Showing papers on "Acetonitrile published in 2014"

Organic synthetic transformations using organic dyes as photoredox catalysts

Abstract: The oxidizing ability of organic dyes is enhanced significantly by photoexcitation. Radical cations of weak electron donors can be produced by electron transfer from the donors to the excited states of organic dyes. The radical cations thus produced undergo bond formation reactions with various nucleophiles. For example, the direct oxygenation of benzene to phenol was made possible under visible-light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in an oxygen-saturated acetonitrile solution of benzene and water via electron transfer from benzene to the triplet excited state of DDQ. 3-Cyano-1-methylquinolinium ion (QuCN+) can also act as an efficient photocatalyst for the selective oxygenation of benzene to phenol using oxygen and water under homogeneous and ambient conditions. Alkoxybenzenes were also obtained when water was replaced by alcohol under otherwise identical experimental conditions. QuCN+ can also be an effective photocatalyst for the fluorination of benzene with O2 and fluoride anion. Photocatalytic selective oxygenation of aromatic compounds was achieved using an electron donor–acceptor-linked dyad, 9-mesityl-10-methylacridinium ion (Acr+–Mes), as a photocatalyst and O2 as the oxidant under visible-light irradiation. The electron-transfer state of Acr+–Mes produced upon photoexcitation can oxidize and reduce substrates and dioxygen, respectively, leading to the selective oxygenation and halogenation of substrates. Acr+–Mes has been utilized as an efficient organic photoredox catalyst for many other synthetic transformations.

Switching the reaction course of electrochemical CO₂ reduction with ionic liquids.

Abstract: The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) offers new ways to modulate the electrochemical reduction of carbon dioxide. [emim][Tf2N], when present as the supporting electrolyte in acetonitrile, decreases the reduction overpotential at a Pb electrode by 0.18 V as compared to tetraethylammonium perchlorate as the supporting electrolyte. More interestingly, the ionic liquid shifts the reaction course during the electrochemical reduction of carbon dioxide by promoting the formation of carbon monoxide instead of oxalate anion. With increasing concentration of [emim][Tf2N], a carboxylate species with reduced CO2 covalently bonded to the imidazolium ring is formed along with carbon monoxide. The results highlight the catalytic effects of the medium in modulating the CO2 reduction products.

Electrochemical Reduction of Brønsted Acids by Glassy Carbon in Acetonitrile—Implications for Electrocatalytic Hydrogen Evolution

Abstract: Molecular catalysts for electrochemically driven hydrogen evolution are often studied in acetonitrile with glassy carbon working electrodes and Bronsted acids. Surprisingly, little information is available regarding the potentials at which acids are directly reduced on glassy carbon. This work examines acid electroreduction in acetonitrile on glassy carbon electrodes by cyclic voltammetry. Reduction potentials, spanning a range exceeding 2 V, were found for 20 acids. The addition of 100 mM water was not found to shift the reduction potential of any acid studied, although current enhancement was observed for some acids. The data reported provides a guide for selecting acids to use in electrocatalysis experiments such that direct electrode reduction is avoided.

Investigating the role of solvent-solute interaction in crystal nucleation of salicylic acid from organic solvents.

Abstract: In previous work, it has been shown that the crystal nucleation of salicylic acid (SA) in different solvents becomes increasingly more difficult in the order: chloroform, ethyl acetate acetonitrile, acetone, methanol, and acetic acid. In the present work, vibration spectroscopy, calorimetric measurements, and density functional theory (DFT) calculations are used to reveal the underlying molecular mechanisms. Raman and infrared spectra suggest that SA exists predominately as dimers in chloroform, but in the other five solvents there is no clear evidence of dimerization. In all solvents, the shift in the SA carbonyl peak reflecting the strength in the solvent–solute interaction is quite well correlated to the nucleation ranking. This shift is corroborated by DFT calculated energies of binding one solvent molecule to the carboxyl group of SA. An even better correlation of the influence of the solvent on the nucleation is provided by DFT calculated energy of binding the complete first solvation shell to the S...

A New Look at the Stability of Dimethyl Sulfoxide and Acetonitrile in Li-O2 Batteries

Abstract: Dimethyl sulfoxide (DMSO) and acetonitrile (MeCN) have recently been highlighted as promising electrolyte solvents for Li-O2 batteries. Possible reactions between these two solvents and Li2O2 are here discussed using X-ray photoelectron spectroscopy to analyze surface of the Li2O2 powder after direct contact with the solvents for different times of exposure. The results indicated that Li2O2 decomposes DMSO solvents, whereas no indication of degradation of MeCN by Li2O2 was observed.

