84 for the free ion and ion-pair reactions, respectively, may be observed because, on average, the SCα bonds are identical in the free ion and ion-pair transition states. When a more electron-donating substituent is added to the nucleophile, an earlier transition state is found in both the ion-pair and free ion reactions. However, the substituent effect is smaller in the ion-pair reactions, presumably because the change in the negative charge on the sulfur atom with substituent is greater in the free ion than in the ion-pair. The substituent effect on transition state structure suggested by the KIEs is not predicted by any of the theories that are used to predict substituent effects on SN2 reactions. Both the secondary alpha deuterium and primary leaving group nitrogen KIEs and the Hammett ρ values indicate that the transition state is earlier when the solvent is changed from DMF to methanol as the "solvation rule for SN2 reactions" predicts. This probably occurs because an earlier, more ionic, transition state is more highly solvated (more stable) in methanol.Key words: nucleophilic substitution, SN2, isotope effect, transition state, substituent, ion-pair.
The phosphoryl and the carbonyl group as hydrogen bond acceptors
Authors: Modro Agnes M , Modro Tom A
Published: April 2011

Formation constants for the hydrogen-bonded complexes between phenol (donor) and selected carbonyl or phosphoryl compounds (acceptors) were determined from IR Δν shifts. The P=O group is about two orders of magnitude stronger as an acceptor than the C=O group, but no evidence for the enolization of a β-diphosphoryl system was found. The IR absorption band of the phosphoryl group (νP=O) showed weak response to the involvement in the hydrogen bonding, and the shifts of the donor molecule (νOH) provide a better measure of the formation constants. Key words: carbonyl and phosphoryl groups as hydrogen bond acceptors, enolization of a β-difunctional system, determination of Kassoc from the IR Δνshifts.
Proton and hydrogen atom tunnelling in hydrolytic and redox enzyme catalysis
Authors: Kuznetsov A M , Ulstrup Jens
Published: April 2011

We discuss a broad theoretical frame for hydrogen transfer in chemical and biological systems. Hydrogen tunnelling, coupling between the tunnel modes and the environment, and fluctuational barrier preparation for hydrogen tunnelling are in focus and given precise analytical forms. Specific rate constants are provided for three limits, i.e., the fully diabatic, the partially adiabatic, and the fully adiabatic limits. These limits are all likely to represent real chemical or biological hydrogen transfer systems. The rate constants are referred particularly to the driving force and temperature dependence of the kinetic isotope effect (KIE). The origin of these correlations is different in the three limits. It is rooted in the tunnel factor and weak excitation of the heavier isotopes in the former two limits, giving a maximum for thermoneutral processes. A new observation is that the adiabatic limit also accords with a KIE maximum for thermoneutral processes but the KIE is here reflected solely in the activation Gibbs free energy differences, in this case rooted in the low-frequency environmental nuclear dynamics. Three systems of biological hydrogen tunnelling, viz. lipoxygenase, yeast alcohol dehydrogenase, and bovine serum amine oxygenase, offer unusual new cases for analysis and have been analysed using the theoretical frames. All the systems show large KIEs and strong indications of hydrogen tunnelling. They also represent different degrees of fluctuational barrier preparation, with lipoxygenase as the most rigid and bovine serum amine oxygenase as the softest system.Key words: generalized Born-Oppenheimer scheme, kinetic isotope effect, gated proton transfer, partially adiabatic proton transfer, proton tunnelling in enzyme catalysis.
A new silicon-mediated nitrogen to oxygen rearrangement
Authors: Talami Stefano , Stirling Charles JM
Published: April 2011

Migration of silicon from nitrogen to oxygen has been quantified with respect to the distance between migration termini, substituents at silicon, and substitution on the chain linking oxygen to nitrogen.Key words: silicon migration, nitrogen to oxygen, penta coordinate intermediate.
Solvent isotope effects in the oxidation of dipeptides by aqueous chlorine
Authors: Armesto X L , Canle L. M , García V , Santaballa J A
Published: April 2011

A kinetic study of the mechanism of oxidation of Ala-Gly and Pro-Gly by aqueous chlorine has been carried out. Among other experimental facts, the deuterium solvent isotope effects were used to clarify the mechanisms involved. In a first stage, N-chlorination takes place, and then the (N-Cl)-dipeptide decomposes through two possible mechanisms, depending on the acidity of the medium. The initial chlorination step shows a small isotope effect. In alkaline medium, two consecutive processes take place: first, the general base-catalyzed formation of an azomethine (β ca. 0.27), which has an inverse deuterium solvent isotope effect (kOH-/kOD-~ 0.8). In a second step, the hydrolysis of the azomethine intermediate takes place, which is also general base-catalyzed, without deuterium solvent isotope effect, the corresponding uncatalyzed process having a normal deuterium solvent isotope effect (kH2O/kD2O~ 2). In acid medium, the (N-Cl)-dipeptide undergoes disproportionation to a (N,N)-di-Cl-dipeptide, the very fast decomposition of the latter in deuterium oxide preventing a reliable estimation of the solvent isotope effect.Key words: chlorination, deuterium isotope effects, fractionation factors, peptide oxidation, water treatment.
Hydrolysis and aminolysis of alkyl xanthate esters and cellulose analogues
Authors: Humeres Eduardo , Soldi Valdir , Klug Marilene , Nunes Mauricéa , Oliveira Célia MS , Barrie Patrick J
Published: April 2011

