Showing papers on "Elementary reaction published in 2009"

Complementary structure sensitive and insensitive catalytic relationships.

Abstract: The burgeoning field of nanoscience has stimulated an intense interest in properties that depend on particle size. For transition metal particles, one important property that depends on size is catalytic reactivity, in which bonds are broken or formed on the surface of the particles. Decreased particle size may increase, decrease, or have no effect on the reaction rates of a given catalytic system. This Account formulates a molecular theory of the structure sensitivity of catalytic reactions based on the computed activation energies of corresponding elementary reaction steps on transition metal surfaces. Recent progress in computational catalysis, surface science, and nanochemistry has significantly improved our theoretical understanding of particle-dependent reactivity changes in heterogeneous catalytic systems. Reactions that involve the cleavage or formation of molecular π-bonds, as in CO or N2, must be distinguished from reactions that involve the activation of σ-bonds, such as CH bonds in methane. Th...

Development of a Group Contribution Method To Predict Aqueous Phase Hydroxyl Radical (HO•) Reaction Rate Constants

Abstract: The hydroxyl radical (HO•) is a strong oxidant that reacts with electron-rich sites of organic compounds and initiates complex chain mechanisms. In order to help understand the reaction mechanisms, a rule-based model was previously developed to predict the reaction pathways. For a kinetic model, there is a need to develop a rate constant estimator that predicts the rate constants for a variety of organic compounds. In this study, a group contribution method (GCM) is developed to predict the aqueous phase HO• rate constants for the following reaction mechanisms: (1) H-atom abstraction, (2) HO• addition to alkenes, (3) HO• addition to aromatic compounds, and (4) HO• interaction with sulfur (S)-, nitrogen (N)-, or phosphorus (P)-atom-containing compounds. The GCM hypothesizes that an observed experimental rate constant for a given organic compound is the combined rate of all elementary reactions involving HO•, which can be estimated using the Arrhenius activation energy, Ea, and temperature. Each Ea for thos...

Surface science investigations of oxidative chemistry on gold.

Abstract: Because of gold's resistance to oxidation and corrosion, historically chemists have considered this metal inert. However, decades ago, researchers discovered that highly dispersed gold particles on metal oxides are highly chemically active, particularly in low-temperature CO oxidations. These seminal findings spurred considerable interest in investigations and applications of gold-based materials. Since the discovery of gold's chemical activity at the nanoscale, researchers found that bulk gold also has interesting catalytic properties. Thus, it is important to understand and contrast the intrinsic chemical properties of bulk gold with those of nanoparticle Au. Despite numerous studies, the structure and active site of supported Au nanoclusters and the active oxygen species remain elusive, and model studies under well-controlled conditions could help identify these species. The {111} facet has the lowest surface energy and is the most stable and prevalent configuration of most supported gold nanoparticles. Therefore, a molecular-level understanding of the physical properties and surface chemistry of Au(111) could provide mechanistic details regarding the nature of Au-based catalysts and lead to improved catalytic processes. This Account focuses on our current understanding of oxidative chemistry on well-defined gold single crystals, predominantly from recent investigations on Au(111) that we have performed using modern surface science techniques. Our model system strategy allows us to control reaction conditions, which assists in the identification of reaction intermediates, the determination of the elementary reaction steps, and the evaluation of reaction energetics for rate-limiting steps. We have employed temperature-programmed desorption (TPD), molecular beam reactive scattering (MBRS), and Auger electron spectroscopy (AES) to evaluate surface oxidative chemistry. In some cases, we have combined these results with density functional theory (DFT) calculations. By controlling the reaction parameters that determine product selectivity, we have examined the chemical properties of bulk gold. Based on our investigations, the surface-bound oxygen atoms are metastable at low temperature. We also demonstrate that the oxygen atoms and formed hydroxyls are responsible for some of the distinct chemical behavior of gold and participate in surface reactions either as a Bronsted base or a nucleophilic base. We observe similar reaction patterns on gold surfaces to those on copper and silver surfaces, suggesting that the acid-base reactions that have been observed on copper and silver may also occur on gold. Our model chemical studies on gold surfaces have provided intrinsic fundamental insights into high surface area gold-based catalysts and the origin of the reactive oxygen species.

