Showing papers in "Israel Journal of Chemistry in 1999"

PRISM: Piecewise Reusable Implementation of Solution Mapping. An Economical Strategy for Chemical Kinetics

Abstract: In a chemical kinetics calculation, a solution-mapping procedure is applied to parametrize the solution of the initial-value ordinary differential equation system as a set of algebraic polynomial equations. To increase the accuracy, the parametrization is done piecewise, dividing the multidimensional chemical composition space into hypercubes and constructing polynomials for each hypercube. A differential equation solver is used to provide the solution at selected points throughout a hypercube, and from these solutions the polynomial coefficients are determined. Factorial design methods are used to reduce the required number of computed points. The polynomial coefficients for each hypercube are stored in a data structure for subsequent reuse, since over the duration of a flame simulation it is likely that a particular set of concentrations and temperature will occur repeatedly at different times and positions. The method is applied to H2–air combustion using an 8-species reaction set. After N2 is added as an inert species and enthalpy is considered, this results in a 10-dimensional chemical composition space. To add the capability of using a variable time-step, time-step is added as an additional dimension, making an 11-dimensional space. Reactive fluid dynamical simulations of a 1-D laminar premixed flame and a 2-D turbulent non-premixed jet are performed. The results are compared to identical control runs which use an ordinary differential equation solver to calculate the chemical kinetic rate equations. The resulting accuracy is very good, and a factor of 10 increase in computational efficiency is attained.

Proton Solvation and Proton Mobility

Abstract: Experimental evidence for proton solvation and proton mobility is analyzed and the results are compared with recent simulations. Three factors contribute to differences in proton solvation energies: hydrogen-bond cleavage, changes in hydrogen-bond lengths, and proton derealization. These factors are estimated from experimental attributes. In dilute acidic aqueous solutions H3O+ is more stable than H5O2+ by about 0.6 kcal/mol. This estimate, together with the activation energy for proton mobility, supports the 121 mechanism for proton mobility in which a protonated water monomer is transformed, by second-shell hydrogen-bond cleavage, to a protonated dimer and back to another protonated monomer.

Oxidation of Dimethyl Ether and its Interaction with Nitrogen Oxides

Abstract: The oxidation of dimethyl ether (DME) under flow reactor conditions has been studied experimentally and in terms of a detailed chemical kinetic model. The experiments were performed at atmospheric pressure in the temperature range 600–1500 K and at different air/fuel ratios. Of particular interest was the interaction of DME with nitrogen oxides. The results show that the oxidation of DME occurs readily at temperatures above 1000 K, largely independent of the stoichiometry. Addition of NO under stoichiometric and fuel-rich conditions does not affect the oxidation chemistry for DME, but above 1100 K a minor amount of the NO is reduced to HCN and N2 in reburn-type reactions. Addition of NO or NO2 under oxidizing conditions significantly enhances the oxidation rate of DME and shifts the temperature for onset of oxidation to lower values, a phenomenon similar to that of NOx-sensitized oxidation of hydrocarbons. The proposed chemical kinetic model provides a good description of DME oxidation in the absence of nitrogen oxides. Under the conditions of the present study, the conversion of DME proceeds mainly through the high-temperature mechanism, with little importance of the intermediate peroxy species. In the presence of NO or NO2, the reaction CH3 + NO2 ⇌ CH3O + NO, followed by dissociation of CH3O, readily provides H atoms and thereby promotes the oxidation. At lower temperatures the mechanism involves CH3OCH2O2 and CH3O2 radicals. While the effect of NOx generally is described satisfactorily by the model, deviations at lower temperatures may indicate inadequacies in the reaction subset for these peroxy species.

