Showing papers on "Acetonitrile published in 2003"

Solute Rotation and Solvation Dynamics in a Room-Temperature Ionic Liquid

Abstract: Steady-state spectra, rotation times, and time-resolved emission spectra of the probe 4-aminophthalimide (4-AP) in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim+][PF6-]) were measured over the temperature range 298−355 K. The steady-state spectroscopy indicates that the solvation energetics of 4-AP in [bmim+][PF6-] are comparable to those of 4-AP in highly polar but aprotic solvents such as dimethylformamide and acetonitrile (π* ∼ 0.8, ∼ 0.4). The rotation of 4-AP in [bmim+][PF6-] and in more conventional aprotic solvents generally conforms to the expectations of simple hydrodynamic models. Other than the fact that [bmim+][PF6-] is highly viscous, nothing distinguishes the rotation of 4-AP in this ionic liquid from its rotation in more conventional polar aprotic solvents. Time-dependent emission spectra, recorded with an instrumental response of 25 ps, indicate that solvation dynamics in [bmim+][PF6-] occur in two well-separated time regimes. Near to room temperature, the obser...

Reactivities of Some Electrogenerated Organic Cation Radicals in Room-Temperature Ionic Liquids: Toward an Alternative to Volatile Organic Solvents?

Abstract: Four different ionic liquids, based on 1-alkyl-3-methylimidazolium or quaternary ammonium cations, were used as reaction media for several typical electrochemical reactions with a strict control of the residual water concentration. The oxidation of organic molecules (anthracene, naphthalene, durene, 1,4-dithiafulvene, and veratrole) for which the cation radicals undergo first and second-order kinetics reactions were investigated in ionic liquids and compared with their behavior in acetonitrile. From the analysis of the voltammetric current responses, the reaction mechanism was established and the thermodynamic and kinetics parameters were extracted. The most interesting result is that the nature of investigated mechanisms is almost unchanged in ionic liquids as compared with conventional organic media. A decrease of the electron-transfer kinetics from the aromatic molecules to the electrode (around 1 order of magnitude) is observed for all of the studied molecules, indicating an higher solvent reorganizat...

Solvent-stabilized molecular capsules.

Abstract: Pyrrogallolarenes 2 were prepared by acid-catalyzed condensation of pyrrogallol with aldehydes. Compound 2a crystallizes from a methanol solution of quinuclidine hydrochloride to give a dimeric molecular capsule surrounding one disordered quinuclidinium cation. The molecules of 2a are connected by direct hydrogen bonds and by bridging methanol and water molecules. The chloride anion is positioned outside the capsule and is hydrogen bonded to the hydroxy groups of 2a. The shortest distance between the cation and anion was found to be 6.7 A. Crystallization of 2b from aqueous acetonitrile resulted in a dimeric capsule linked by a polar belt of 16 hydrogen bonding water molecules. Four acetonitrile molecules occupy the cavity of this dimeric capsule and assume two binding sites that differ in hydrogen bonding and electronic environment. Compounds 2 also form hydrogen-bonded dimeric molecular capsules in alcohols and aqueous acetonitrile solutions. These assemblies readily encapsulate tetramethylammonium, tetramethylphosphonium, quinuclidinium, and tropylium cations to give complexes stable on the NMR time scale at 233 K.

Electrochemical behavior of a new precursor for the design of poly[Ni(salen)]-based modified electrodes

Abstract: We describe the potentiodynamic preparation and subsequent characterization of poly[Ni(3-MeOsaltMe)] films (surface concentration, 3 < i/nmol cm -2 < 350) in acetonitrile media. Coulometric and gravimetric (electrochemical quartz crystal microbalance, EQCM) data allow one to monitor the deposition process and show that the resultant films are physically and chemically stable. Combined EQCM/probe beam deflection measurements were used to quantify the individual contributions (fluxes and population changes) of anions, cations, and solvent to the overall redox switching process. The first redox cycle for a film “rested” in the reduced state results in accumulation of anion (charge) and solvent in the film. The subsequent steadystate response is dominated by a combination of anion and solvent transfers; this takes place in two stages, the second of which involves significantly more solvent entry, to an extent dependent upon time scale. After a sequence of redox cycles, the initial thermodynamically “irreversible” behavior is restored by “resting” the film. Solvation effects are critical determinants of film redox chemistry and dynamics: in addition to controlling the feasibility and stability of deposition, they control film ion transport rate.