A new skeletal PRF kinetic model for HCCI combustion

Abstract: A skeletal kinetic model for Primary Reference Fuel (PRF) with 33 species and 38 reactions has been developed for Homogeneous Charge Compression Ignition (HCCI) combustion and introduced in this paper. Low-temperature reaction scheme of the new model was based on the reduced kinetic model for PRF by Tanaka et al. [S. Tanaka, F. Ayala, J.C.Keck, Combust. Flame , 133 (2003) 467–481] and several modifications were added. The main features of the modifications are consideration of intermediates, olefins and aldehydes, and consideration of beta-scission of alkyl radicals in parallel to the low-temperature reactions. High-temperature reaction mechanism is very simple, consists of thermal decomposition of alkyl radicals to ethylene and oxidation of ethylene and formaldehyde to products. Beta-scission and thermal decomposition of alkyl radicals to ethylene consist of five elementary reactions and one global reaction, and oxidations of ethylene and formaldehyde consist of 10 elementary reactions and two additional global reactions. Shock tube ignition delay data, intermediate profiles from gas-sampling experiments in a HCCI engine were used to develop and validate the new PRF model. And the validated results showed good agreements to the experimental data for both shock tube experiments and also gas sampling experiments in a HCCI engine.

Intrinsic kinetic equation for oxygen reduction reaction in acidic media: the double Tafel slope and fuel cell applications

Abstract: According to Sergio Trasatti, “A true theory of electrocatalysis will not be available until activity can be calculated a priori from some known properties of the materials.” Toward this goal, we developed intrinsic kinetic equations for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) using as the kinetic parameters the free energies of adsorption and activation for elementary reactions. Rigorous derivation retained the intrinsic connection between the intermediates' adsorption isotherms and the kinetic equations, affording us an integrated approach for establishing the reaction mechanisms based upon various experimental and theoretical results. Using experimentally deduced free energy diagrams and activity-and-barriers plot for the ORR on Pt(111), we explained why the Tafel slope in the large overpotential region is double that in the small overpotential region. For carbon-supported Pt nanoparticles (Pt/C), the polarization curves measured with thin-film rotating disk electrodes also exhibit the double Tafel slope, albeit Pt(111) is several times more active than the Pt nanoparticles when the current is normalized by real surface area. An analytic method was presented for the polarization curves measured with H2 in proton exchange membrane fuel cells (PEMFCs). The fit to a typical iR-free polarization curve at 80 °C revealed that the change of the Tafel slope occurs at about 0.77 V that is the reversible potential for the transition between adsorbed O and OH on Pt/C. This is significant because it predicts that the Butler–Volmer equation can only fit the data above this potential, regardless the current density. We also predicted a decrease of the Tafel slope from 70 to 65 mV dec−1 at 80 °C with increasing oxygen partial pressure, which is consistent with the observation reported in literature.

Temperature-dependent transitions between normal and inverse isotope effects pertaining to the interaction of H-H and C-H bonds with transition metal centers.

Abstract: Deuterium kinetic isotope effects (KIEs) serve as versatile tools to infer details about reaction mechanisms and the nature of transition states, while equilibrium isotope effects (EIEs) associated with the site preferences of hydrogen and deuterium enable researchers to study aspects of molecular structure. Researchers typically interpret primary deuterium isotope effects based on two simple guidelines: (i) the KIE for an elementary reaction is normal (kH/kD > 1) and (ii) the EIE is dictated by deuterium preferring to be located in the site corresponding to the highest frequency oscillator. In this Account, we evaluate the applicability of these rules to the interactions of H−H and C−H bonds with a transition metal center. Significantly, experimental and computational studies question the predictability of primary EIEs in these systems based on the notion that deuterium prefers to occupy the highest frequency oscillator. In particular, the EIEs for (i) formation of σ-complexes by coordination of H−H and ...