The hydrolysis and aminolysis of a series of S-substituted O-alkylxanthate esters was studied in 20% v/v aqueous methanol at 35°C. The pH-rate profiles of the hydrolyses showed water and hydroxide-ion-catalyzed reactions. The reaction of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) and p-nitrobenzyl cellulose xanthate (CelXNB) with polyalanine and lysozyme produced a covalent bond between the polypeptide and the cellulose matrix, as shown by solid-state13C NMR. However, the nature of the bonding could not be identified. The reaction of nucleophiles (H2O, OH-, RNH2) and xanthic esters was consistent with an addition-elimination mechanism through a tetrahedral intermediate. Brønsted plots against the pKaof the nucleophile (βnu) or the nucleofuge of the substrate (βlg) were used to characterize the rate-determining step. The pKavalues of the nucleophiles ranged between -1.74 and 15.74, and for the nucleofuges, they were in the range of 10.50-0.92. For nucleophiles with pKavalues up to about 10, βlgwas 0.10-0.15, and βnuchanged from 0.48 to 0.35 for the strongest electron-withdrawing nucleofuge. It was concluded that the water-catalyzed hydrolyses, and also aminolyses with moderately basic amines, occur with rate-determining formation of the tetrahedral intermediate. For strong bases such as hydroxide ion, the disappearance of the intermediate becomes the slowest step. The reaction of cellulose xanthic esters with external nucleophiles as hydroxide ion and amines shows simple first-order kinetics and is slower than alkyl or sugar xanthates, probably due to the diffusion effect through the tight cybotactic region of cellulose. Key words: hydrolysis, aminolysis, alkyl xanthic esters, cellulose xanthic esters, sugar xanthic esters.
Dicarbonylcyclopentadienyliridium, (η-C5H5)Ir(CO)2, as a ligand
Authors: Jiang Faming , Biradha Kumar , Leong Weng Kee , Pomeroy Roland K , Zaworotko Michael J
Published: April 2011

The following complexes, in which CpIr(CO)2 acts as a 2e donor ligand, have been prepared: Cp(OC)2IrW(CO)5 (1), Cp(OC)2IrRu(CO)3(SiCl3)2, Cp(OC)2IrOs(CO)3(GeCl3)(Cl) (2), Cp(OC)2IrOs(CO)3(X)2 (X = Cl, Br (3), I). The characterization of the complexes included the crystal structures of 1, 2, and 3, which reveal that all contain an unbridged metal-metal bond. The carbonyl ligands of 1 in solution undergo exchange on the NMR time scale above -40°C. With the exception of 2, all the complexes dissociate in solution at room temperature, some rapidly so. Only in the case of 3 is an equilibrium with the dissociation products established. The results indicate that CpIr(CO)2 is a weak ligand.Key words: tungsten, osmium, iridium, binuclear.
A versatile approach to the synthesis of oligonucleotide analogs containing neutral 5'-thioformacetal internucleoside linkages
Authors: Ducharme Yves , Harrison Kimberly A
Published: April 2011

Activation of nucleoside donors 5 by sulfuryl chloride followed by the addition of 5'-thionucleoside acceptors 3 yields 5'-thioformacetal dinucleotide analogs 6 with in situ trapping of liberated methanesulfenyl chloride with cyclohexene. Purine as well as pyrimidine derivatives can be part of a coupling reaction as nucleoside donors or acceptors. The dimethoxytrityl protecting group is compatible with the present coupling methodology allowing differential 3',5'-end protection. The synthesis of longer oligonucleotides is also possible as shown by the preparation of trinucleotide analog 23.Key words: oligonucleotides, 5'-thioformacetal.
Effect of cyclodextrin complexation on the photochemistry of the lignin model α-guaiacoxyacetoveratrone
Authors: Okano L T , Ovans R , Zunic V , Moorthy J N , Bohne C
Published: April 2011

The photodecomposition of α-guaiacoxyacetoveratrone (GAV) in homogeneous solvents and in aqueous cyclodextrin solutions was studied by following the fluorescence emission of the photoproducts formed. The same qualitative behavior as previously observed for the photodegradation quantum yield measurements was reproduced in the fluorescence studies in dry or wet methanol and acetonitrile. The yield for GAV photodegradation in water is smaller than in the organic solvents. Complexation of GAV to β- and γ-cyclodextrins leads to an increase in the photodecomposition yield, especially during the early part of the photodecomposition kinetics. The transients formed in the photolysis of GAV in water are similar to those observed in acetonitrile, and the lifetime of triplet GAV in water is 130 ns. In addition, solvated electrons were formed when GAV was photolyzed in water. Complexation of GAV to cyclodextrins leads to an increase in the triplet yield compared to the radical yield and a moderate shortening of the triplet lifetime.Key words: α-guaiacoxyacetoveratrone, cyclodextrin, excited singlet states, excited triplet states, radicals.
Electrospray ionization mass spectrometry and on-line capillary zone electrophoresis - mass spectrometry for the characterization of citrate synthase
Authors: McComb Mark E , Donald Lynda J , Perreault Hélène
Published: April 2011

