Quantum reactive scattering of H + hydrocarbon reactions

A practical quantum-dynamical method is described for predicting accurate rate constants for general chemical reactions. The ab initio potential energy surfaces for these reactions can be built from a minimal number of grid points (average of 50 points) and expressed in terms of analytical functionals. All the degrees of freedom except the breaking and forming bonds are optimised using the MP2 method with a cc-pVTZ basis set. Single point energies are calculated on the optimised geometries at the CCSD(T) level of theory with the same basis set. The dynamics of these reactions occur on effective reduced dimensionality hyper-surfaces accounting for the zero-point energy of the optimised degrees of freedom. Bonds being broken and formed are treated with explicit hyperspherical time independent quantum dynamics. Application of the method to the H + CH(4)--> H(2)+ CH(3), H + C(2)H(6)--> H(2)+ C(2)H(5), H + C(3)H(8)--> H(2)+n-C(3)H(7)/H(2)+i-C(3)H(7) and H + CH(3)OH --> H(2)+ CH(3)O/H(2)+ CH(2)OH reactions illustrate the potential of the approach in predicting rate constants, kinetic isotope effects and branching ratios. All studied reactions exhibit large quantum tunneling in the rate constants at lower temperatures. These quantum calculations compare well with the experimental results.     

Dynamics of the F atom reaction with propene

The F+C2H3CH3 reaction has been investigated using the crossed molecular beam technique. Three reaction channels have been observed in this reaction: H+C3H5F, CH3+C2H3F, and HF+C3H5. Time-of-flight spectra as well as product laboratory angular distributions have been measured for the HF, C2H3F, and C3H5F products from these three channels. Relative branching ratios of the three observed reaction channels have also been estimated. Experimental results indicate that these different channels exhibit significantly different reaction dynamics.     

New mechanistic insights to the O(3P) + propene reaction from multiplexed photoionization mass spectrometry

The reaction of O((3)P) with propene (C(3)H(6)) has been examined using tunable vacuum ultraviolet radiation and time-resolved multiplexed photoionization mass spectrometry at 4 Torr and 298 K. The temporal and isomeric resolution of these experiments allow the separation of primary from secondary reaction products and determination of branching ratios of 1.00, 0.91 ± 0.30, and 0.05 ± 0.04 for the primary product channels CH(3) + CH(2)CHO, C(2)H(5) + HCO, and H(2) + CH(3)CHCO, respectively. The H + CH(3)CHCHO product channel was not observable for technical reasons in these experiments, so literature values for the branching fraction of this channel were used to convert the measured product branching ratios to branching fractions. The results of the present study, in combination with past experimental and theoretical studies of O((3)P) + C(3)H(6), identify important pathways leading to products on the C(3)H(6)O potential energy surface (PES). The present results suggest that up to 40% of the total product yield may require intersystem crossing from the initial triplet C(3)H(6)O PES to the lower-lying singlet PES.     

A temperature-dependent relative-rate study of the OH initiated oxidation of n-butane: the kinetics of the reactions of the 1- and 2-butoxy radicals

The kinetics of the reactions of 1-and 2-butoxy radicals have been studied using a slow-flow photochemical reactor with GC-FID detection of reactants and products. Branching ratios between decomposition, CH3CH(O*)CH2CH3 --> CH3CHO + C2H5, reaction (7), and reaction with oxygen, CH3CH(O*)CH2CH3+ O2 --> CH3C(O)C2H5+ HO2, reaction (6), for the 2-butoxy radical and between isomerization, CH3CH2CH2CH2O* --> CH2CH2CH2CH2OH, reaction (9), and reaction with oxygen, CH3CH2CH2CH2O* + O2 --> C3H7CHO + HO2, reaction (8), for the 1-butoxy radical were measured as a function of oxygen concentration at atmospheric pressure over the temperature range 250-318 K. Evidence for the formation of a small fraction of chemically activated alkoxy radicals generated from the photolysis of alkyl nitrite precursors and from the exothermic reaction of 2-butyl peroxy radicals with NO was observed. The temperature dependence of the rate constant ratios for a thermalized system is given by k7/k6= 5.4 x 10(26) exp[(-47.4 +/- 2.8 kJ mol(-1))/RT] molecule cm(-3) and k9/k8= 1.98 x 10(23) exp[(-22.6 +/- 3.9 kJ mol(-1))/RT] molecule cm(-3). The results agree well with the available experimental literature data at ambient temperature but the temperature dependence of the rate constant ratios is weaker than in current recommendations.     