Force-activated reactivity switch in a bimolecular chemical reaction.

Abstract: The effect of mechanical force on the free-energy surface that governs a chemical reaction is largely unknown. The combination of protein engineering with single-molecule force-clamp spectroscopy allows us to study the influence of mechanical force on the rate at which a protein disulfide bond is reduced by nucleophiles in a bimolecular substitution reaction (S(N)2). We found that cleavage of a protein disulfide bond by hydroxide anions exhibits an abrupt reactivity 'switch' at ∼500 pN, after which the accelerating effect of force on the rate of an S(N)2 chemical reaction greatly diminishes. We propose that an abrupt force-induced conformational change of the protein disulfide bond shifts its ground state, drastically changing its reactivity in S(N)2 chemical reactions. Our experiments directly demonstrate the action of a force-activated switch in the chemical reactivity of a single molecule.

Potential Energy Surface of Methanol Decomposition on Cu(110)

Abstract: Combining the dimer saddle point searching method and periodic density functional theory calculations, the potential energy surface of methanol decomposition on Cu(110) has been mapped out. Each elementary step in the methanol decomposition reaction into CO and hydrogen occurs via one of three possible mechanisms: O−H, C−H, or C−O bond scission. Multiple reaction pathways for each bond scission have been identified in the present work. Reaction pathway calculations are started from an initial (reactant) state with methanol adsorbed in the most stable geometry on Cu(110). The saddle point and corresponding final state of each reaction or diffusion mechanism were determined without assuming the reaction mechanism. In this way, the reaction paths are determined without chemical intuition. The harmonic pre-exponential factor of each identified reaction is calculated from a normal-mode analysis of the stationary points. Then, using harmonic transition state theory, the rate constant of each identified reaction...

Probing the dynamics of polyatomic multichannel elementary reactions by crossed molecular beam experiments with soft electron-ionization mass spectrometric detection

Abstract: In this Perspective we highlight developments in the field of chemical reaction dynamics. Focus is on the advances recently made in the investigation of the dynamics of elementary multichannel radical–molecule and radical–radical reactions, as they have become possible using an improved crossed molecular beam scattering apparatus with universal electron-ionization mass spectrometric detection and time-of-flight analysis. These improvements consist in the implementation of (a) soft ionization detection by tunable low-energy electrons which has permitted us to reduce interfering signals originating from dissociative ionization processes, usually representing a major complication, (b) different beam crossing-angle set-ups which have permitted us to extend the range of collision energies over which a reaction can be studied, from very low (a few kJ mol−1, as of interest in astrochemistry or planetary atmospheric chemistry) to quite high energies (several tens of kJ mol−1, as of interest in high temperature combustion systems), and (c) continuous supersonic sources for producing a wide variety of atomic and molecular radical reactant beams. Exploiting these new features it has become possible to tackle the dynamics of a variety of polyatomic multichannel reactions, such as those occurring in many environments ranging from combustion and plasmas to terrestrial/planetary atmospheres and interstellar clouds. By measuring product angular and velocity distributions, after having suppressed or mitigated, when needed, the problem of dissociative ionization of interfering species (reactants, products, background gases) by soft ionization detection, essentially all primary reaction products can be identified, the dynamics of each reaction channel characterized, and the branching ratios determined as a function of collision energy. In general this information, besides being of fundamental relevance, is required for a predictive description of the chemistry of these environments via computer models. Examples are taken from recent on-going work (partly published) on the reactions of atomic oxygen with acetylene, ethylene and allyl radical, of great importance in combustion. A reaction of relevance in interstellar chemistry, as that of atomic carbon with acetylene, is also discussed briefly. Comparison with theoretical results is made wherever possible, both at the level of electronic structure calculations of the potential energy surfaces and dynamical computations. Recent complementary CMB work as well as kinetic work exploiting soft photo-ionization with synchrotron radiation are noted. The examples illustrated in this article demonstrate that the type of dynamical results now obtainable on polyatomic multichannel radical–molecule and radical–radical reactions might well complement reaction kinetics experiments and hence contribute to bridging the gap between microscopic reaction dynamics and thermal reaction kinetics, enhancing significantly our basic knowledge of chemical reactivity and understanding of the elementary reactions which occur in real-world environments.