The Use of a Vanadium Species As a Catalyst in Photoinduced Water Oxidation

Abstract: The first water oxidation catalyst containing only vanadium atoms as metal centers is reported. The compound is the mixed-valence [(VIV5VV1)O7(OCH3)12]− species, 1. Photoinduced water oxidation catalyzed by 1, in the presence of Ru(bpy)32+ (bpy = 2,2′-bipyridine) and Na2S2O8, in acetonitrile/aqueous phosphate buffer takes place with a quantum yield of 0.20. A hole scavenging reaction between the photochemically generated Ru(bpy)33+ and 1 occurs with a bimolecular rate constant of 2.5 × 108 M–1 s–1. The time-resolved formation of the oxidized molecular catalyst 1+ in bimolecular reactions is also evidenced for the first time by transient absorption spectroscopy. This result opens the way to the use of less expensive vanadium clusters as water oxidation catalysts in artificial photosynthesis schemes.

Solvation dynamics in a prototypical ionic liquid + dipolar aprotic liquid mixture: 1-butyl-3-methylimidazolium tetrafluoroborate + acetonitrile.

Abstract: Solvation energies, rotation times, and 100 fs to 20 ns solvation response functions of the solute coumarin 153 (C153) in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) + acetonitrile (CH3CN) at room temperature (20.5 °C) are reported. Available density, shear viscosity, and electrical conductivity data at 25 °C are also collected and parametrized, and new data on refractive indices and component diffusion coefficients presented. Solvation free energies and reorganization energies associated with the S0 ↔ S1 transition of C153 are slightly (≤15%) larger in neat [Im41][BF4] than in CH3CN. No clear evidence for preferential solvation of C153 in these mixtures is found. Composition-dependent diffusion coefficients (D) of Im41+ and CH3CN as well as C153 rotation times (τ) are approximately related to solution viscosity (η) as D, τ ∝ ηp with values of p = −0.88, −0.77, and +0.90, respectively. Spectral/solvation response functions (Sν(t)) are bimodal at all compositions, consisting of ...

Are all polar molecules hydrophilic? Hydration numbers of nitro compounds and nitriles in aqueous solution.

Abstract: The hydration numbers of typical aprotic polar substances bearing dipole moments larger than 3 D, such as nitro compounds and nitriles, were precisely determined in aqueous solution using high frequency dielectric relaxation techniques up to a frequency of 50 GHz at 25 °C. The hydration number is one of the most quantitative parameters for determining the hydrophilicity or hydrophobicity of a compound. The hydration numbers of various nitriles, such as acetonitrile, propionitrile and n-butyronitrile bearing cyano groups, were determined to be ca. 0, irrespective of the species of molecule. Moreover, the hydration numbers of various nitro compounds, such as nitromethane, nitroethane and 1-nitropropane, were also evaluated to be ca. 0. These findings clearly reveal that neither cyano nor nitro functional groups form strong hydrogen bonds to water molecules. Consequently, neither nitro compounds nor nitriles are hydrophilic, despite their high polarities due to their large dipole moments. Rather, these compounds are “hydroneutral,” with hydrophilicities intermediate between those of hydrophilic and hydrophobic molecules. The molecular motions of the examined highly polar molecules in aqueous solution were well described with single Debye-type rotational relaxation modes without strong interactions between the solute and water molecules, but with relatively strong interactions between the polar solute molecules due to the Kirkwood factor being less than unity. This small Kirkwood factor indicated that both nitro and cyano groups have a tendency to align in an anti-parallel intermolecular configuration due to their strong dipole–dipole interactions as a result of their dipole moments greater than 3 D.

On the Oxidation of the Three‐Dimensional Aromatics [B12X12]2− (X=F, Cl, Br, I)

