Showing papers on "Acetonitrile published in 1991"

Shock Tube Pyrolysis of Pyrrole and Kinetic Modeling

Abstract: The kinetics of pyrolysis of pyrrole dilute in argon have been studied in a single pulse shock tube, using capillary column GC, together with GC/MS and FTIR for product identification, over the temperature range 1200–1700 K, total pressures of 7.5–13.5 atm and nominal mixture compositions of pyrrole of 5000 and 700 ppm (nominal concentrations of 5 × 10−7 and 7 × 10−8 mol cm−3). Time-resolved measurements of the rate of disappearance of pyrrole behind reflected shock waves have been made by absorption spectroscopy at 230 nm, corresponding to the lowest 1π* 1π transition of pyrrole at pressures of 20 atm and mixture compositions between 1000–2000 ppm pyrrole (1.7–3.0 × 10−7 mol cm−3) over the temperature range of 1300 to 1700 K. At the lower end of the studied temperature range, the isomers of pyrrole, allyl cyanide and cis- and trans-crotononitrile, were the principal products, together with hydrogen cyanide and propyne/allene. At elevated temperatures, acetylene, acetonitrile, cyanoacetylene, and hydrogen became important products. The rate of overall disappearance of pyrrole, as measured by absorption spectrometry, was found to be first order in pyrrole concentration, with a rate constant kdis(pyrrole) = 1014.1±0.7 exp(−74.1 ± 3.0 kcal mol−1/RT) s−1 between 1350–1600 K and at a pressure of 20 atm. First order dependence of pyrrole decomposition and major product formation was also observed in the single pulse experiments over the range of mixture compositions studied. A 75-step reaction model is presented and shown to substantially fit the observed temperature profiles of the major product species and the reactant profile. In the model the initiation reaction is postulated to be the reversible formation of pyrrolenine, (2H-pyrrole). Pyrrolenine can undergo ring scission at the C2N bond forming a biradical which can rearrange to form allyl cyanide and crotononitrile or undergo decomposition to form HCN and C3H4 or acetylene and a precursor of acetonitrile. The model predicts an overall rate of disappearance of pyrrole in agreement with the experimental measurements.

Raman spectroscopic study of solvation structure in acetonitrile/water mixtures

Abstract: : Raman spectroscopy is used to probe the CN stretching frequency of acetonitrile as a function of concentration in water. The CN band is modeled as the sum of two Gaussians. The concentration dependence of area and width for each of the Gaussian components provides experimental support of an equilibrium between two forms of acetonitrile in solution. In addition, the concentration dependence of each of the bands correlates well with the thermodynamically related Kirkwood-Buff integrals. The frequency shift of the CN band exhibits a linear dependence on the dielectric constant of protic solvents. There has been considerable effort to define and understand the fundamental molecular interactions important in liquid chromatography . Although the solvophobic theory is commonly invoked to explain retention in reversed-phase liquid chromatography (RPLC), recent studies have pointed out shortcomings in this model.

Carbon-carbon double-bond formation in the intermolecular acetonitrile reductive coupling promoted by a mononuclear titanium(II) compound. Preparation and characterization of two titanium(IV) imido derivatives

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Preferential solvation of a dipolar solute in mixed binary solvent: a study of UV-visible spectroscopy

Abstract: The preferential solvation in mixed binary solvent, e.g., ethanol+acetonitrile, ethanol+propionitrile, and ethanol+butyronitrile, has been studied by monitoring the solvatochromic charge-transfer band of N-alkylpyridinium iodide. It appears that alcohols are preferred over the nitriles. A model for describing the preferential solvation of a dipolar in binary solvent mixture has been developed and discussed in relation to other existing approaches. The role of (a) solvent-solvent interaction and (b) difference in size of the component solvent molecules in determining the preferential solvation has also been discussed

Electrochemistry of zeolite-encapsulated cobalt salen complexes in acetonitrile and dimethyl sulphoxide solutions

Abstract: CoII complexes of N,N′-bis (salicylaldehyde)ethylenediimine (salen) have been entrapped in the supercages of zeolite Y. The redox properties of these complexes are investigated by cyclic voltammetry. The results show the existence of two different kind of entrapped CoIII salen forms within the zeolite. The accessibility and redox activity of the intrazeolite complexes are discussed.