Influence of water on elementary reaction steps in electrocatalysis

Abstract: We studied simple reaction pathways of molecules interacting with Pt(111) in the presence of water and ions using density functional theory within the generalized gradient approximation. We particularly focus on the dissociation of H2 and O2 on Pt(111) which represent important reaction steps in the hydrogen evolution/oxidation reaction and the oxygen reduction reaction, respectively. Because of the weak interaction of water with Pt(111), the electronic structure of the Pt electrode is hardly perturbed by the presence of water. Consequently, processes that occur directly at the electrode surface, such as specific adsorption or the dissociation of oxygen from the chemisorbed molecular oxygen state, are only weakly influenced by water. In contrast, processes that occur further away from the electrode, such as the dissociation of H2, can be modified by the water environment through direct molecule–water interaction.

On the mechanism of decomposition of the benzyl radical

Abstract: The C7H7 potential energy surface was studied from first principles to determine the benzyl radical decomposition mechanism. The investigated high temperature reaction pathway involves 15 accessible energy wells connected by 25 transition states. The analysis of the potential energy surface, performed determining kinetic constants of each elementary reaction using conventional transition state theory, evidenced that the reaction mechanism has as rate determining step the isomerization of the 1,3-cyclopentadiene, 5-vinyl radical to the 2-cyclopentene,5-ethenylidene radical and that the fastest reaction channel is dissociation to fulvenallene and hydrogen. This is in agreement with the literature evidences reporting that benzyl decomposes to hydrogen and a C7H6 species. The benzyl high-pressure decomposition rate constant estimated assuming equilibrium between the rate determining step transition state and benzyl is k1(T) = 1.44 × 1013T0.453exp(−38400/T) s−1, in good agreement with the literature data. As fulvenallene reactivity is mostly unknown, we investigated its reaction with hydrogen, which has been proposed in the literature as a possible decomposition route. The reaction proceeds fast both backward to form again benzyl and, if hydrogen adds to allene, forward toward the decomposition into the cyclopentadienyl radical and acetylene with high-pressure kinetic constants k2(T) = 8.82 × 108T1.20exp(1016/T) and k3(T) = 1.06 × 108T1.35exp(1716/T) cm3/mol/s, respectively. The computed rate constants were then inserted in a detailed kinetic mechanism and used to simulate shock tube literature experiments.

The mechanism of the interstellar isomerization reaction HOC+→HCO+ catalyzed by H2: New Insights from the reaction electronic flux

Abstract: A theoretical study of the mechanism of the isomerization reaction HOC+→HCO+ is presented. The mechanism was studied in terms of reaction force, chemical potential, reaction electronic flux (REF), and bond orders. It has been found that the evolution of changes in REF along the intrinsic reaction coordinate can be explained in terms of bond orders. The energetic lowering of the hydrogen assisted (catalyzed) reaction has been identified as being due to the stabilization of the H3+ transition state complex and the stepwise bond dissociation and formation of the H–O and H–C bonds, respectively.