Abstract: The perhalogenated closo-dodecaborate dianions [B12 X12 ](2-) (X=H, F, Cl, Br, I) are three-dimensional counterparts to the two-dimensional aromatics C6 X6 (X=H, F, Cl, Br, I). Whereas oxidation of the parent compounds [B12 H12 ](2-) and benzene does not lead to isolable radicals, the perhalogenated analogues can be oxidized by chemical or electrochemical methods to give stable radicals. The chemical oxidation of the closo-dodecaborate dianions [B12 X12 ](2-) with the strong oxidizer AsF5 in liquid sulfur dioxide (lSO2 ) yielded the corresponding radical anions [B12 X12 ](⋅-) (X=F, Cl, Br). The presence of radical ions was proven by EPR and UV/Vis spectroscopy and supported by quantum chemical calculations. Use of an excess amount of the oxidizing agent allowed the synthesis of the neutral perhalogenated hypercloso-boranes B12 X12 (X=Cl, Br). These compounds were characterized by single-crystal X-ray diffraction of dark blue B12 Cl12 and [Na(SO2 )6 ][B12 Br12 ]⋅B12 Br12 . Sublimation of the crude reaction products that contained B12 X12 (X=Cl, Br) resulted in pure dark blue B12 Cl12 or decomposition to red B9 Br9 , respectively. The energetics of the oxidation processes in the gas phase were calculated by DFT methods at the PBE0/def2-TZVPP level of theory. They revealed the trend of increasing ionization potentials of the [B12 X12 ](2-) dianions by going from fluorine to bromine as halogen substituent. The oxidation of all [B12 X12 ](2-) dianions was also studied in the gas phase by mass spectrometry in an ion trap. The electrochemical oxidation of the closo-dodecaborate dianions [B12 X12 ](2-) (X=F, Cl, Br, I) by cyclic and Osteryoung square-wave voltammetry in liquid sulfur dioxide or acetonitrile showed very good agreement with quantum chemical calculations in the gas phase. For [B12 X12 ](2-) (X=F, Cl, Br) the first and second oxidation processes are detected. Whereas the first process is quasi-reversible (with oxidation potentials in the range between +1.68 and +2.29 V (lSO2 , versus ferrocene/ferrocenium (Fc(0/+) ))), the second process is irreversible (with oxidation potentials ranging from +2.63 to +2.71 V (lSO2 , versus Fc(0/+) )). [B12 I12 ](2-) showed a complex oxidation behavior in cyclic voltammetry experiments, presumably owing to decomposition of the cluster anion under release of iodide, which also explains the failure to isolate the respective radical by chemical oxidation.

Adsorptive characterization of the ZIF-68 metal-organic framework: a complex structure with amphiphilic properties.

Abstract: In this experimental study, the adsorption behavior of the ZIF-68 heterolinked zeolitic imidazolate framework has been explored. Vapor phase adsorption isotherms of linear C1–C6 alcohols, C6 alkane isomers, aromatics (benzene, toluene, xylene isomers, 1,3,5-trimethylbenzene, and 1,3,5-triisopropylbenzene), and polar adsorbates (water, acetonitrile, and acetone) are reported and discussed. The complex pore structure of ZIF-68, with two one-dimensional channels, each with a different polarity, displays an overall hydrophobic character. Its two-pore system results in S-shaped isotherms for small polar adsorbates (small alcohols, acetone, and acetonitrile), while longer alcohols and nonpolar molecules, such as aromatics and C6 alkane isomers, lead to type I adsorption isotherms. Bulky molecules, with a kinetic diameter significantly larger than the pore windows, are adsorbed in large amounts, which gave reason to think that this ZIF-68 material has a certain degree of framework flexibility to enlarge the free...

Synergic Exfoliation of Graphene with Organic Molecules and Inorganic Ions for the Electrochemical Production of Flexible Electrodes.

Abstract: A facile and efficient method based on electrochemistry for the production of graphene-based materials for electronics is demonstrated. Uncharged acetonitrile molecules are intercalated in graphite by electrochemical treatment, owing to the synergic action of perchlorate ions dissolved in acetonitrile. Then, acetonitrile molecules are decomposed with microwave irradiation, which causes gas production and rapid graphite exfoliation, with an increase in the graphite volume of up to 600 %. Upon further processing and purification, highly dispersible nanosheets are obtained that can be processed into thin layers by roll-to-roll transfer or into thicker electrodes with excellent capacitance stability upon extensive charging/discharging cycles. The good exfoliation yield (>50 % of monolayers), minimal oxidation damage and good electrochemical stability of the nanosheets obtained were confirmed by scanning force and electron microscopy, as well as Raman spectroscopy and galvanostatic analyses.

Electrodeposition from Acidic Solutions of Nickel Bis(benzenedithiolate) Produces a Hydrogen-Evolving Ni–S Film on Glassy Carbon

Abstract: Films electrodeposited onto glassy carbon electrodes from acidic acetonitrile solutions of [Bu4N][Ni(bdt)2] (bdt = 1,2-benzenedithiolate) are active toward electrocatalytic hydrogen production at potentials 0.2–0.4 V positive of untreated electrodes. This activity is preserved when the electrode is rinsed and transferred to a fresh acid solution. X-ray photoelectron spectra indicate that the deposited material contains Ni and S, and time-of-flight secondary ion mass spectrometry shows that electrodeposition decomposes the Ni(bdt)2 assembly. Correlations between voltammetric and spectroscopic results indicate that the deposited material is active, i.e., that catalysis is heterogeneous rather than homogeneous. Control experiments establish that obtaining the observed catalytic response requires both Ni and the 1,2-benzenedithiolate ligand to be present during deposition.