Excited-State Proton Transfer in Bifunctional Compounds

Abstract: When the acidic and basic groups of an amphoteric compound become stronger on excitation, excited-state proton transfers (ESPT) are driven by the photoinduced synergy between the two functions. This is exemplified here by 7- and 8-hydroxyquinolines (7- and 8-HQ), whatever the initially excited form. For instance, the -OH group of the 7-hydroxyquinolinium form undergoes photoinduced deprotonation even in 8M HClO4! This reveals its outstanding photoacidity, confirmed by the value of the deprotonation rate constant k1 at infinite dilution, 5.5 × 1010 s−1. The decrease of k1 on increasing the ionic strength pointed out the key number of 4 water molecules as proton acceptors. In neutral medium, a very efficient photoinduced tautomerization is observed. For 7-HQ in water, a mechanism consisting of three competitive paths was established: A proton translocation via a bridge of water molecules within less than 10 ps for the cis-isomer, and two stepwise competitive reactions for the trans-isomer. Concerning the weakly fluorescent 8-HQ, the photoinduced tautomerization occurs even in organic media. In most solvents, an intrinsic ESPT takes place within intramolecularly H-bonded molecules, but preferential solvation by residual water of the solvent may lead to open structures where ESPT is impaired. In alkanes, a biprotonic ESPT is expected to occur within very stable dimers (Kdim= 7.0 × 107). In all cases, the coupling between proton transfers and an intramolecular electron transfer leads to the ketonic structure of the tautomer. This might be a general feature in phototautomerization reactions.

Quantum Properties of the Excess Proton in Liquid Water

Abstract: The classical and quantum equilibrium properties of an excess proton in bulk phase water are examined computationally with a special emphasis on the influence of an explicit quantum dynamical treatment of the nuclei on the calculated observables. The potential model used, our recently developed multistate empirical valence bond (MS-EVB) approach is described. The MS-EVB model takes into account the interaction of an exchange charge distribution of the charge-transfer complex with the polar solvent, which qualitatively changes the nature of the solvated complex. The impact and importance of the exchange term on the stability of the solvated H5O2+ (Zundel) cation relative to the H9O4+ (Eigen) cation in the liquid phase is demonstrated. Classical and quantum path-integral molecular dynamics (PIMD) simulations of an excess proton in bulk phase water reveal that quantization of the nuclear degrees of freedom results in an increased stabilization of the solvated Zundel cation relative to the Eigen cation, and that species intermediate between the two are also probable. Quantum effects lead to a significant broadening of the probability distributions used to characterize the two species, and a definite differentiation and sharp characterization of the species connected to the excess proton in liquid water is found to be difficult.

The Sulfinatodehalogenation Reaction

Abstract: Sulfinatodehalogenation is a simple and efficient reaction for the synthesis of polyfluoroalkane-sulfinates and sulfonates. The reaction can also be applied for the polyfluoroalkylation of organic compounds. A brief account is given of its development and scope, reaction mechanism, and applications in organic synthesis.

An Extended Empirical Valence Bond Model for Describing Proton Mobility in Water

Abstract: In order to study the microscopic nature of the hydrated proton and its transport mechanism, we have introduced a multistate empirical valence bond model, fitted to ab initio results. This model was applied to the study, at low computational cost, of the structure and dynamics of an excess proton in liquid water. The quantum character of the proton is included by means of an effective parametrization of the model using preliminary path-integral calculations. The mechanism of proton transfer is interpreted as the translocation of a special O–H+–O bond along the hydrogen network, i.e., a series of reactions of the form H5O2+ + H2O ⇌ H2O + H5O2+, rather than H3O+ + H2O H2O + H3O+ as usually described. The translocation of the special bond can be described as a diffusion process with a jump time of 1 ps. A time-dependent correlation function analysis of the special pair relaxation yields two timescales, 0.3 and 3.5 ps. The first time is attributed to the interconversion between a delocalized (H5O2+-like) and a localized (H9O4+-like) form of the hydrated proton within a given special pair. The second one is the relaxation time of the special pair, including return trajectories. The computed diffusion constant, as well as the isotopic substitution effect, are in good agreement with experiment. The hydration structure around the excess proton is discussed in terms of various radial distribution functions around the water molecules involved in the special pair and those in the first solvation shell. The hydrogen-bond-dynamics which accompanies the translocation process is studied statistically. The “Moses mechanism” proposed by Noam Agmon for proton mobility in water is partially verified by our simulations.