Cobalt catalyzed autoxidation of monoterpenes in acetic acid and acetonitrile solutions

Abstract: Oxidation of limonene, α-pinene and β-pinene with dioxygen in acetic acid and acetonitrile solutions containing catalytic amounts of CoCl 2 has been studied. Limonene and α-pinene give both allylic oxidation and epoxidation products in a molar ratio of near 1/1, with chemoselectivities for corresponding products being higher in acetonitrile than those in acetic acid. On the other hand, oxidation of β-pinene leads essentially to allylic products, i.e. highly valuable pinocarveol, pinocarvone, myrtenol and myrtenal. In acetic acid, a combined selectivity for these products does not exceed 40% due to the concomitant substrate isomerization and acetic acid addition, while in acetonitrile, good selectivities of up to 85% at a 40–50% substrate conversion have been achieved.

Synthesis and Structural Characterization of Double Metal Cyanides of Iron and Zinc: Catalyst Precursors for the Copolymerization of Carbon Dioxide and Epoxides

Abstract: Several synthetic approaches for the preparation of double metal cyanide (DMC) derivatives of iron(II) and zinc(II) are described. These include (1) metathesis reactions of ZnCl2 or ZnI2 with KCpFe(CN)2CO in aqueous solution, (2) reactions of KCpFe(CN)2CO and its phosphine-substituted analogues with Zn(CH3CN)4(BF4)2 and subsequent displacement of acetonitrile at the zinc centers by the addition of a neutral (phosphine) or anionic (phenoxide) ligand, and (3) reactions of the protonated HCpFe(CN)2(phosphine) complexes with Zn(N(SiMe3)2)2, followed by the addition of phenols. All structures are based on a diamond-shaped planar arrangement of the Fe2(CN)4Zn2 core with various appended ligands at the metal sites. Although attempts to replace the iodide ligands in [CpFe(μ-CN)2PPh3ZnI(THF)]2 with acetate using silver acetate failed, two novel cationic mixed-metal cyanide salts based on the [CpFe(PPh3)(μ-CN)2Zn(NC5H5)]22+ framework were isolated from pyridine solution and their structures were defined by X-ray cr...

Selective recognition of halide anions by calix[4]pyrrole: A detailed thermodynamic study

Abstract: Several analytical techniques ( 1 H NMR, conductance measurements, and titration calorimetry) have been used to assess the interaction of calix[4]pyrrole and halide anions in dipolar aprotic solvents (acetonitrile and N,N-dimethylformamide). Solubility data for calix[4]pyrrole in various solvents at 298.15 K were determined. These data were used to calculate the standard Gibbs energies of solution. Taking acetonitrile as the reference solvent, the transfer Gibbs energies of this ligand to various solvents were calculated. Chemical shift changes (Δδ) of the pyrrole proton relative to the free ligand resulting from the addition of the anion salts to the ligand in CD 3 CN at 298 K follow the sequence F - > Cl - > Br > I - . Conductance measurements were performed (i) to establish the stoichiometry of the anionic calix[4]pyrrole complexes and (ii) to assess the range of concentration at which the free and complex anion salts are predominantly in their ionic forms in acetonitrile and N,N-dimethylformamide at 298.15 K. Titration calorimetry was used to determine the stability constant, K s , (hence the standard Gibbs energy) and the enthalpy. Combination of Gibbs energy and enthalpy data yields the entropy of complexation. A linear correlation is found between the log K, and the Δδ values. The results show that calix[4]pyrrole is able to recognize selectively the halide anions in these solvents The selectivity of the ligand for one anion relative to another is quantitatively evaluated through the calculation of the selectivity factor. It is shown that the ligand behavior is representative of flexible ligands in which calix[4]pyrrole compete successfully with the solvent for the anion to an extent that the higher selectivity of the ligand is for the smallest anion (fluoride). The thermodynamics of complexation is discussed in terms of the solvation properties of the reactants and the products in acetonitrile and N,N-dimethylformamide.

Fe(II) and Fe(III) mononuclear complexes with a pentadentate ligand built on the 1,3-diaminopropane unit. Structures and spectroscopic and electrochemical properties. Reaction with H2O2.

Abstract: Two new iron complexes, [L53FeIICl]PF6 (1·PF6) and [(L53H+)FeIIICl3]PF6 (2·PF6), were synthesized (L53 = N-methyl-N,N‘,N‘-tris(2-pyridylmethyl)propane-1,3-diamine), and their molecular structures were determined by X-ray crystallography. Their behavior in solution was studied by UV−vis spectroscopy and electrochemistry. Upon addition of a base to an acetonitrile solution of 2, the new unsymmetrical dinuclear complex [L53FeIIIOFeIIICl3]+ was detected. Treating 1 with hydrogen peroxide has allowed us to detect the low spin [L53FeIIIOOH]2+. Its spectroscopic properties (UV−vis, EPR and resonance Raman) are similar to those reported for related FeOOH complexes obtained with amine/pyridine ligands. Using stopped-flow absorption spectroscopy, the formation and degradation of [L53FeIIIOOH]2+ has been monitored, and a mechanism is proposed to reproduce the kinetic data.