The enzyme citrate synthase from E. coli is a protein with a molecular weight (Mr) of 47 885 Da (wild type). This enzyme has been studied extensively, and its amino acid sequence has been characterized. This model protein has been used in this work for development and validation of methods involving capillary electrophoresis (CE) and electrospray ionization mass spectrometry (ESI-MS). The Mr determinations were conducted using sample infusion ESI-MS, and the tryptic digestion products of wild-type citrate synthase were characterized by on-line CE-ESI-MS coupled with a sheathless interface. On-line experiments were conducted on two different mass spectrometers, a Quattro-LC triple quadrupole instrument equipped with a Z-SprayTM source (Micromass), and a reflecting time-of-flight (TOF) mass spectrometer built in-house in the Time-of-Flight Laboratory, Department of Physics, University of Manitoba. This is the first article to be written on the interfacing of a Z-SprayTM source with CE. Unmodified fused silica capillaries gold-coated sheathless interfaces were used. The on-line CE separations yielded theoretical plate numbers greater than 104 on average. Selected ion electrophorograms (SIE) of the tryptic peptides recorded on the Quattro-LC displayed S/N ratios ranging from ca. 14 to 120 on raw data. These SIE enabled identification of each peptide. The use of reflecting time-of-flight mass spectrometry (TOFMS) afforded mass resolution values of ca. 6000 (m/deltamFWHM), which enabled isotopic resolution of the peptide components. CE-ESI-MS and CE-ESI-TOFMS experiments enabled the generation of a complete tryptic map of citrate synthase.Key words: capillary electrophoresis, electrospray ionization, mass spectrometry, citrate synthase, tryptic digestion, triple quadrupole analyzer, time-of-flight analyzer.
A novel photorearrangement of 1,3-diketonatoborinate: a photoinduced intramolecular alkylation on a conjugated carbonyl system
Authors: Ishiyama Jun-ichi , Chow Yuan L
Published: April 2011

Irradiation of borinates derived from 9-borabicyclo[3,3,1]nonane and 1,3-diketones at their π-π* transition band caused a rearrangement from their triplet excited state to give boronate intermediates that could be hydrolyzed to the corresponding aldols. The primary photolysis involves the scission of one of the BC bonds followed by a 1,3- or 1,5-migration and the formation of a more stable BO bond by radical pathways. The triplet state reaction was established by quenching and heavy atom effect experiments. The overall reaction pattern is an irreversible 1,2- or 1,4-addition of an alkyl-boron group to an α,β-conjugated ketone system by radical processes. Evidence showed that the non-isolated boronates also underwent secondary photoreactions followed by hydrolysis to afford enones corresponding to dehydration products of aldols; mechanisms are suggested for these secondary steps. The photoreaction was more facile in nonpolar solvents than in polar solvents; different solvents also gave different product compositions. In the presence of dilute acetic acid, the photolysis gave a high yield of aldols, suggesting a rapid acetolysis of the BC bond in boronates to acetylborates. Under oxygen, the photolysis was complicated by radical oxidation to give totally different products.Key words: borinate photochemistry, charge transfer absorption, alkylborane addition, boronate intermediates, BC bond homolytic scission
Mechanisms for the formation of epoxide and chlorine-containing products in the oxidation of ethylene by chromyl chloride: a density functional study
Authors: Torrent Maricel , Deng Liqun , Duran Miquel , Solà Miquel , Ziegler Tom
Published: April 2011

The reaction between CrO2Cl2and ethylene leading to the formation of epoxide and chlorohydrin precursors or directly to 1,2-dichloroethane has been studied by density functional theory. The formation of the epoxide precursor (Cl2(O)Cr-OC2H4) was found to take place via a [3+2] addition of ethylene to two Cr=O bonds followed by rearrangement of the five-membered diol to the epoxide product. The alternative mechanisms involving a direct addition of oxygen to ethylene or the [2+2] addition of the olefin to a Cr=O bond were found to have much higher activation energies. The formation of the chlorohydrin precursor (Cl(O)Cr-OCH2=CHCl) was found to take place via a [3+2] addition to one CrCl and one Cr=O bond. Pathways involving initial [2+2] addition to a CrCl or Cr=O bond had much higher activation barriers. The generation of 1,2-dichloroethane is highly unfavorable with an endothermicity of 44.7 kcal/mol and an even higher activation barrier. It is suggested that the formation of epoxide and chlorohydrin from the respective precursors requires the addition of H2O.Key words: reaction mechanisms, epoxide, oxidation of ethylene, chromyl chloride, DFT.
Modelling and calculation of dipole moment functions for XH bonds
Authors: Daub Christopher D , Henry Bryan R , Sage Martin L , Kjaergaard Henrik G
Published: April 2011

Two studies of aspects of modelling dipole moment functions of XH bonds in small molecules for use in calculating overtone intensities have been undertaken. The first study deals with the fitting of ab initio calculations of the dipole moment at discrete points to a functional form. The two methods that are compared are the use of least-squares regression and the use of interpolating polynomials. The interpolating polynomial method is deemed superior due to its greater efficiency in terms of the number of points necessary to obtain reasonable results. The second study attempts to explain the indifference of calculated overtone intensities to the inclusion of electron correlation in the ab initio calculation of the dipole moment function. It is found that in most cases the influence of electron correlation can be modelled as a function with a very small matrix element, which results in a very small contribution to the overtone intensity.Key words: dipole moment function, vibrational overtone intensities, electron correlation, ab initio calculations.
Symmetry in double-diffusive convection
Authors: Pettitt Brian , Danchura Werner
Published: April 2011