Photoinduced Double Proton Transfer: Inter‐ and Intramolecular Cases

Abstract: Photoinduced two-proton tautomerization has been studied in two types of chromophores: (i) alcohol complexes of azaaromatic molecules possessing both proton donor and acceptor groups; (ii) constitutional isomers of porphyrin. The reaction path for the intermolecular process may involve solvent reorientation around the excited chromophore. In this case, rapid internal conversion is activated, efficiently competing with proton transfer. Another possibility arises if cyclic, doubly-hydrogen-bonded complexes exist already in the ground state. Excitation of such species leads to a fast tautomerization, which is not stopped even at low temperatures. Excited-state double proton transfer also has been observed in cyclic dimers in the crystal phase. In porphyrin isomers, the rate of the process is a function of the strength of the intramolecular hydrogen bond. Dependence of the phototautomerization rate on temperature and on the nature of the surrounding matrix has also been studied.

Acid Ionization of HBr in a Small Water Cluster

Abstract: The acid ionization by which HBr transfers a proton to a water molecule is studied for HBr in small water clusters as a first stage in a study of the corresponding Arctic atmospheric process. Electronic structure calculations, involving relatively large basis sets and including electron correlation effects, indicate that HBr is acid-ionized in a cluster of four water molecules. The most stable structure in the general HBr(H2O)4 system is a diamond-like (trigonal bipyramidal) structure in which the bromide and hydronium ion species are bridged by three water molecules, and infrared spectral and ionization potential characteristics potentially useful for experimental study are provided. This structure is calculated to remain the thermodynamically most stable one up to about 180 K, where HBr + (H2O)4 becomes more stable. Aspects of the formation of the acid-ionized cluster from both HBr + (H2O)4 and H2O + HBr(H2O)3 are discussed. Possible implications for HBr acid ionization at the surface of ice, believed to be important for Arctic atmosphere ozone depletion mechanisms, are briefly discussed.

The Deuterium Isotope Effect as a Tool to Investigate Enzyme Catalysis: Proton‐Transfer Control Mechanisms in Cytochrome c Oxidase

Abstract: Cytochrome c oxidase is a membrane-bound redox-driven proton pump. The coupling of the exergonic electron-transfer reactions from cytochrome c to oxygen to proton translocation across the membrane requires control of internal electron- and proton-transfer reactions. In this work, we focus on the kinetics of electron and proton transfer during those reaction steps that are coupled to proton pumping in cytochrome c oxidase. The results show that during the first pumping step (peroxy oxoferryl transition), proton transfer regulates intramolecular electron transfer. The proton transfer takes place in two steps: (1) Internal proton transfer from a protonatable group, proposed to be Glu(I-286), in the so-called D-pathway, to an oxygen intermediate at the binuclear center (τ ≅ 100 μs); (2) Rapid re-protonation of Glu(I-286) from the bulk solution (τ < 100 μs). Only after proton uptake from solution the last (fourth) electron is transferred “one step closer” towards the binuclear center (from CuA to heme a). During the second proton-pumping step (oxoferryl oxidized), this electron is transferred to the binuclear center, linked to the uptake of a proton through the D-pathway. The electron-transfer rate displays a kinetic-isotope effect (kH/kD) of 6 ± 1 (in a pH range in which the pH dependence of the rate is small), which indicates that the electron transfer is rate-limited by the proton transfer. The entry into the D-pathway (around Asp(I-132)) is composed of a cluster of negatively-charged amino acid residues together with a number of histidines, forming a so-called proton-collecting antenna designed to allow rapid protonation of groups within the proton-transfer pathway.

