Reactions of Ethynyloxy Radical with Hydroperoxyl Radical: Bridging Theoretical Reaction Dynamics and Chemical Modeling of Combustion

A detailed and accurate combustion reaction mechanism is crucial for understanding the nature of fuel combustion. In this work, a theoretical study of reaction HCCO+HO2 using M06-2X/6-311++G(d,p) for geometry optimization and combined methods based on spin-unrestricted CCSD(T)/CBS level of theory with basis set extrapolation from MP2/aug-cc-pVnZ (n=T and Q) for energy calculations were performed. The temperature- and pressure-dependent rate coefficients at 300-2000 K and 0.01-100 atm, suitable for combustion conditions, were derived using the Rice-Ramsberger-Kassel-Marcus/Master-Equation approach. Furthermore, temperature-dependent thermochemistry data of key species for the HCCO+HO2 system has also been studied. Finally, an updated ketene model is developed by supplementing the most recent theoretical work and the theoretical work in this paper. This updated model was tested to simulate the speciation of ketene oxidation in available experimental research. It is shown that the updated model for predicting ketene oxidation exhibits a high level of agreement with experimental data across a wide range of species profiles. An analysis was conducted to identify the crucial reactions that influence ketene ignition. This paper's research findings are essential for enhancing the combustion mechanism of ketene and other hydrocarbons and oxygenated hydrocarbon fuels.

Kinetic parameters for gas-phase reactions: experimental and theoretical challenges

This article aims to illustrate the added value provided to experimental kinetics investigations by complementary theoretical kinetics studies, using as examples (i) reactions of two major hydrocarbon flame radicals, HCCO and C(2)H, and (ii) reactions of several oxygenated organic compounds with hydroxyl radicals of interest to atmospheric chemistry. The first part, on HCCO and C(2)H kinetics, does not attempt to give an extensive literature review, but rather addresses some major experimental techniques, mainly specific ones, that have allowed a great part of the available reactivity databases on these two species to be established. For several key reactions, it is shown how potential energy surfaces and statistical rate predictions based thereon have provided insight into the molecular mechanisms and have allowed estimates of product distributions as well as reliable extrapolations of experimental rate coefficients and branching ratios to higher temperatures. The second part addresses current issues in atmospheric chemistry relating mainly to hydroxyl radical reactions with oxygenated organics, and focuses on the experimental characterization of the often unusual temperature dependence of their rate coefficients and on the theoretical rationalization thereof, through the formation of hydrogen-bonded pre-reactive complexes and resulting tunnelling-enhanced H-abstraction. Finally, the development of general structure-activity relationships for OH reactions with organics, H-abstractions as well as OH-additions for unsaturated compounds, is briefly discussed.

CH and C2 Measurements Imply a Radical Pool within a Pool in Acetylene Flames

Measured CH and C2 profiles show a striking resemblance as a function of time in a series of seven well-characterized fuel-rich (phi=1.2-2.0) non-sooting acetylene flames. This implied commonality and interrelationship are unexpected as these radicals have dissimilar chemical kinetic natures. As a result, a rigorous examination was undertaken of the behavior of each of the hydrocarbon species known to be present, C, CH, CH2, CH3, CH4, CHO, CHOH, CH2O, CH2OH, CH3O, CH3OH, C2, C2H, C2H2, CHCO, CH2CO, and C2O. This emphasized the main region where CH and C2 are observed (50-600 micros) and reduced the kinetic reactions to only those that operate efficiently and are dominant. It was immediately apparent that this region of the flame reflects the nature of a hydrogen flame heavily doped with CO and CO2 and containing traces of hydrocarbons. The radical species, H, OH, O, along with H2, H2O, and O2, form an important controlling radical pool that is in partial equilibrium, and the concentrations of each of the hydrocarbon radicals are minor to this, playing secondary roles. As a result, the dominant fast reactions are those between the hydrocarbons and the basic hydrogen/oxygen radicals. Hydrocarbon-hydrocarbon reactions are unimportant here at these equivalence ratios. CH and C2 are formed and destroyed on a sub-microsecond time scale so that their flame profiles are the reflection of a complex kinetically dynamic system. This is found to be the case for all of the hydrocarbon species examined. As might be expected, these rapidly form steady-state distributions. However, with the exceptions of C, CHO, CHOH, and CH2O, which are irreversibly being oxidized, the others all form an interconnected hydrocarbon pool that is under the control of the larger hydrogen radical pool. The hydrocarbon pool can rapidly adjust, and the CH and C2 decay together as the pool is drained. This is either by continuing oxidation in less rich mixtures, or in richer flames where this is negligible by the onset of hydrocarbon-hydrocarbon reactions. The implications of such a hydrocarbon pool are significant. It introduces a buffering effect on their distribution and provides the indirect connection between CH and C2. Moreover, because they are members of this radical pool, flame studies alone cannot answer questions concerning their specific importance in combustion other than their contributing role to this pool. The presence of such a pool modifies the exactness that is needed for kinetic mechanisms, and knowledge of every species in the system no longer is necessary. Furthermore, as rate constants become refined, it will allow for the calculation of the relative concentrations of the hydrocarbon species and facilitate reduced kinetic mechanisms. It provides an explanation for previous isotopically labeled experiments and illustrates the difficulty of exactly identifying in flames the role of individual species. It resolves the fact that differing kinetic models can show similar levels of accuracy and has implications for sensitivity analyses. It finally unveils the mechanism of the flame ionization detector and has implications for the differing interpretations of diamond formation mechanisms.