A theoretical treatment of the double-diffusive convection instability is presented, with a focus on the isothermal case of two solutes in a common solvent, initially forming a distinct interface. Linear stability analysis is used to determine the wave number of the marginal mode and symmetry properties of the corresponding fluid flow for realistic values of two control parameters: (i) the ratio of the diffusion coefficient of the solute in the lower solution to that in the upper solution (τd), and (ii) the ratio of the excess densities (relative to the solvent) of the upper to the lower solutions (τp). It is shown that the instability has two regimes: (i) τd-3/2 < τρ < τd-1 in which diffusion precedes convection, and (ii) τd-1 < τρ < 1 in which convection is immediate. The two regimes are separated by a point (τρ = τd-1) through which the wavelength diverges and the fluid velocity undergoes a continuous symmetry change. Above this point, columns of alternating up-and-down flow fade continuously into the solutions above and below the original interface. Below it, the flow within a column undergoes at least one reversal. The horizontal nodal plane associated with such a reversal should be observable.Key words: double-diffusive convection, fluid instability, pattern formation, solutions.
The effect of preparation and aging conditions on the internal environment of sol-gel derived materials as probed by 7-azaindole and pyranine fluorescence
Authors: Flora Kulwinder K , Dabrowski Monika A , Musson Sean P , Brennan John D
Published: April 2011

Sol-gel derived silicate materials were prepared by a two-stage processing method similar that commonly used for entrapment of biomolecules. In the first step, tetraethylorthosilicate (TEOS) was hydrolyzed by sonicating in the presence of water (molar ratio of H2O:Si = 4:1) and 0.044 M HCl with no added alcohol. The hydrolyzed TEOS was then mixed with varying amounts of a buffer solution (1:1 up to 4:1 vol:vol of silane:buffer) to induce gelation, and the monoliths were aged at room temperature either in air or in buffer for several days, followed by prolonged drying in air over several months. The emission spectra of the entrapped probes 7-azaindole (7AI) and 8-hydroxy-1,3,6-trisulfonated pyrene (pyranine) were examined at various times during the aging process to monitor the effects of the H2O:Si ratio on the evolution of the internal environment of the silicate materials. The results showed that (i) the internal environment of the silicate materials continued to evolve long after the drying of the matrix (as determined by mass changes) was complete; (ii) the internal structure of the silicate materials, as monitored by both 7AI fluorescence and infrared spectroscopy, was affected by the proportion of water used for gelation of dry-aged materials, but was similar for all wet-aged samples, and (iii) both wet-aged and dry-aged monoliths had a distribution of final environments present both during and after drying. The implications of these findings for protein entrapment are discussed.Key words: fluorescence, sol-gel, silicate materials, aging conditions, biosensor.
Collagen remodeling in the extracellular matrix of the cardiomyopathic Syrian hamster heart as assessed by FTIR attenuated total reflectance spectroscopy
Authors: Bromberg Pamela S , Gough Kathleen M , Dixon Ian MC
Published: April 2011

Collagen type I and III deposition in the cardiac extracellular matrix contributes significantly to myocardial dysfunction. Diffuse and focal fibrosis is believed to accompany human congestive cardiomyopathy (CCM) associated with congestive heart failure (CHF). The left ventricle collagen remodeling that occurs in the hamster model of CCM is marked by left ventricle fibrosis, hypertrophy and dilation, proceeded by CHF post 150 days of age. The objectives of our study were to (i) evaluate changes in collagen deposition in the right (RV) and left (LV) ventricular tissue of cardiomyopathic (CMP) and control (CON) myocardium using FTIR ATR spectroscopy, (ii) classify the normal and diseased heart tissue using linear discriminant analysis (LDA) aided by a genetic algorithm (GA) selection of spectroscopically diagnostic regions in the mid-IR region, (iii) rationalize the spectroscopic differences between left/right ventricle tissue as well as CON/CMP tissue according to the pathophysiology documented for the UM-X7.1 strain of CMP hamsters. The presence of collagen in the tissue was confirmed spectroscopically by observation of the collagen IR fingerprint in the 1000-1800 cm-1 region. Difference spectroscopy was utilized to substantiate which tissue under comparison exhibited pronounced collagen content. Multivariate analysis (LDA) was carried out on user-selected spectral subregions and compared to class separation based on spectral subregions chosen nonsubjectively by a GA. Our study confirmed that the animals experienced LV collagen remodeling denoted by focal rather than diffuse fibrosis. In addition, RV collagen remodeling, denoted by decreased RV collagen content, appeared to accompany the increased LV collagen deposition found for the CMP animals.Key words: FTIR spectroscopy, collagen, cardiomyopathy, genetic algorithm, linear discriminant analysis.
Étude de l'adsorption des hydrocarbures aromatiques polycycliques sur l'hydroxyde d'aluminium par chromatographie liquide à haute pression
Authors: Hassani Chakib Ameziane , Ménard Hugues
Published: April 2011

A study by high performance liquid chromatography of adsorption isotherm of HAP in toluene on Al(OH)3 heated between 25 and 700°C shows that the rate of adsorption is maximal on the Al(OH)3 heated at 300°C. The measurement of electrokinetic potential and the surface area of adsorbants approve this result. The determination of adsorption enthalpies of different HAP on the aluminium hydroxide has shown the existence of physical adsorption. The adsorbing power of Al(OH)3 heated at 300°C is compared with that of other materials known or suspected very carcinogens.Key words: adsorption isotherm of HAP, aluminium hydroxide.
A 19F nuclear magnetic resonance study of the conjugate Brönsted-Lewis superacid HSO3F-SbF5. Part 1
Authors: Zhang Dingliang , Heubes Markus , Hägele Gerhard , Aubke Friedhelm
Published: April 2011