A complete look at the multi-channel dissociation of propenal photoexcited at 193 nm: branching ratios and distributions of kinetic energy

We observed fifteen photofragments upon photolysis of propenal (acrolein, CH(2)CHCHO) at 193 nm using photofragment translational spectroscopy and selective vacuum-ultraviolet (VUV) photoionization. All the photoproducts arise from nine primary and two secondary dissociation pathways. We measured distributions of kinetic energy of products and determined branching ratios of dissociation channels. Dissociation to CH(2)CHCO + H and CH(2)CH + HCO are two major primary channels with equivalent branching ratios of 33%. The CH(2)CHCO fragment spontaneously decomposes to CH(2)CH + CO. A proportion of primary products CH(2)CH from the fission of bond C-C of propenal further decompose to CHCH + H but secondary dissociation HCO → H + CO is negligibly small. Binary dissociation to CH(2)CH(2) (or CH(3)CH) + CO and concerted three-body dissociation to C(2)H(2) + CO + H(2) have equivalent branching ratios of 14%-15%. The other channels have individual branching ratios of ～1%. The production of HCCO + CH(3) indicates the formation of intermediate methyl ketene (CH(3)CHCO) and the production of CH(2)CCH + OH and CH(2)CC + H(2)O indicate the formation of intermediate hydroxyl propadiene (CH(2)CCHOH) from isomerization of propenal. Distributions of kinetic energy release and dissociation mechanisms are discussed. This work provides a complete look and profound insight into the multi-channel dissociation mechanisms of propenal. The combination of a molecular beam apparatus and synchrotron VUV ionization allowed us to untangle the complex mechanisms of nine primary and two secondary dissociation channels.     

Photodissociation and photoisomerization of alpha-fluorotoluene and 4-fluorotoluene in a molecular beam

The photodissociation of jet-cooled alpha-fluorotoluene and 4-fluorotoluene at 193 and 248 nm was studied using vacuum ultraviolet (vuv) photoionization/multimass ion imaging techniques as well as electron impact ionization/photofragment translational spectroscopy. Four dissociation channels were observed for alpha-fluorotoluene at both 193 and 248 nm, including two major channels C6H5CH2F-->C6H5CH2 (or C7H7)+F and C6H5CH2F-->C6H5CH (or C7H6)+HF and two minor channels C6H5CH2F-->C6H5CHF+H and C6H5CH2F-->C6H5+CH2F. The vuv wavelength dependence of the C7H7 fragment photoionization spectra indicates that at least part of the F atom elimination channel results from the isomerization of alpha-fluorotoluene to a seven-membered ring prior to dissociation. Dissociation channels of 4-fluorotoluene at 193 nm include two major channels C6H4FCH3-->C6H4FCH2+H and C6H4FCH3-->C6H4F+CH3 and two minor channels C6H4FCH3-->C6H5CH2 (or C7H7)+F and C6H4FCH3-->C6H5CH (or C7H6)+HF. The dissociation rates for alpha-fluorotoluene at 193 and 248 nm are 3.3 x 10(7) and 5.6 x 10(5) s(-1), respectively. The dissociation rate for 4-fluorotoluene at 193 nm is 1.0 x 10(6) s(-1). An ab initio calculation demonstrates that the barrier height for isomerization from alpha-fluorotoluene to a seven-membered ring isomer is much lower than that from 4-fluorotoluene to a seven-membered ring isomer. The experimental observed differences of dissociation rates and relative branching ratios between alpha-fluorotoluene and 4-fluorotoluene may be explained by the differences in the six-membered ring to seven-membered ring isomerization barrier heights, F atom elimination threshold, and HF elimination threshold between alpha-fluorotoluene and 4-fluorotoluene.     