A diffusional bimolecular propensity function

Abstract: We derive an explicit formula for the propensity function (stochastic reaction rate) of a generic bimolecular chemical reaction in which the reactant molecules move about by diffusion, as solute molecules in a bath of much smaller and more numerous solvent molecules. Our derivation assumes that the solution is macroscopically well stirred and dilute in the solute molecules. It effectively extends the physical rationale for the chemical master equation and the stochastic simulation algorithm from well-stirred dilute gases to well-stirred dilute solutions, with the former becoming a limiting case of the latter. This extension is important for cellular systems, where the solvent molecules are typically water and the solute (reactant) molecules are much larger organic structures, whose relatively low populations often require a discrete-stochastic formalism. In the course of our derivation, we illuminate some limitations on the ability of the classical diffusion equation to accurately describe how a diffusing molecule moves on spatial and temporal scales that are relevant to collision-induced chemical reactions.

Imaging Consecutive Steps of O2 Reaction with Hydroxylated TiO2(110): Identification of HO2 and Terminal OH Intermediates

Abstract: We report the results of a combined experimental and theoretical investigation of the reaction of molecular oxygen with a partially hydroxylated TiO2(110) surface. The consecutive steps of both primary and secondary site-specific reactions have been tracked with high-resolution scanning tunneling microscopy (STM). We have directly imaged stable, adsorbed hydroperoxyl (HO2) species, which is believed to be a key intermediate in many heterogeneous photochemical processes but generally metastable and “elusive” until now. We also found terminal hydroxyl groups, which are another critical but never previously directly observed intermediates. Conclusive evidence that O2 reacts spontaneously with a single bridging OH group as an initial reaction step is provided. The experimental results are supported by density functional theory (DFT) calculations that have determined the energies and configurations of these species. Reported observations provide a base for a consistent description of the elementary reaction st...

Reaction dynamics of phenyl radicals in extreme environments: a crossed molecular beam study.

Abstract: Polycyclic aromatic hydrocarbons (PAHs)organic compounds that consist of fused benzene ringsand their hydrogen-deficient precursors have attracted extensive interest from combustion scientists, organic chemists, astronomers, and planetary scientists. On Earth, PAHs are toxic combustion products and a source of air pollution. In the interstellar medium, research suggests that PAHs play a role in unidentified infrared emission bands, diffuse interstellar bands, and the synthesis of precursor molecules to life. To build clean combustion devices and to understand the astrochemical evolution of the interstellar medium, it will be critical to understand the elementary reaction mechanisms under single collision conditions by which these molecules form in the gas phase. Until recently, this work had been hampered by the difficulty in preparing a large concentration of phenyl radicals, but the phenyl radical represents one of the most important radical species to trigger PAH formation in high-temperature environments. However, we have developed a method for producing these radical species and have undertaken a systematic experimental investigation. In this Account, we report on the chemical dynamics of the phenyl radical (C(6)H(5)) reactions with the unsaturated hydrocarbons acetylene (C(2)H(2)), ethylene (C(2)H(4)), methylacetylene (CH(3)CCH), allene (H(2)CCCH(2)), propylene (CH(3)CHCH(2)), and benzene (C(6)H(6)) utilizing the crossed molecular beams approach. For nonsymmetric reactants such as methylacetylene and propylene, steric effects and the larger cones of acceptance drive the addition of the phenyl radical to the nonsubstituted carbon atom of the hydrocarbon reactant. Reaction intermediates decomposed via atomic hydrogen loss pathways. In the phenyl-propylene system, the longer lifetime of the reaction intermediate yielded a more efficient energy randomization compared with the phenyl-methylacetylene system. Therefore, two reaction channels were open: hydrogen losses from the vinyl and from the methyl groups. All fragmentation pathways involved tight exit transition states. In the range of collision energies investigated, the reactions are dictated by phenyl radical addition-hydrogen atom elimination pathways. We did not observe ring closure processes with the benzene ring. Our investigations present an important step toward a systematic investigation of phenyl radical reactions under single collision conditions similar to those found in combustion flames and in high-temperature interstellar environments. Future experiments at lower collision energies may enhance the lifetimes of the reaction intermediates, which could open up competing ring closure channels to form bicyclic reaction products.

No electron left behind: a rule-based expert system to predict chemical reactions and reaction mechanisms.