The Brönsted-Lewis superacid HSO3F-SbF5 or "magic acid" is re-investigated by modern 19F NMR methods over a wide concentration range. The system is found to be considerably more complex than had been assumed previously. A total of 13 different anions are identified of which only five have previously been identified in magic acid. With increasing SbF5 contents the concentration of monomeric anions like [SbF6]-, [SbF5(SO3F)]-, cis- and trans-[SbF4(SO3F)2]-, and mer-[SbF3(SO3F)3]- gradually decreases. Except for [Sb2F11]-, which is present in very small concentrations only, the formation of oligomers involves exclusively μ-fluorosulfato bridges. In addition to donor (SO3F)- and acceptor (SbF5) complex formation to give [SbF5(SO3F)]- and possibly ligand redistribution, the solvolysis of SbF5 or SbF4(SO3F) in HSO3F appears to be the principal formation reaction for polyfluorosulfatofluoroantimonate(V) anions. In glass (NMR tubes) the solvolysis product HF is converted to the oxonium ion [H3O]+, which has previously been identified by 1H NMR and structurally characterized as [H3O][Sb2F11] by us.Key words: magic acid, conjugate superacid, fluorosulfuric acid, 19F NMR spectra.
Nuclear magnetic resonance investigations of azo-hydrazone, acid–base equilibria of FD& Yellow No. 5 (tartrazine)
Authors: Mazzola Eugene P , Bell Sandra J , Turujman Saleh A , Barrows Julie N , Bailey, Jr. John E , McMahon Joseph B , Germann Tania K , Reynolds William F
Published: April 2011

Azo-hydrazone, acid-base equilibria were examined for the water-soluble, food color additive FD&C Yellow No. 5 (Y5), with the observation of dynamic NMR effects near, and at, pH 10, where the food colorant converts from a predominantly hydrazone tautomer (acidic to moderately basic pH) to a prevailing azo-anion resonance hybrid (high pH). Potentiometric titrations further indicate that the pKa of Y5 is ca. 10. These data suggest that observed NMR line broadening is due to slow proton transfer between a hydrazone-NH and water.Key words: 1H, 13C, and 15N NMR, azo-hydrazone/acid-base equilibria, FD&C Yellow No. 5.
31P chemical shielding as a function of bond angle: phosphorus chemical shielding surface of phosphinoborane R2PBR'2
Authors: Ditty Michael JT , Power William P
Published: April 2011

The phosphorus chemical shielding surface of phosphinoborane R2PBR'2 has been investigated via molecular orbital theory calculation and experimental measurement of selected derivatives. Ab initio calculations of phosphorus chemical shielding tensors were determined for the phosphinoboranes H2PBH2 and (CH3)2PB(CH3)2. Changes in the angle from planarity, i.e., that between the P-BR'2 plane and the bisector of the RPR angle, are reflected in the orientations and magnitudes of the three principal components of the phosphorus chemical shielding tensor. To determine the validity of the calculated phosphorus chemical shielding surface, three phosphinoborane compounds with different angles from planarity were synthesized and studied by solid-state 31P NMR spectroscopy. The 31P NMR powder patterns provided experimental magnitudes of the principal components of the phosphorus chemical shielding tensor for each compound, which compared well to the calculated predictions; where orientations of these tensors could be determined experimentally, they also agreed well with the calculated results. The combined experimental and theoretical results provide a good description of the effects on changes in bond angle on phosphorus chemical shielding as the molecule is distorted from a planar to folded geometry.Key words: chemical shielding, solid-state NMR, phoshinoborane, ab initio calculations.
Improved hydrophobic zeolites to fill silicone membranes for ethyl acetate extraction from water by pervaporation
Authors: Yang Hu , Ping Zhenghua , Long Yingcai , Nguyen Quang Trong
Published: April 2011

Two types of hydrophobic zeolites Y were prepared by treating the NaY-type zeolite with SiCl4, with or without subsequent hydrothermal treatment. X-ray diffraction analysis showed that the framework of the two modified zeolites Y are more perfect than that of the NaY one, and the Si/Al ratio is greatly enhanced by hydrothermal treatment. The determined sorption isotherms indicate an improvement of the ester-to-water sorption ratio with the increase in Si/Al ratio. When the zeolite-filled silicone membranes are used in separation of ethyl acetate - water mixture by pervaporation, both the total permeation flux and the selectivity to the ester are improved by the use of the hydrophobic zeolite Y as the filler in the membrane. The stronger the hydrophobicity of the zeolite Y, the higher the filled membrane selectivity.Key words: hydrophobic zeolites Y, silicone-filled membranes, ethyl acetate - water mixture, sorption, pervaporation.
Symmetry, disorder, and dynamics in solid crown ether complexes: 2H NMR studies of 15-crown-5·NaI, 15-crown-5·NaClO4, and 21-crown-7·KI
Authors: Buchanan G W , Gerzain M , Ratcliffe C I
Published: April 2011