Coordination number of Li+ in nonaqueous electrolyte solutions determined by molecular rotational measurements.

Abstract: The coordination number of Li+ in acetonitrile solutions was determined by directly measuring the rotational times of solvent molecules bound and unbound to it. The CN stretch of the Li+ bound and unbound acetonitrile molecules in the same solution has distinct vibrational frequencies (2276 cm–1 vs 2254 cm–1). The frequency difference allows the rotation of each type of acetonitrile molecule to be determined by monitoring the anisotropy decay of each CN stretch vibrational excitation signal. Regardless of the nature of anions and concentrations, the Li+ coordination number was found to be 4–6 in the LiBF4 (0.2–2 M) and LiPF6 (1–2 M) acetonitrile solutions. However, the dissociation constants of the salt are dependent on the nature of anions. In 1 M LiBF4 solution, 53% of the salt was found to dissociate into Li+, which is bound by 4–6 solvent molecules. In 1 M LiPF6 solution, 72% of the salt dissociates. 2D IR experiments show that the binding between Li+ and acetonitrile is very strong. The lifetime of t...

Submerged liquid plasma – low energy synthesis of nitrogen-doped graphene for electrochemical applications

Abstract: In this study, micro-plasma discharge is produced by applying a high electric potential between graphite and Pt electrodes in acetonitrile solvent. The electrons generated in the micro-plasma discharge collide with acetonitrile and produce ˙H and ˙CH2CN radicals. The radicalized graphene layer exfoliated from the graphite electrode reacts with nascent hydrogen (˙H) and acetonitrile (˙CH2CN) radicals and partially restores its aromaticity and conjugation. Raman spectra of the product confirm the synthesis of nitrogen-functionalized graphene (N-FG), which has a marginal increase in disorderness compared to that of pure graphite and remarkable dispersibility in both hydrophilic and hydrophobic solvents. The excellent fluorescence properties of N-FG confirm the presence of fluorophores such as –NH and –NC– at the radicalized graphene sites, as supported by ultraviolet-visible spectroscopy and X-ray photoelectron spectroscopy studies. The functional groups present in N-FG lead to excellent electrochemical performance, with distinct redox peaks in cyclic voltammetry and a high specific capacitance of 291 F g−1 at a scan rate of 5 mV s−1. N-FG exhibits excellent cycling stability, with a marginal reduction of specific capacitance (<10% reduction) at the end of 1000 cycles.

Cobalt-bisglyoximato diphenyl complex as a precatalyst for electrocatalytic H2 evolution

Abstract: Electrochemical investigation of the title CoII compound in acetonitrile with a strong acid (HClO4) showed no sign of proton catalysis at the CoII/CoI wave, but instead revealed the formation of Co based nanoparticles at the surface of the carbon electrode. Catalytic proton reduction of the resulting nanometer sized cobalt particles at pH 7 was found to occur efficiently. Partial coverage of the carbon substrate by the particles leads to an apparent exchange current density as high as those obtained at a pure cobalt electrode or cobalt films.

Electrocatalytic Proton Reduction by a Dicobalt Tetrakis-Schiff Base Macrocycle in Nonaqueous Electrolyte

Abstract: A series of dicobalt complexes, Co2L2+ and Co2LAc+, where L is a N6O2 coordinating bis(phenolate) tetrakis-Schiff base ligand, have been synthesized and characterized via electrochemical and spectroscopic techniques. [Co2LAc](ClO4) crystallizes in the monoclinic space group P21/n, and the structure reveals a highly distorted octahedral geometry for the CoII ions, which are bridged by an acetate with a Co–Co distance of 3.2 A. Cyclic voltammetry (CV) of Co2L2+ and Co2LAc+ in anhydrous acetonitrile reveals large anodic/cathodic peak splitting for the CoII/III redox transitions and a multielectron wave for the CoII/I reductions. The CVs for Co2L2+ and Co2LAc+ were also compared to those of Zn2LAc+ and H4L2+ to identify the ligand-center oxidations and reductions. Addition of trifluoroacetic acid (TFA) or acetic acid (AcOH) to the electrolyte solutions of Co2L2+ results in an irreversible reduction wave that is consistent with electrocatalytic H+ reduction. The catalytic rate law shows a first order dependenc...