Evidence for 1La, 1Lb Dual State Emission in 1‐naphthol and 1‐methoxynaphthalene Fluorescence in Liquid Solutions

Abstract: The steady-state fluorescence spectra of 1-methoxynaphthalene and 1-naphthol were measured in pure organic solvents and in binary mixtures of water and several organic solvents. The 1-isomers exhibited a much larger fluorescence Stokes-shift than the corresponding 2-isomers. The emission spectra of 1-methoxynaphthalene and 1-naphthol in pure nonpolar organic solvents exhibited roughly the same structured spectral features, while the fluorescence spectra in water and formamide exhibited much broader red-shifted and less structured spectral features. In accord with previous observations, we attribute these spectral changes to two emitting states, 1Lb and 1La, whose relative fluorescence intensities are sensitive to solvent polarity. Our analysis of the fluorescence spectra of 1-naphthol and 1-methoxynaphthalene by Pekarian functions have demonstrated that the 1La state becomes the dominant emitting state in polar solvents. In addition, the 1La state was found to be further stabilized in hydrogen-bond-donating solvents. In contrast to previous suggestions, the onset of the excited-state proton transfer reaction from 1-naphthol occurred at higher solvent polarities than those required for the establishment of a dominantly 1La emitting state.

Intermolecular Excited-State Proton Transfer in Clusters of 1-Naphthol with Water and with Ammonia

Abstract: The excited-state proton transfer reactions of 1-naphthol(H2O)n and 1-naphthol(NH3)n clusters are compared. Both exisiting and new data suggest that the two systems may react in strikingly different fashions, roughly corresponding to non-adiabatic and adiabatic limiting cases, respectively. In water clusters, the reaction is kinetically limited by solvent motion, and there is a strong dynamic interaction involving the water and two lowest electronic excited states, which thereby invert. In ammonia clusters, the excited states appear to be premixed by the the stronger base, and the reaction may proceed in a rapid manner, resembling intracluster vibrational relaxation, with relatively little solvent reorganization.

Quantum–Classical Simulation of Proton Migration in Water

Abstract: Proton transport in water is treated within a mixed quantum–classical molecular dynamics scheme. Therein the migration of a positive charge is treated as a two-step process: (i) the displacement of a hydronium ion, followed by (ii) proton transfer between a hydronium ion and an adjacent water molecule. Both the hydronium ion and the water molecules are treated as flexible molecules. In the H5O2+-complex, the proton no longer belongs to a single water molecule, but is delocalized between the two oxygen atoms. The proton transfer is treated quantum mechanically as a function of a transfer coordinate within a single complex. The quantum description is changed to another pair of water molecules after strong proton localization to one of the water molecules in the complex and according to distance criteria with respect to adjacent water molecules. It is demonstrated that the proposed formalism is well suited for an effective simulation of the proton migration in water. In particular, the lifetimes of Eigen complexes and the proton diffusion coefficient (as obtained from simulations) are in very good agreement with corresponding experimental data.

Hydrogen-Bond Dynamics and Solvation of Electronically Excited States as Determined by Femtosecond Vibrational Spectroscopy

Abstract: The ultrafast dynamics of site-specific hydrogen bonds between an organic chromophore serving as hydrogen acceptor and a hydrogen-donating species in solution is studied by femtosecond vibrational spectroscopy. This new method gives specific insight into microscopic structural changes of hydrogen bonds initiated by electronic excitation of the chromophore. We study H-bonded complexes of coumarin 102 with the solvent CHCl3, and with phenol. Upon electronic excitation, the intermolecular hydrogen bond between proton donor and acceptor is cleaved within 200 fs, followed by a slower reorientation dynamics of the donor molecules extending into the picosecond regime. For CHCl3, this slower rearrangement is interpreted in terms of polar solvation. In complexes consisting of a coumarin molecule and two phenol moieties, the slower dynamics is related to geometry changes of the phenol–phenol hydrogen bond. The consequences of those results for a microscopic picture of polar solvation are discussed.