2H NMR line shapes have been obtained as a function of temperature for partially deuteriated 15-crown-5·NaI, 15-crown-5·NaClO4 and 21-crown-7·KI. Sudden changes in the line shapes above 330 K correspond to phase transitions: DSC shows transitions at 338, 347 K for 15-crown-5·NaI, and 356, 374, 383 K for 15-crown-5·NaClO4. The 2H NMR line shapes for the room temperature (RT) phases show the onset of a motion of the macrocycles above 200K, which is rapid a little above room temperature. Through detailed analysis of the complicated line shapes it has been shown that the most consistent interpretation of the dynamics in these RT phases is a "merry-go-round" type of motion similar to that found for 18-crown-6 and 12-crown-4 complexes in which O-CH2-CH2 units exchange sites around the ring, simultaneously adjusting their conformation to fit the site. In the high-temperature phases of all three complexes all the ring atoms are dynamically equivalent. This can only happen through a combination of increased symmetry and disorder for which possible models have been devised.Key words: 2H NMR, crown ether complexes, dynamics, disorder.
A nuclear magnetic resonance investigation of the micellar properties of a series of sodium cyclohexylalkanoates
Authors: MacInnis Judith A , Palepu R , Marangoni D Gerrard
Published: April 2011

The micellar properties of a family of surfactants, the sodium cyclohexylalkanoates, have been investigated in aqueous solution using multinuclear NMR spectroscopy. C-13 chemical shift measurements have been used to determine both the cmc values and the micellar aggregation numbers (Ns values) of these surfactants. The cmc values and the degrees of counterion binding were estimated from 23Na chemical shift measurements. The critical micelle concentrations (cmc's) and the aggregation numbers determined from the NMR experiments indicate that these amphiphiles have high cmc's and low aggregation numbers when compared to other single-headed surfactants (most notably the sodium alkanoates). The conformational changes incurred by the carbon atoms upon micelle formation have been deduced from the 13C chemical shift differences (δsurf,mic - δsurf,aq). These results are used to discuss the formation of the aggregates of the sodium cyclohexylalkanoate surfactants as a function of the length of the alkanoate side chain.Key words: micelles, surfactants, NMR spectroscopy, chemical shifts, aggregation numbers, degree of counterion binding, conformational changes.
Crystal structure and solution studies of the product of the reaction of β-mercaptoethanol with vanadate
Authors: Bhattacharyya Sudeep , Batchelor Raymond J , Einstein Frederick WB , Tracey Alan S
Published: April 2011

Reaction of β-mercaptoethanol with vanadate under slightly alkaline conditions provided a crystalline complex that was characterized by X-ray diffraction and FTIR spectroscopy. The complex was dimeric in structure with a central [VO]2 core and a pentacoordinate, crudely trigonal bipyramidal arrangement about each vanadium atom with a sulphur occupying a pseudo-axial position. A single 51V NMR signal was observed for this complex when dissolved in water, chloroform or acetonitrile. A large influence of acetonitrile on the vanadium chemical shift suggested the possibility of reaction with acetonitrile. FTIR showed the presence of two complexes in acetonitrile solution but only one in chloroform or water. Mixed solvent studies were carried out in an effort to further characterize the solution complexes. Crystal structure of [{VO2(OC2H4S)}2][NEt4]2: monoclinic, space group P21/n,. a = 8.3451(17), b = 16.954(4), c = 10. 2064(25) Å; β = 101. 271(18)°; V = 1416.2 Å3; Z = 2; RF = 0.048 for 1355 data (Io 2..5σ (Io) and 147 variables.Key words: mercaptoethanol, vanadate, vanadium NMR, X-ray diffraction, FTIR, thiolate.
Thermodynamic properties of aqueous diethanolamine (DEA),N,N-dimethylethanolamine (DMEA), and their chloride salts: apparent molar heat capacities and volumes at temperatures from 283.15 to 328.15 K
Authors: Collins Christopher , Tobin Joelle , Shvedov Dmitri , Palepu Rom , Tremaine Peter R
Published: April 2011

Apparent molar heat capacities Cp,ϕand apparent molar volumes Vϕfor aqueous diethanolamine (HOC2H4)2NH, diethanolammonium chloride (HOC2H4)2NH2Cl, N,N'-dimethylethanolamine (HOC2H4)(CH3)2N, and N,N'-dimethylethanolammonium chloride (HOC2H4)(CH3)2NHCl were determined from 283.15 to 328.15 K with a Picker flow microcalorimeter and vibrating tube densimeter. The experimental results have been analyzed in terms of Young's Rule with the Guggenheim form of the extended Debye-Hückel equation and appropriate corrections for chemical relaxation effects. These calculations lead to standard partial molar heat capacities and volumes for the neutral amines, (HOC2H4)2NH(aq) and (HOC2H4)(CH3)2N(aq), and the ions (HOC2H4)2NH2+(aq) and (HOC2H4)(CH3)2NH+(aq) over the experimental temperature range. Key words: standard partial molar volumes, standard partial molar heat capacities, diethanolamine, dimethyethanolamine, aqueous alkanolamine ionization.
Dimerization of conjugated 1-indanones
Authors: Leblanc Yves , Dufresne Claude , Dhawan Rajiv , Ollerenshaw Jason , Littke Adam , Trimble Laird A , Tsou Nancy N
Published: April 2011

Conjugated 1-indanones dimerize under basic conditions to provide spirodimers. For example when 2-(E)-carbomethoxymethylene-1-indanone (5) was treated with Cs2CO3 in CH3CN, two spirodimers, 6 and 7, were produced via two modes of reaction (mode A and mode B). With 2-(E)-cyanomethylene-1-indanone (9) only mode A dimerized spiro products (10 and 11) were observed whereas with 2-chloromethylidene-1-indanone (12) a 1,2-dimer (13) was obtained instead of the expected spirodimer.Key words: dimerization, conjugated 1-indanones, spirodimers.
A new short synthetic approach to chlorocyclopentenones related to cryptosporiopsin via a ring expansion strategy: first synthesis of a fungitoxic metabolite from Sporormia affinis
Authors: Kabanyane Sidima , Decken Andreas , Yu Chao-Mei , Strunz George M
Published: April 2011