Surface-enhanced Raman spectroscopy studies on the interaction of imidazole with a silver electrode in acetonitrile solution

Abstract: Surface-enhanced Raman scattering (SERS) spectrum of imidazole adsorbed at a silver electrode in nonaqueous acetonitrile solution has been investigated as a function of applied potential. The elect...

A general purpose reporter for cations: absorption, fluorescence and electrochemical sensing of zinc(II)

Abstract: A fluorescent sensor has been synthesized that comprises a pyrene moiety tethered to a [2,2′:6′,2″]-terpyridyl ligand via a diethynylated thiophene linker. This multicomponent supermolecule is highly emissive in solution but addition of Zn2+ cations results in a drastic decrease in the fluorescence yield. Complexation between Zn2+ cations and the vacant terpyridyl ligand has been confirmed by NMR spectroscopy and by electrospray mass spectrometry. The ES-MS results provide strong support for the formation of both 1 ∶ 1 and 1 ∶ 2 (metal ∶ ligand) complexes in solution. Complexation is further evidenced by a pronounced shift in the absorption spectrum, by the appearance of a long-wavelength band in the fluorescence spectrum and by cyclic voltammetry. On the basis of UV-visible spectrophotometric titrations, overall binding constants of logβ1 ≈ 5.1 and logβ2 ≈ 9.9 were derived for the 1 ∶ 1 and 1 ∶ 2 complexes, respectively, in acetonitrile solution. The possible application of this system as a generic chemical sensor for cationic species is mentioned.

Cyclodextrin phosphanes as first and second coordination sphere cavitands.

Abstract: The binding properties of two alpha-cyclodextrins, each containing two C(5)-linked "CH(2)PPh(2)" units, L 1 (A,D-substituted) and L 2 (A,C-substituted), have been investigated. Both ligands readily form transition-metal chelate complexes in which the metal centres are immobilised at the cavity entrance. Although diphosphane L 1 displays a marked tendency to behave only as a trans-spanning ligand, the ligand possesses a certain degree of flexibility, for example, allowing the stabilisation of a trigonal silver(I) complex in which the bite angle drops to 143 degrees. Another feature of L 1 concerns its ability to function as an hemilabile ligand. Together with four methoxy groups anchored onto the primary face, the two P(III) centres of L 1 form a circularly arranged P(2)O(4) 12-electron donor set able to complex an Ag(+) ion in a dynamic way, each of the four oxygen atoms coordinating successively to the silver ion. Furthermore, the particular structures of L 1 and L 2, characterised by the presence of P(III) units lying close to the cavity entrance, lead upon complexation to complexes whereby the first coordination sphere is partly entrapped in the cyclodextrin. Thus, when treated with metal chlorides, both ligands systematically produce complexes in which the Mbond;Cl unit is maintained inside the cyclodextrin through weak Cl.H-5 interactions. The chelate complex [Ag(L 1)]BF(4) reacts with acetonitrile in excess to afford a mixture of two equilibrating complexes, [Ag(acetonitrile)(L 1)]BF(4) and [Ag(acetonitrile)(2)(L 1)]BF(4), whose coordinated nitriles lie inside the cyclodextrin cavity. The inner-cavity ligands can be substituted by a benzonitrile molecule. The present study provides the first identification of an [Ag(acetonitrile)(2)(phosphane)(2)](+) ion. The unexpected stabilisation of this species probably rests on a cavity effect, the cyclodextrin walls favouring recombination of the complex after facile dissociation of the nitrile ligands.

Prediction of electrode potentials of some quinone derivatives in acetonitrile

Abstract: The electrode potentials of some quinone derivatives in acetonitrile have been calculated. The calculations are carried out at the SCF level with the inclusion of entropic and thermochemical corrections to yield free energies of redox reactions. The Polarizable Continuum Model (PCM) is used to describe the solvent. The model produces electrode potential values in a relatively good agreement with experimental data. The results show that the method is likely to be useful in the prediction of electrode potentials of organic molecules in different aprotic solvents.