Temperature Dependence of Excited-State Proton Transfer and Geminate Recombination Processes in Water and in Glycerol-Doped Ice

Abstract: The reversible proton dissociation and geminate recombination of photoacids was studied as a function of temperature in neat water, binary water mixture containing 0.6 mol% glycerol, and doped ice containing 0.6 mol% glycerol. The deuterium isotope effect on both condensed phases was also studied. 8-hydroxypyrene-1,3,6 trisulfonate trisodium salt was used as the electronically-excited-state proton emitter. The experimental data are analyzed by the Debye–Smoluchowski equation solved numerically with boundary conditions to account for the reversibility of the reaction. We propose a qualitative model to describe the unusual temperature dependence of the proton transfer rate in the liquid phase. We also propose a model for proton transfer in solid ice based on L-defects transport as proton acceptors. While in the liquid phase at t > 10°C the proton dissociation rate constant is almost temperature independent, in glycerol-doped ice we find a large temperature dependence.

Electrophilic Fluorination of Aromatics with Selectfluor™ and Trifluoromethanesulfonic Acid1

Abstract: 1-(chloromethyl)-4-fluoro-1,4-diazabicyclo[2.2.2]octane bis (tetrafluoroborate) [Selectfluor™ F-TEDA-BF4 (TEDA = triethylenediamine)] in the presence of trifluoromethanesulfonic acid has been found to be a very effective reagent system for the direct electrophilic fluorination of a wide variety of aromatic compounds under mild reaction conditions to the corresponding fluoroaromatics in good to excellent yields.

Hydrogen-Bond Rearrangement and Intermolecular Proton Transfer in Protonated Methanol Clusters

Abstract: Vibrational predissociation spectra (VPS) of hydrogen-bonded and non-hydrogen-bonded (free) OH stretches within the frequency range of 2700–3800 cm−1 are closely analyzed to determine the isomeric structures and proton location in protonated methanol clusters, H+(CH3OH)n. The present report emphasizes protonated methanol tetramers and pentamers, which are predicted by ab initio calculations to exist in linear and cyclic forms. Only one isomer with linear structure was identified in the VPS of protonated methanol tetramers in a supersonic expansion. Both linear and cyclic configurations were found for protonated methanol pentamers, and an isomeric transition between these two structures due to hydrogen-bond rearrangement was observed at a cluster temperature of 190 K. Ab initio calculations indicated that the excess proton in these clusters can be either localized on one methanol unit, as in cyclic H+(CH3OH)4 and linear H+(CH3OH)5 isomers, or delocalized between two methanol molecules, as in linear H+(CH3OH)4 and cyclic H+(CH3OH)5 isomers. Such unique proton delocalization behavior is revealed in the VPS of cyclic H+(CH3OH)5, which is recognized as a symmetric five-membered pentamer by its characteristic free-OH stretching absorption at 3647 cm−1 and hydrogen-bonded OH stretches at 3448 and 3461 cm−1. Using protonated methanol pentamers as a model system, a proton transfer mechanism is suggested to involve intracluster proton transfer mediated by a sequence of hydrogen-bond breaking and reforming processes. This appealing mechanism can be closely associated with the exceptionally high proton mobility in liquid methanol.

Influence of Electrostatic Interactions on Complexes with Short O···O Hydrogen Bonds in Basic Salts of Pyridine Betaines and Acid Salts of ω-Phenyloalkanocarboxylic Acids

Abstract: The isostructural complexes [C5H5N+(CH2)nCOO]2HX and [C6H5(CH2)nCOO]2HK (n = 1–4), which differ in their counterions and charge on the ring, were synthesized, and their powder FT-IR spectra analyzed. All complexes containing a charged pyridine ring are of Hadži type iii, characterized by an intense broad (continuum) absorption below 1600 cm−1 typical of a short-strong hydrogen bond (SSHB) with a delocalized proton and a single vC=O band. The positively charged nitrogen atoms interact electrostatically with the X− ion and, additionally, with one of the oxygen atoms of the carboxylic group, producing a more or less symmetric environment of the H-bonded proton, and stabilizing the SSHB. The broad absorption of [C6H5CH2COO]2HK is very similar to that of other pyridine complexes. Upon addition of methylene groups the broad absorption moves to higher wavenumbers, the O···O distance is elongated, and the H-bonded proton becomes more localized. In the spectrum of [C6H5(CH2)4COO]2HK the vC=O and vasCOO bands were found at 1704 and 1641 cm−1, respectively, which shows that the H-bonded proton is asymmetrically located. The observed variation of absorption with the number of CH2 groups reflects changes of contacts between the K+ ion and COO− groups.