A metabolite of the coprophilous fungus, Sporormia affinis, 2-trans-allyl-5-chloro-1-hydroxy-4-oxo-2-cyclopentene-1-carboxylic acid methyl ester, 2, was synthesized in racemic form, by an eight-step sequence, in 11% overall yield. Departing from previous ring-contraction strategies for synthesis of chlorocyclopentenones in the cryptosporiopsin series, the key step in the new synthesis was ring expansion of a substituted cyclobutanone, the product of [2+2] cycloaddition of dichloroketene with methyl 2-trimethylsilyloxyhex-2-enoate.Key words: Sporormia affinis, cryptosporiopsin, chlorocyclopentenone, dichloroketene, [2+2] cycloaddition, ring expansion, selenium dioxide.
Excited state intramolecular proton transfer in 2-(2'-hydroxyphenyl)-1H-naphth-[2,3-d]-imidazole: effects of solvents and pH
Authors: Das Somes K , Krishnamoorthy G , Dogra Sneh K
Published: April 2011

The solvent dependent study of absorption, fluorescence, and fluorescence excitation spectra of 2-(2'-hydroxyphenyl)-1H-naphth-[2,3-d]-imidazole (HPNI) have indicated the presence of different rotamers and tautomers in the Soand S1states. Similarity of the absorption and fluorescence spectra of HPNI in protic solvents with those of 2-(2'-methoxyphenyl)-1H-naphth-[2,3-d]-imidazole (MPNI) suggests that the normal emission is observed from the rotamers 2 and 4, whereas the tautomer emission is observed from the tautomer 3, formed by the excited state intramolecular proton transfer (ESIPT) in the rotamer 1 (Scheme 1). Ground state geometries of rotamers 1, 2, and the tautomer 3 were optimized using AM1 method. The results show that the rotamer 2 is the most stable and its stability further increases in polar and protic solvents due to the dipolar solvation interaction. The increase in the fluorescence quantum yield of the normal band when methanol or water is added to dioxane is due to: (i) the formation of rotamer 4, where ESIPT is not possible and (ii) the decrease in the rate of non-radiative decay process. Very large red shifted fluorescence bands of the monocations of HPNI and MPNI in different solvents have been assigned to the charge transfer band.Key words: 2-(2'-hydroxyphenyl)-1H-naphth-[2,3-d]-imidazole, ESIPT, prototropic reactions, fluorescence, 2-(2'-methoxyphenyl)-1H-naphth-[2,3-d]-imidazole.
Silaheterocycles 36 (1). Trichlorovinylsilane, lithium-tert-butyl, and 1,3-enynes: A versatile combination for the competitive formation of silacyclobutanes and silacyclobutenes
Authors: Auner N , Grasmann M , Herrschaft B , Hummer M
Published: April 2011

Equimolar amounts of trichlorovinylsilane (9), lithium-tert-butyl, and 1,3-enynes were reacted to yield the corresponding isomeric silacyclobutanes and silacyclobutenes competitively. As reaction pathway the mixture 9/tBuLi is discussed to give a silene equivalent, Cl2Si=CHCH2tBu (10), yielding the four-membered ring silacycles by formal [2 + 2] cycloaddition reactions of 10 with the C=C double or the C=C triple bond of the 1,3-enyne. The relative ratio of the products formed depends on the polarity of the multiple bonds in the enyne, which is mainly determined by the substituent pattern. Thus, from the organosubstituted 1,3-enynes R1 C=C-C(R2)=CR3R4 (R1 = Me, Et, SiMe3, Ph; R2 = H, Me, Ph; R3=Me, OMe, Ph; R4=H; and R1C=C-R' (R' = 1-cyclohexenyl, cyclohexanevinylidyne)) and 9/tBuLi the silacyclobutanes 12, 13, and 15 and the silacyclobutenes 14,16-24, and 27 (from 10 and 3-hexyne) are prepared in a one-step synthesis and isolated from the reaction mixtures. The silacyclobutanes and -butenes are thermally stable and can be distilled under vacuo up to temperatures of about 150°C without decomposition. The experimental results are confirmed qualitatively by semiempiric calculations at the AM-1 level and their analysis using FMO theory. The solid state structure of the silacyclobutene 19 (C17H30Cl2Si2) has been determined by single crystal X-ray diffractometry. 19 is orthorhombic, space group P212121, a = 1030.94(2) pm, b = 1244.6(2) pm, c = 1605.5(3) pm, Z = 4.Key words: neopentylsilene, dichloroneopentylsilene, silacyclobutenes, silacyclobutanes, 1,3-enynes.
Diastereoselective alkylations of chiral, phosphorus-stabilized carbanions: N-alkyl substituent effects in P-alkyl-1,3,2-diazaphosphorinane 2-oxides
Authors: Denmark Scott E , Kim Jung-Ho
Published: April 2011