Abstract: Predicting the course and major products of arbitrary reactions is a fundamental problem in chemistry, one that chemists must address in a variety of tasks ranging from synthesis design to reaction discovery. Described here is an expert system to predict organic chemical reactions based on a knowledge base of over 1500 manually composed reaction transformation rules. Novel rule extensions are introduced to enable robust predictions and describe detailed reaction mechanisms at the level of electron flows in elementary reaction steps, ensuring that all reactions are properly balanced and atom-mapped. The core reaction prediction functionalities of this expert system are illustrated with applications including: (1) prediction of detailed reaction mechanisms; (2) computer-based learning in organic chemistry; (3) retrosynthetic analysis; and (4) combinatorial library design. Select applications are available via http://cdb.ics.uci.edu.

Transient reaction analysis and steady-state kinetic study of selective catalytic reduction of NO and NO + NO2 by NH3 over Fe/ZSM-5

Abstract: The reaction kinetics of selective catalytic reduction (SCR) by NH3 on NO (standard SCR) and on NO + NO2 (fast SCR) over Fe/ZSM-5 were investigated using transient and steady-state analyses. In the standard SCR, the N2 production rate was transiently promoted in the absence of gaseous NH3; this enhancement can be attributed to the negative reaction order of NH3 (between −0.21 and −0.11). The steady-state data for the standard SCR could be fit to a Langmuir–Hinshelwood-type reaction between NOad and Oad to form NO2. In the fast SCR, however, the promotion behavior in the absence of gaseous NH3 was not observed and the apparent NH3 order changed from positive to negative with NH3 concentration. The steady-state rate analysis combined with elementary reaction modeling suggested that competitive adsorption between NO2 and NH3 was occurring due to strong NO2 adsorption; this must be the main reason for the absence of the promotion effect.

Density functional theory study on water-gas-shift reaction over Molybdenum disulfide

Abstract: Density functional theory calculations have been carried out to investigate the adsorption of reaction intermediates appearing during water–gas-shift reaction at the sulfur covered MoS2 (1 0 0) surfaces, Mo-termination with 37.5% S coverage and S-termination with 50% S coverage using periodic slabs. The pathway for water–gas-shift reaction on both terminations has been carefully studied where the most favorable reaction path precedes the redox mechanism, namely the reaction takes place as follows: CO + H2O → CO + OH + H → CO + O + 2H → CO2 + H2. The most likely reaction candidates for the formate species HCOO formation is the surface CO2 reaction with H as a side reaction of CO2 desorption on S-termination with 50% S coverage. The formed HCOO species will react further with adsorbed hydrogen yielding H2COO followed by breaking its C–O bond to form the surface CH2O and O species.

Theoretical study of the 5-aminotetrazole thermal decomposition.

Abstract: The thermal decomposition of 5-aminotetrazole was studied theoretically using the G3 multilevel procedure and DFT B3LYP technique. The unimolecular primary decomposition reactions of the three most stable isomers of 5-ATZ were studied in the gas phase and in the melt using a simplified model of the latter. The influence of the melt on the elementary reaction barrier was taken into account by the calculation of the solvation free energies using the PCM model. In contrast to all previous publications, we considered the bimolecular reactions of 5-ATZ and demonstrated that they are very important especially in the condensed phase. It was found that the imino form undergoes fast isomerization to the amino form in the H-bonded dimers and does not participate in the 5-ATZ thermolysis. On the contrary, amino and, probably, the 2H isomer are the main isomers of 5-ATZ in the melt and gas phase. The N2 elimination reaction was found to be the dominant unimolecular channel of the amino and 2H isomer decomposition in both the gas phase and melt. The significant lowering of the activation barriers of decomposition reactions in H-bonded dimers was found. In agreement with the existing experimental data, HN3 elimination dominates for some of the considered complexes. It was concluded that the initial stages of thermolysis of 5-ATZ cannot be satisfactory described by the simple unimolecular reactions proposed in the literature.

Theoretical kinetic study of the reactions of cycloalkylperoxy radicals.