Theoretical Studies of the Grotthuss Mechanism in Biological Proton Wires

Abstract: The structural, dynamic, and thermodynamic properties of proton wires are examined. Detailed molecular aspects of both hop and turn steps of the Grotthuss mechanism for proton conduction along single-file chains of water molecules in model nonpolar pores and in the gramicidin A channel are reviewed, and new simulation results are presented. The Grotthuss mechanism is discussed and analyzed in terms of hydrogen-bonded network structure and connectivity, and the properties of proton wires are contrasted to those of bulk water. General considerations regarding the biological relevance of such studies are proposed.

Hydrogen Bond Compression during Triple Proton Transfer in Crystalline Pyrazoles. A Dynamic 15N NMR Study

Abstract: Using dynamic solid state 15 N CPMAS NMR spectroscopy (CP ≡ cross polarization, MAS ≡ magic-angle spinning), the kinetics of degenerate inter- molecular triple proton and deuteron transfers in the cyclic trimers of 15 N-labeled polycrystalline 4-nitropyrazole (4NO 2 P) and 4-bromopyrazole (4BrP) have been studied as a function of temperature and are compared to the kinetics of triple proton transfer in bulk solid 3,5-dimethylpyrazole (DMP) studied previously. The results show that the transfer kinetics in the new trimers are much faster than in DMP. However, the kinetic HHH/HHD/HDD/DDD isotope effects of 4NO 2 P are similar to those of DMP. These effects indicate a single barrier for the triple proton transfers where all three protons lose zero-point energy in the transition state, as expected for a structure with three compressed hydrogen bonds. At low temperatures, strong deviations from an Arrhenius-behavior are observed which are described in terms of a modified Bell tunneling model and a concerted proton motion. The barrier for the triple proton transfer in 4NO 2 P and 4BrP is substantially smaller than in DMP. As there is no correlation with the electronic properties of the substituents, we assign this finding to steric effects where the bulky methyl groups of DMP in the 3- and 5- positions hinder the hydrogen bond compression, in contrast to 4NO 2 P and 4BrP exhibiting substitutents in the 4-position. These results lead to a minimum energy pathway of the proton transfer following in the absence of steric hindering the hydrogen bond correlation line q 1 = f(q 2 ), established previously, where q 1 represents the deviation of the proton from the hydrogen bond center and q 2 the N...N distance. Tunneling occurs at constant N...N distances.

Covalent and Ionic States of Strong Acids at the Ice Surface

Abstract: The relationship between the degree of ionization and the environment of a strong acid is of basic scientific interest. Often this relationship reduces to the interdependence of ion/acid hydration and proton transfer. Despite the presence of pure water, the surface of crystalline ice, particularly at cryogenic temperatures, is one of limited (controlled?) availability of water of hydration. Here, the detailed nature of the ice surface and the states of strong acids adsorbed to ice at cryogenic temperatures are examined. These subjects are of special current interest since the ability to model the complex chemistry that occurs on the surfaces of water-rich particles in the atmosphere, particularly in the stratosphere over the polar regions, requires a valid concept of the acid-ice interface. Our combined spectroscopic and simulation studies have identified the surface of free-standing ice particles as badly disordered, with a range of water-ring sizes and an increased level of H-bond saturation relative to an ordered ice surface. FT-IR results are reported for the interaction of the surface of such ice particles with submonolayer amounts of adsorbed DCl, DBr, and HNO3 and for multilayer exposure to DCl. The DCl and DBr adsorbed states demonstrate behavior familiar from observations on strongly bound molecular adsorbates. Two methods have been devised for exposure of the nanocrystals to HNO3 One gives an ionic state initially, while the initial state of the other approach is molecular. In both instances, the system is observed to evolve, with time/warming, towards a common mixed molecular–ionic adsorbed state.