A systematic study of the diastereoselective alkylation of anions derived from racemic N-substituted P-alkyl 1,3,2-diazaphosphorinane 2-oxides was carried out with variation of the N-substituent. High diastereoselectivity for the methylation of a P-benzyl anion has been achieved with N-neopentyl derivative 5d. Similarly, a P-ethyl anion derived from N-neopentyl derivative 6d showed high diastereoselectivity upon benzylation. The observed difference in alkylation diastereoselectivity between P-ethyl and P-benzyl anions for various N-alkyl substituents is discussed.Key words: phosphonamide-stabilized carbanions, alkylation, asymmetric, stereoselective, organolitihium.
Genetic analysis of microcin H47 immunity
Authors: Rodríguez Eliana , Laviña Magela
Published: April 2011

Microcin H47 is a bactericidal antibiotic produced by a natural Escherichia coli isolate. The genetic system encoding microcin production and immunity consists of at least seven clustered genes. Four of these are devoted to microcin biosynthesis and two genes are required for its secretion into the extracellular medium. The product of the seventh gene, mchI, determines the cell's self-immunity. This gene was shown to encode a highly hydrophobic 69-residue peptide. Analysis of the MchI amino acid sequence, as well as the characterization of MchIPhoA hybrid proteins, indicated that the microcin immunity product is probably exported out of the cytoplasm and remains an integral membrane peptide. This localization of the immunity peptide points to the cellular membrane as the site of action of microcin H47. Key words: microcin H47, microcin immunity, phoA gene fusion, membrane peptide.
Effects of bacterial colonization of tomato roots on subsequent colonization by Pseudomonas fluorescens MelRC2Rif
Authors: Ikeda K , Toyota K , Kimura M
Published: April 2011

We investigated the effects of root colonization by soil microbes on the subsequent colonization by a bacterial inoculant, Pseudomonas fluorescens MelRC2Rif. When a soil bacterial community was introduced onto tomato roots, subsequent colonization by MelRC2Rif was significantly lowered. However, soil fungi had no suppressive effects, suggesting that bacteria may be involved in the suppression. When individual bacterial strains were examined, most strains except for P. fluorescens showed no significant suppressive effect on subsequent colonization by MelRC2Rif. In contrast, four out of six P. fluorescens strains significantly suppressed subsequent colonization by MelRC2Rif. Gram-negative bacteria, which had little suppressive effect on colonization by MelRC2Rif with individual inoculation, elicited suppression in some combination treatments. Simultaneous inoculation of individual bacterial strains with MelRC2Rif indicated that suppressive effect by these strains occurred only when MelRC2Rif was inoculated later to tomato roots. These results suggest that a less competitive bacterial strain may interfere with the colonization of a competitive strain on tomato rhizospheres. Key words: ecological niche, bacterial colonization, rhizosphere, competition.
Investigation of serine hydroxymethyltransferase in methanogens
Authors: Lin Zhaosheng , Sparling Richard
Published: April 2011

The cofactor specificity of serine hydroxymethyltransferase (SHMT) activities was tested in extracts of several methanogens using tetrahydromethanopterin (H4MPt) from Methanobacterium thermoautotrophicum Marburg, tetrahydrosarcinapterin (H4SPt) from Methanosarcina barkeri, and tetrahydrofolate (H4folate) as the potential C1 carrier. In Methanosphaera stadtmanae and Methanococcus thermolithotrophicus, the activities were H4MPt dependent. In Methanospirillum hungatei GP1, Methanosaeta concilii, Methanolobus tindarius, and Methanosarcina barkeri Fusaro, the activities were strictly H4folate dependent. H4SPt was reactive with the SHMT of Methanosphaera stadtmanae but not with that of Methanosarcina barkeri. In both Methanosarcina barkeri and Methanospirillum hungatei, pyridoxal phosphate stimulated SHMT activity. The apparent Km values for H4folate and L-serine were 0.086 and 0.29 mM in Methanosarcina barkeri and 0.065 and 0.31 mM in Methanospirillum hungatei, respectively. Key words: tetrahydromethanopterin, tetrahydrofolate, Archaea, serine hydroxymethyltransferase, methanogen.
Feeding behaviour and grazing impacts of a Euplotes sp. on attached bacteria
Authors: Lawrence J R , Snyder R A
Published: April 2011

The locomotory and feeding responses of a Euplotes sp. to attached populations of Vibrio natriegens and Pseudomonas fluorescens in a continuous flow system were analyzed by computer image analysis of video microscopy recordings. Upon entry into the chamber, the ciliates moved in long continuous arcs 300 µm in length during which time no bacteria were consumed. As feeding began, the average path length shortened, the arcs became tighter, and the ciliates changed direction more frequently. The feeding activity of the Euplotes appeared to be gregarious, being concentrated in patches within the biofilm of attached bacteria. It was also noted that the feeding effort targeted patches previously visited by other Euplotes, despite reduced bacterial density relative to the surrounding field of attached bacteria. This focused and intense feeding activity resulted in localized zones of nearly complete clearance within the attached bacterial populations. Loss of bacteria and averaged ciliate presence within feeding patches were determined from digitized time series images and discrimination thresholds for particle size. These data were used to determine grazing rates indicating that Euplotes sp. removed 120 V. natriegens cells·ciliate1·h1 and up to 882 P. fluorescens cells·ciliate1·h1. However, surface clearance rates for Euplotes sp. grazing on V. natriegens and P. fluorescens were 0.02 and 0.03 mm2·ciliate1·h1, respectively, indicating that surface grazing pressure was fairly consistent within the patches of intense feeding activity. The effect of such intense localized feeding behaviour on attached or biofilm bacteria would be to increase spatial and temporal heterogeneity within biofilms. Key words: digital image analysis, Euplotes, grazing, biofilms.