Theoretical mechanistic study on the radical-radical reaction of ketenyl with nitrogen dioxide

The radical-radical reaction between the ketenyl radical (HCCO) and nitrogen dioxide (NO(2)) played a very important role in atmospheric and combustion chemistry. Motivated by recent laboratory characterization about the reaction kinetics of ketenyl radical with nitrogen dioxide, in this contribution, we applied the coupled cluster and density functional theory to explore the mechanism of the title reaction. These calculations indicate that the title reaction proceeds mostly through singlet pathways, less go through triplet pathways. It is found that the HCCO + NO(2) reaction initially favors formation of adduct OCCHNO(2) (1) with no barrier. Subsequently, starting from isomer 1, the most feasible pathway is ring closure of 1 to isomer O-cCCHN(O)O (2) followed by CO(2) extrusion to product HCNO + CO(2) (P(1)), which is the major product with predominant yields. Much less competitively, 1 can take the successive 1,3-H- and 1,3-OH-shift interconversion to isomer OCCNOHO (3(a), 3(b), 3(c)) and then to isomer OCOHCNO (4(a), 4(b)), which can finally take a concerted H-shift and C-C bond fission to give HCNO + CO(2) (P(1)). The least competitive pathway is the ring-closure of isomer 3(a) to form isomer O-cCCN(OH)O (5(a), 5(b)) followed by dissociation to HONC + CO(2) (P(2)) through the direct side CO(2) elimination. Because the intermediates and transition states involved in the most favorable channel all lie below the reactants, the title reaction is expected to be rapid, as is confirmed by experiment. Therefore, it can be significant for elimination of nitrogen dioxide pollutants. The present results can lead us to a deep understanding of the mechanism of the title reaction and can be helpful for understanding NO(x)-combustion chemistry.

Theoretical Study on Reaction Mechanism of the Cyanogen Radical with Nitrogen Dioxide

The complex singlet potential energy surface for the reaction of CN with NO2, including 9 minimum isomers and 10 transition states, is explored computationally using a coupled cluster method and a density functional method. The most favorable association of CN with NO2 was found to be a barrierless carbon-to-nitrogen approach process forming an energy-rich adduct a (NCNO2) followed by C-N bond rupture along with C-O bond formation to give b1 (trans-NCONO), which can easily convert to b2 (cis-NCONO). Our results show that the product P1 (NCO + NO) is the major product, while the product P2 (CNO + NO) is a minor product. The other products may be of significance only at high temperatures. Product P1 (NCO + NO) can be obtained through path 1 P1: R --> a --> b1 (b2) --> P1 (NCO + NO), whereas the product P2 (CNO + NO) can be formed through path P2: R --> a --> b1 --> b2 --> c1 (c2) --> P2 (CNO + NO). Because the intermediates and transition states involved in the above two channels are all lower than the reactants in energy, the CN + NO2 reaction is expected to be rapid, as is confirmed by experiment. Therefore, it may be suggested as an efficient NO2-reduction strategy. These calculations indicate that the title reaction proceeds mostly through singlet pathways and less go through triplet pathways. The present results can lead us to understand deeply the mechanism of the title reaction and can be helpful for understanding NO2-combustion chemistry.