Influence of potential on metal-adsorbate structure: solvent-independent nature of infrared spectra for platinum(111) carbon monoxide

Abstract: In situ infrared spectra are reported in the C-O stretching (ν CO )region for saturated CO adlayers on Pt(111) in four nonaqueous solvents; acetonitrile, dimethylformamide, dichloromethane, and tetrahydrofuran, each containing tetraalkylammonium cations, over wide electrode potential ranges, ca. −2.0 to 1.5 V vs SCE

Papain in organic solvents : determination of conditions suitable for biocatalysis and the effect on substrate specificity and inhibition

Abstract: Synthesis of N-CBZ-(N-Carbobenzoxy)-1-amino-acid methyl esters from N-CBZ-amino acids and methanol has been used as an assay to examine the properties of papain in organic solvents containing small amounts of water. Papain is active in solvents ranging in polarity from acetonitrile to tetrachloromethane. The optimal activity in each solvent varied only about three to four fold, but the amount of added water required to achieve it varied from 4% (v/v) in acetonitrile to 0.05% (v/v) in tetrachloromethane. The enzyme was generally more stable in hydrophobic solvents and at lower water contents. The apparent K(m) value of CBZ-glycine was 26 times higher in acetonitrile than in toluene due to differential partitioning of the substrate between aqueous and organic phases. The substrate specificity of the enzyme was qualitatively little different from that in aqueous solution, with amino acid derivatives still the best substrates. Nitrile analogs of substrates inhibited the enzyme, as they do in aqueous solution, and inhibition by a variety of substituted aromatic hydrocarbons showed that the main specificity of papain for hydrophobic side chains at its S(2) subsite, was little affected. The results show that papain can catalyze reactions under a variety of conditions in organic solvents but its substrate specificity is little changed from that in aqueous media.

Lithium bromide in acetonitrile and water: a neutron scattering study

Abstract: The structure around lithium ions in solutions of lithium bromide in acetonitrile and water has been studied by neutron diffraction. For this purpose the isotopic first-order difference method has been applied to the lithium ion. For a 0.58 M acetonitrile solution it has been found that the bromide anion enters into the first solvation shell around the lithium ion, whereas in the case of a 1.88 M aqueous solution the first hydration shell of the cation is not disturbed by the anion. The solvation number is approximately 3 in the case of acetonitrile and approximately 4.5 in the case of water.

Isolation and characterization of methyl hypofluorite (CH3OF)

Abstract: Methyl hypofluorite (CH 3 OF) has been prepared by the reaction of elemental fluorine with methanol in acetonitrile or propionitrile at low temperature. It was removed from the reaction mixture in a stream of nitrogen and purified by fractional distillation. The compound is moderately long-lived, although the liquid has exploded upon rapid warming. The liquid compound has a freezing point of about −142°C, and vapor pressure measurements indicate a normal boiling point of −32.6±0.9°C and an enthalpy of vaporization of 23.37±0.26 kJ/mol. The infrared spectrum of the vapor is consistent with the molecule being isostructural with CH 3 OH and CF 3 OF. The compound has also been characterized by mass spectrometry and by 1 H, 19 F, and 13 C NMR spectrometries. This novel reagent adds across various double bonds to form the corresponding fluoro methoxy adducts, which are not easily accessible by other methods

Transport properties of nonelectrolyte mixtures. IX. Viscosity coefficients for acetonitrile and for three mixtures of toluene+acetonitrile from 25 to 100°c at pressures up to 500 MPa

Abstract: A two-coil self-centering falling-body viscometer has been used to measure viscosity coefficients for acetonitrile and three binary mixtures of toluene+ acetonitrile at 25, 50, 75, and 100°C and pressures up to 500 MPa. The results for acetonitrile can be interpreted by an approach based on hard-sphere theory, with a roughness factor of 1.46. The binary-mixture data are well represented by the Grunberg and Nissan equation with a mixing parameter which is pressure and temperature dependent but composition independent.

Electron-transfer distance in intermolecular diffusion-limited reactions

Abstract: Diffusion-limited fluorescence quenching of ZnTPP(1S1) and quinone (benzoquinone BQ and chloranil CA) have been studied in toluene and acetonitrile at 5, 25 and 45 °C. The steady-state and time-domain experimental studies indicate that the ‘optimum’ electron-transfer distance is larger by 3–5 A for the ZnTPP–CA reaction than that in the ZnTPP–BQ reaction.