An Electrophysiological Comparison of Voltage-Gated Proton Channels, Other Ion Channels, and Other Proton Channels

Abstract: Voltage-gated proton selective channels occupy an ill-defined region between “normal” ion channels and a variety of proton-conducting pathways inside proteins, including, but not limited to, membrane-bound proteins. Voltage-gated H+ channels closely resemble other voltage-gated ion channels in their voltage- and time-dependent gating, but differ in their extreme selectivity, their miniscule single-channel conductance, and the high activation enthalpy for conduction. Furthermore, in contrast with the “multiple occupancy” hypothesized to account for aspects of permeation through other ion channels, it seems unlikely that H+ channels can be occupied by more than one proton at a time. Voltage-gated H+ channels functionally resemble other proton-conducting pathways in proteins, but until their structure has been determined, this similarity will remain speculative. The present restriction to functional measurements is less of a handicap than might be expected—the history of ion channel research shows that deductions based on electrophysiological measurements often closely predict the eventually determined structure. Existing evidence supports the idea that protons permeate the membrane through voltage-gated H+ channels by hopping across a hydrogen-bonded chain that consists of at least some amino acid side groups in addition to water molecules.

Kinetics of Hexafluoropropylene Oxide Pyrolysis Studied by Gas‐Phase NMR. Kinetic Measurements Made Easy

Abstract: Gas-phase NMR complemented with a high-temperature probe makes possible quantitative kinetic studies of homogeneous vapor-phase reactions with unprecedented ease, as demonstrated with a study of the thermal fragmentation of hexafluoropropylene oxide The reaction follows strictly first-order kinetics, is unaffected by surface chemistry, and the resulting Arrhenius activation parameters compare well with those obtained by more laborious classical methods of gas-phase reaction kinetics The fragmentation kinetics is also unaffected by the presence of oxygen The profiles for the evolution of the final products in this case show very graphically that singlet difluorocarbene, one of the initial products of the fragmentation of hexafluoropropylene oxide, does not react with oxygen, a ground-state triplet, at chemically significant rates

Copper(II) Mediated Homo‐Coupling of 1,2‐difluorovinylstannanes

Abstract: The reaction of substituted 1,2-difluorovinylstannanes with anhydrous Cu(OAc)2 in dimethylformamide, under an oxygen atmosphere, at room temperature stereospecifically gives the corresponding symmetrical 1,3-dienes in good to excellent yields. Moisture drastically reduces the yield of homo-coupled product and produces significant amounts of RCF=CFH. The Cu(II) promoted homo-coupling of the fluorinated vinylstannanes is superior to the Cu(I)Cl mediated homo-coupling of the corresponding hydrocarbon vinylstannanes. When 1,2-difluorovinylstannanes are treated with Cu(I)Cl, the reaction is sluggish and significant amounts of RCF=CFH are formed.

Laser studies of small radicals in rich methane flames : oh, hco, and 1ch2

Abstract: Chemical mechanisms for combustion processes are often developed for nearly stoichiometric flames, a perfect balance of fuel and oxidizer. As the fuel content is increased and flames become richer, these models will eventually break down because they lack reactions leading to the formation of larger hydrocarbons and ultimately soot. In this contribution, we investigate the behavior of two methane combustion models, the GRI 2.11 and Prada–Miller mechanisms, in a series of rich flames of stoichiometry 1.0, 1.2, 1.4, and 1.6. Using non-intrusive laser diagnostics, laser-induced fluorescence, and cavity ring-down spectroscopy, the concentration profiles as a function of height above burner for OH, HCO, and 1CH2 have been measured in these rich low-pressure methane flames. The experimental results are compared to the calculated profiles. In general, good agreement is found for all species over the range of stoichiometries investigated. The agreement of the OH and HCO profiles with the models is particularly striking. For 1CH2, the width, position, and relative height of the profiles match the model predictions for flames of stoichiometry 1.0 and 1.2. Discrepancies are noted for the richer flames investigated, those with stoichiometrics of 1.4 and 1.6. An analysis of the models suggests that additional pathways for 1CH2 formation and removal should be considered. Even under fairly rich conditions, stoichiometry of 1.6, both models perform well for the radicals investigated here. There is no indication of the requirement for >C2 chemistry to reproduce the present data.