Determination of Acidity Constants of 4-(2-Pyridylazo)resorcinol in Binary Acetonitrile + Water Mixtures

Abstract: The acid−base properties of 4-(2-pyridylazo)resorcinol in binary mixtures of acetonitrile + water are studied by a multiwavelength spectrophotometric method. To evaluate the pH−absorbance data, a resolution method based on the combination of soft- and hard-modeling is applied. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. The corresponding pKa values have been determined in all binary mixtures of acetonitrile + water. There is a linear relationship between acidity constants and the mole fraction of acetonitrile in the solvent mixtures. The effect of solvent properties on acid−base behavior is discussed.

Self-consistent acidity scales of neutral and cationic brønsted acids in acetonitrile and tetrahydrofuran

Abstract: The reaction o f 2,3,7,8,12,13,17,18-octaethylporphyrin (H 2OEP) with copper(II) triflate and copper(II) perchlorate in acetonitrile was studied using spectrophotom etry. The reaction product is the so-called sitting-atop complex where two pyrrolenine nitrogen atoms o f the porphyrin coordinate to the incoming metal ion and two protons on the pyrrole nitrogen atoms still remain. The composition o f the sitting-atop complex was determined by the mole ratio method, and it was found tha t the H2OEP molecule binds two copper(II) ions in the product. The mechanism o f the reaction was confirmed to be a series o f second-order reactions with the first and second step o f the reactions being the outer sphere complex form ation between the H2OEP molecule and copper(II) ion and the rate determining s itting-atop complex form ation reaction, respectively, based on the mole ratio m ethod. The reaction is relatively fast, and the second-order rate constants for the reaction of H 2OEP with copper(II) ion was determined to be к = (3 .2+0.3) x 106 M -1 s _ 1 ( Г = 25.0 °C) for the copper(II) triflate and к = (3 .0+0.2) x 106 M -1 s “ 1 ( T 25.0 °C) for the copper(II) perchlorate under the second-order conditions. The pATa values o f the m onoand diprotonated forms o f the conjugate acid o f several porphyrins including H 2O EP were determined by spectrophotom etric titration in acetonitrile. The higher reactivity o f H 2O EP toward copper(II) ion as com pared with other porphyrins such as 5,10,15,20-tetraphenylporphyrin was attributed to its higher basicity. © 2002 Elsevier Science B.V. All rights reserved.

Heteroconjugation-based capillary electrophoretic separation of phenolic compounds in acetonitrile and propylene carbonate.

Abstract: A mixture of methyl- and hydroxy-substituted phenols was separated by capillary electrophoresis in pure acetonitrile and propylene carbonate. Interactions between undissociated phenolic compounds and the background electrolytes were investigated. In the present work, benzyltriethylammonium chloride, tetrabutylammonium acetate, and two room temperature-molten salts, 1-butyl-3-methyl imidazolium trifluoroacetate and 1-butyl-3-methyl imidazolium heptafluorobutanoate, were used as background electrolytes. The formation of a negative complex between background electrolyte anion and neutral phenolic compound was observed and the formation constant calculated. The formation constants for anion-analyte complexes were approximately the same in propylene carbonate and in acetonitrile. In both solvents the formation constants were the highest for acetate and the lowest for trifluoroacetate. The separation of analytes was slightly influenced by the nature of the solvent: in acetonitrile the resolution between peaks was higher for 1,3-dihydroxyphenol and 1,3,5-trihydroxyphenol, in propylene carbonate 3-methylphenol and phenol were better separated. It was demonstrated that traces of water influence the mobilities of anion-phenol complexes in propylene carbonate.

Reactions of nitriles at semiconductor surfaces

Abstract: Motivated by the recent interest in using nitriles for organic modification of semiconductor surfaces, we have used density functional theory and cluster models to study the reactions of nitriles on semiconductor surfaces. In particular, we have calculated the energetics for the surface reactions of acetonitrile, 2-propenenitrile, and 3-butenenitrile on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces. For acetonitrile, our calculations predict that although it forms multiple products on the Si(100)-2 x 1 surface, the molecule does not react on the Ge(100)-2 × 1 surface. For 2-propenenitrile, the calculations show that whereas the C=C [2 + 2] cycloaddition product is thermodynamically most stable, formation of the hetero-[4 + 2] product is kinetically favored. We also have investigated the possibility of surface-catalyzed isomerization of 3-butenenitrile to 2-butenenitrile on the Si(100)-2 x 1 and Ge(100)-2 x 1 surface. We find that there are multiple surface-catalyzed isomerization pathways on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces. However, our calculated kinetics predicts that the amount of 2-butenenitrile formed by surface-catalyzed isomerization is small.