Product Study of the Photolysis of Ketene and Ethyl Ethynyl Ether at 193.3 nm

The product distributions of the excimer laser photolysis of ketene (CH2CO) and ethyl ethynyl ether (C2H5OCCH) at lambda = 193.3 nm (ArF) were studied using a time-of-flight mass spectrometer (TOFMS) as an analytical tool. Ketene was photolyzed in bath gases consisting of mixtures of He and H2/D2 at various mixing ratios at constant total pressures of 4 Torr and temperature of about 300 K. Singlet methylene (1CH2) produced in the photolysis of ketene was almost instantaneously converted either to triplet methylene (3CH2) or to methyl radicals in collisions with He and H2 or D2. By extrapolating the methyl and methylene signals to zero time after photolysis, initial concentrations of these radicals were obtained. Analyzing the initial 3CH2 and CH3 concentrations as functions of hydrogen-to-helium ratios as well as simulating the observed traces of reactant and product species resulted in 1CH2 + CO (66 +/- 8)%, as the main product channel of the ketene photolysis with smaller contributions from HCCO + H (17 +/- 7)% and 3CH2 + CO (6 +/- 9)%. Hydrogen atoms, acetylene, ethylene, ethyl, and ketenyl radicals, and small amounts of ketene were observed as primary products of the ethyl ethynyl ether photolysis. Quantification of C2H2, C2H4, C2H5, and CH2CO product leads to a HCCO yield of (91 +/- 14)%.

Kinetics of the HCCO + NO2 Reaction

The kinetics of the HCCO + NO2 reaction were investigated using a laser photolysis/infrared diode laser absorption technique. Ethyl ethynyl ether (C2H5OCCH) was used as the HCCO radical precursor. Transient infrared detection of the HCCO radical was used to determine a total rate constant fit to the following expression: k1= (2.43 +/- 0.26) x 10(-11) exp[(171.1 +/- 36.9)/T] cm3 molecule(-1) s(-1) over the temperature range of 298-423 K. Transient infrared detection of CO, CO2, and HCNO products was used to determine the following branching ratios at 298 K: phi(HCO + NO + CO) = 0.60 +/- 0.05 and phi(HCNO + CO2) = 0.40 +/- 0.05.

Pulsed laser photolysis and quantum chemical-statistical rate study of the reaction of the ethynyl radical with water vapor

The rate coefficient of the gas-phase reaction C(2)H + H(2)O-->products has been experimentally determined over the temperature range 500-825 K using a pulsed laser photolysis-chemiluminescence (PLP-CL) technique. Ethynyl radicals (C(2)H) were generated by pulsed 193 nm photolysis of C(2)H(2) in the presence of H(2)O vapor and buffer gas N(2) at 15 Torr. The relative concentration of C(2)H radicals was monitored as a function of time using a CH* chemiluminescence method. The rate constant determinations for C(2)H + H(2)O were k(1)(550 K) = (2.3 +/- 1.3) x 10(-13) cm(3) s(-1), k(1)(770 K) =(7.2 +/- 1.4) x 10(-13) cm(3) s(-1), and k(1)(825 K) = (7.7 +/- 1.5) x 10(-13) cm(3) s(-1). The error in the only other measurement of this rate constant is also discussed. We have also characterized the reaction theoretically using quantum chemical computations. The relevant portion of the potential energy surface of C(2)H(3)O in its doublet electronic ground state has been investigated using density functional theory B3LYP6-311 + + G(3df,2p) and molecular orbital computations at the unrestricted coupled-cluster level of theory that incorporates all single and double excitations plus perturbative corrections for the triple excitations, along with the 6-311 + + G(3df,2p) basis set [(U)CCSD(T)6-311 + + G(3df,2p)] and using UCCSD(T)6-31G(d,p) optimized geometries. Five isomers, six dissociation products, and sixteen transition structures were characterized. The results confirm that the hydrogen abstraction producing C(2)H(2)+OH is the most facile reaction channel. For this channel, refined computations using (U)CCSD(T)6-311 + + G(3df,2p)(U)CCSD(T)6-311 + + G(d,p) and complete-active-space second-order perturbation theory/complete-active-space self-consistent-field theory (CASPT2/CASSCF) [B. O. Roos, Adv. Chem. Phys. 69, 399 (1987)] using the contracted atomic natural orbitals basis set (ANO-L) [J. Almlof and P. R. Taylor, J. Chem. Phys.86, 4070 (1987)] were performed, yielding zero-point energy-corrected potential energy barriers of 17 kJ mol(-1) and 15 kJ mol(-1), respectively. Transition-state theory rate constant calculations, based on the UCCSD(T) and CASPT2/CASSCF computations that also include H-atom tunneling and a hindered internal rotation, are in perfect agreement with the experimental values. Considering both our experimental and theoretical determinations, the rate constant can best be expressed, in modified Arrhenius form as k(1)(T) = (2.2 +/- 0.1) x 10(-21)T(3.05) exp[-(376 +/- 100)T] cm(3) s(-1) for the range 300-2000 K. Thus, at temperatures above 1500 K, reaction of C(2)H with H(2)O is predicted to be one of the dominant C(2)H reactions in hydrocarbon combustion.