CuCN and Trimethylsilyl Chloride‐Catalyzed Regiospecific Grignard Reactions to CF3‐Containing Allylic Derivatives

Abstract: CF3-containing allylic alcohol derivatives were treated with an appropriate Grignard reagent in the presence of catalytic amounts of CuCN and trimethylsilyl chloride (TMSCl) to furnish products via the clean anti-SN2′ mechanism. Experimental results as well as ab initio computational analyses unambiguously demonstrated the important roles of TMSCl as a Lewis basic additive for smooth promotion of reductive elimination and inhibition of the “Cu···F” elimination leading to undesired byproduct formation.

Laser diagnostics of combustion processes : from chemical dynamics to technical devices

Abstract: Laser-based in situ diagnostic techniques with high temporal, spectral, and spatial resolution have become valuable tools to study the molecular dynamics of gas-phase and heterogeneous reactions as well as complex technical combustion processes. Results of recent experiments will be presented in which laser-induced fluorescence, sum-frequency generation surface vibrational spectroscopy, Rayleigh scattering, excimer laser-induced fragmentation fluorescence, and high-resolution tunable diode laser absorption spectroscopy were applied to investigate elementary chemical gas-phase and catalytic combustion reactions, internal combustion engine processes, as well as coal combustion and waste incineration.

Flying Protons in Linked Gramicidin A Channels

Abstract: Different lines of evidence indicate that the mobility of protons in gramicidin A (gA) channels as well as in its SS dioxolane linked dimer (SS dimer) is comparable to the proton mobility in aqueous HCl solutions. Single-channel proton conductances in the SS dimer were measured under different experimental conditions. Bulk conductivities of HCl solutions were also measured. Ratios between calculated proton mobilities in the SS dimer and in aqueous solutions varied between 0.75 and 0.10 in the HCl concentration range of 10–6000 mM. Proton mobility in the SS dimer is profoundly affected by the lipid composition of the membrane. Attenuation of proton conduction in the SS dimer and in solution by methanol have similar characteristics. It is proposed that methanol partitions inside the pore of the SS dimer and attenuates proton conductance.

Effects of model protein environments on the dynamics of proton wires

Abstract: The multiconfigurational molecular dynamics with quantum transitions (MC-MDQT) method is utilized to study the impact of model protein environments on the dynamics of proton wires The MC-MDQT method allows the realtime nonequilibrium quantum dynamical simulation of proton transport along water chains and provides a framework for analyzing the detailed dynamical mechanisms of these multiple proton transfer reactions In this paper, the protein environment is modeled by applying structural restraints to the oxygen atoms of the chain, by applying external electric fields, and by including solvating water molecules hydrogen-bonded to the ends of the water chain Our simulations illustrate that the protein environment could strongly impact the dynamics of proton wires through a combination of structural restraints, fluctuating electric fields, solvation, and hydrogen bonding Our simulations also indicate that quantum effects such as hydrogen tunneling and nonadiabatic transitions play a significant role under certain nonequilibrium conditions

Proton Transfer Processes in Water Clusters

Abstract: Elucidation of some of the basic mechanisms and principles of proton transfer in clusters is achieved by systematic evaluation of experimental and theoretical results that were obtained throughout the last decade from the authors' studies on ionic water clusters and, more recently, on protonated rare gas clusters. Among the processes studied are: Blackbody radiation-induced, stepwise desolvation of clusters when stored in an ion trap; ionic dissociation and recombination of HCl in water clusters; metal oxidation in aqueous clusters; formation of nascent hydrogen in reactions of solvated metal cations with acids; and the behavior of ionic water clusters as acidic or basic micro-solutions. Infrared spectroscopic studies of size-selected water clusters are expected to advance our knowledge of structure and dynamics of water clusters and of excess protons therein.