Abstract: Reactions of alkyl radicals with oxygen are key reactions in the low-temperature oxidation of hydrocarbons, but they have not been extensively studied yet in the case of cycloalkanes. Isomerizations of cycloalkylperoxy radicals and formation of cyclic ethers are especially important. In the present work, a theoretical study of the gas-phase reactions of cyclopentylperoxy and cyclohexylperoxy radicals has been carried out by means of quantum chemical calculations at the CBS-QB3 level. Computations on cyclopentylperoxy decomposition pathways are reported here for the first time. Thermochemical data have been obtained by means of isodesmic reactions, and the contribution of hindered rotors has been explicitly taken into account. Transition state theory has been used to calculate rate constants for all the elementary reactions. Three-parameter Arrhenius expressions have been derived in the temperature range 300-1000 K. Tunneling effects have been accounted for in the case of H-atom transfers. Our results compare well with experimental data and previous calculations available in the literature. In particular, the predicted rate constants for processes involving cyclohexylperoxy radicals, which have been introduced in a reaction mechanism scheme proposed before, exhibit excellent agreement with experiments at low and intermediate temperatures.

The Effects of Water on β-d-Xylose Condensation Reactions

Abstract: Car-Parrinello-based ab initio molecular dynamics simulations (CPMD) combined with metadynamics (MTD) simulations were used to determine the reaction energetics for the beta-D-xylose condensation reaction to form beta-1,4-linked xylobiose in a dilute acid solution. Protonation of the hydroxyl group on the xylose molecule and the subsequent breaking of the C-O bond were found to be the rate-limiting step during the xylose condensation reaction. Water and water structure was found to play a critical role in these reactions due to the proton's high affinity for water molecules. The reaction free energy and reaction barrier were determined using CPMD-MTD. We found that solvent reorganization due to proton partial desolvation must be taken into account in order to obtain the correct reaction activation energy. Our calculated reaction free energy and reaction activation energy compare well with available experimental results.

From elementary reactions to evaluated chemical mechanisms for combustion models

Abstract: Methods of determining rate data for elementary reactions for combustion applications, using experimental and theoretical methods, are briefly reviewed. The approaches are illustrated by reference to recent research in three areas: (i) reactions of OH with C2H4 and C2H2, where theory, tuned by reference to experiment, has provided a substantial contribution to the determination of rate data for these complex reactions, over a wide range of temperatures; (ii) reactions between alkyl radicals and O2, where theory and experiment have been closely allied in discerning the details of mechanisms for small alkyl radicals; much remains to be done with larger radicals; (iii) reactions of methylene and the interactions between chemical reaction and the conversion of the singlet state into the triplet, where theory has played little part thus far. Comments are also made on the process of evaluating rate data for elementary reactions for incorporation in chemical mechanisms for use in combustion models.

A surprisingly complex aqueous chemistry of the simplest amino acid. A pulse radiolysis and theoretical study on H/D kinetic isotope effects in the reaction of glycine anions with hydroxyl radicals.

Abstract: A pulse radiolysis study was carried out of the reaction rate constants and kinetic isotope effects of hydroxyl-radical-induced H/D abstraction from the most-simple α-amino acid glycine in its anionic form in water. The rate constants and yields of three predominantly formed radical products, glycyl (NH2–˙CH–CO2−), aminomethyl (NH2–˙CH2), and aminyl (˙NH–CH2–CO2−) radicals, as well as of their partially or fully deuterated analogs, were found to be of comparable magnitude. The primary, secondary, and primary/secondary H/D kinetic isotope effects on the rate constants were determined with respect to each of the three radicals. The unusual variety of products for such an elementary reaction between two small and simple species indicates a complex mechanism with several reactions taking place simultaneously. Thus, a theoretical modeling of the reaction mechanism and kinetics in the gas- and aqueous phase was performed by using the unrestricted density functional theory with the BB1K functional (employing the polarizable continuum model for the aqueous phase), unrestricted coupled cluster UCCSD(T) method, and improved canonical variational theory. Several hydrogen-bonded prereaction complexes and transition states were detected. In particular, the calculations pointed to a significant mechanistic role of the three-electron two-orbital (σ/σ* N∴O) hemibonded prereaction complexes in the aqueous phase. A good agreement with the experimental rate constants and kinetic isotope effects was achieved by downshifting the calculated reaction barriers by 3 kcal mol−1 and damping the NH(D) stretching frequency by a factor of 0.86.

Molecular Factors of Catalytic Selectivity

Abstract: Selectivity, that is, to produce one molecule out of many other thermodynamically feasible product molecules, is the key concept to develop 'clean manufacturing' processes that do not produce byproducts (green chemistry). Small differences in potential energy barriers for elementary reaction steps control which reaction channel is more likely to yield the desired product molecule (selectivity), instead of the overall activation energy for the reaction that controls turnover rates (activity). Recent studies have demonstrated the atomic- or molecular-level tailoring of parameters such as the surface structures of active sites that give rise to nanoparticle size and shape dependence of turnover rates and reaction selectivities. Here, we highlight seven molecular components that influence reaction selectivities. These include: surface structure, adsorbate-induced restructuring, adsorbate mobility, reaction intermediates, surface composition, charge transport, and oxidation states for model metal single crystal and colloid nanoparticle catalysts. We show examples of their functioning and describe in-situ instruments that permit us to investigate their roles in surface reactions.

A crossed molecular beams study on the formation of the exotic cyanoethynyl radical in titan's atmosphere

Abstract: The reaction of the dicarbon molecule (C2) in its ^(1)Σ_(g) + electronic ground state with hydrogen cyanide HCN(X^(1)Σ^+) is investigated in a crossed molecular beam setup to untangle the formation of the cyanoethynyl radical CCCN(X^(2)Σ^+) in hydrocarbon-rich atmospheres of planets and their moons such as Titan. Combined with electronic structure and rate theory calculations, we show that this elementary reaction is rapid, has no entrance barriers, and yields CCCN via successive rearrangements of the initial HC_(3)N collision complex to the cyanoacetylene intermediate (HCCCN) followed by unimolecular decomposition of the latter without exit barrier. New photochemical models imply that this radical could serve as a key building block to form more complex molecules as observed in situ by the Cassini spacecraft, ultimately leading to organic aerosol particles, which make up the orange-brownish haze layers in Titan's atmosphere.

A consistent molecular hydrogen isotope chemistry scheme based on an independent bond approximation

Abstract: . The isotopic composition of molecular hydrogen (H2) produced by photochemical oxidation of methane (CH4) and Volatile Organic Compounds (VOCs) is a key quantity in the global isotope budget of (H2). The many individual reaction steps involved complicate its investigation. Here we present a simplified structure-activity approach to assign isotope effects to the individual elementary reaction steps in the oxidation sequence of CH4 and some other VOCs. The approach builds on and extends the work by Gerst and Quay (2001) and Feilberg et al. (2007b). The description is generalized and allows the application, in principle, also to other compounds. The idea is that the C-H and C-D bonds – seen as reactive sites – have similar relative reaction probabilities in isotopically substituted, but otherwise identical molecules. The limitations of this approach are discussed for the reaction CH4+Cl. The same approach is applied to VOCs, which are important precursors of H2 that need to be included into models. Unfortunately, quantitative information on VOC isotope effects and source isotope signatures is very limited and the isotope scheme at this time is limited to a strongly parameterized statistical approach, which neglects kinetic isotope effects. Using these concepts we implement a full hydrogen isotope scheme in a chemical box model and carry out a sensitivity study to identify those reaction steps and conditions that are most critical for the isotope composition of the final H2 product. The reaction scheme is directly applicable in global chemistry models, which can thus include the isotope pathway of H2 produced from CH4 and VOCs in a consistent way.