Photodissociation dynamics of 1,2-butadiene at 157 nm

Photodissociation dynamics of 1,2-butadiene at 157 nm has been investigated using a molecular beam apparatus based on photoionization using vacuum ultraviolet synchrotron radiation. Six dissociation pathways have been observed. The observed channels are C4H5+H, C4H4+H2, C3H3+CH3, C2H3+C2H3, C2H4+C2H2, and C4H4+H+H. Among all the dissociation channels, the C3H3+CH3 channel is found to be the dominant process. The product kinetic energy distributions of all dissociation channels have been determined from simulating the experimental time-of-flight spectra. Relative branching ratios for all observed dissociation channels were also estimated based on all detected products.     

Structural isomers of C2N+: a selected-ion flow tube study

Reactivities of the structural isomers CCN+ and CNC+ were examined in a selected-ion flow tube at 300 +/- 5 K. The less reactive CNC+ isomer was identified as the product of the reactions of C(+) + HCN and C(+) + C2N2; in these reactions only CNC+ can be produced because of energy constraints. Rate coefficients and branching ratios are reported for the reactions of each isomer with H2, CH4, NH3, H2O, C2H2, HCN, N2, O2, N2O, and CO2. Ab initio calculations are presented for CCN+ and CNC+; a saddle point for the reaction CCN+ --> CNC+ is calculated to be 195 kJ mol-1 above the CNC+. The results provide evidence that the more reactive CCN+ isomer is unlikely to be present in measurable densities in interstellar clouds.     

Shock tube study of the reaction of CH with N2: overall rate and branching ratio

We have studied the reaction between CH and N2, (1) CH + N2 --> products, in shock tube experiments using CH and NCN laser absorption. CH was monitored by continuous-wave, narrow-line-width laser absorption at 431.1 nm. The overall rate coefficient of the CH + N2 reaction was measured between 1943 and 3543 K, in the 0.9-1.4 atm pressure range, using a CH perturbation approach. CH profiles recorded upon shock-heating dilute mixtures of ethane in argon and acetic anhydride in argon were perturbed by the addition of nitrogen. The perturbation in the CH concentration was principally due to the reaction between CH and N2. Rate coefficients for the overall reaction were inferred by kinetically modeling the perturbed CH profiles. A least-squares, two-parameter fit of the current overall rate coefficient measurements was k1 = 6.03 x 1012 exp(-11150/T [K]) (cm3 mol-1 s-1). The uncertainty in k1 was estimated to be approximately +/-25% and approximately +/-35% at approximately 3350 and approximately 2100 K, respectively. At high temperatures, there are two possible product channels for the reaction between CH and N2, (1a) CH + N2 --> HCN + N and (1b) CH + N2 --> H + NCN. The large difference in the rates of the reverse reactions enabled inference of the branching ratio of reaction 1, k1b/(k1b + k1a), in the 2228-2905 K temperature range by CH laser absorption in experiments in a nitrogen bath. The current CH measurements are consistent with a branching ratio of 1 and establish NCN and H as the primary products of the CH + N2 reaction. A detailed and systematic uncertainty analysis, taking into account experimental and mechanism-induced contributions, yields a conservative lower bound of 0.70 for the branching ratio. NCN was also detected by continuous-wave, narrow-line-width laser absorption at 329.13 nm. The measured NCN time histories were used to infer the rate coefficient of the reaction between H and NCN, H + NCN --> HCN + N, and to estimate an absorption coefficient for the NCN radical.     

Photodissociation dynamics of vinyl fluoride (CH2CHF) at 157 and 193 nm: Distributions of kinetic energy and branching ratios

Using photofragment translational spectroscopy and tunable vacuum-ultraviolet ionization, we measured the time-of-flight spectra of fragments upon photodissociation of vinyl fluoride (CH2CHF) at 157 and 193 nm. Four primary dissociation pathways--elimination of atomic F, atomic H, molecular HF, and molecular H2--are identified at 157 nm. Dissociation to C2H3 + F is first observed in the present work. Decomposition of internally hot C2H3 and C2H2F occurs spontaneously. The barrier heights of CH2CH --> CHCH + H and cis-CHCHF --> CHCH + F are evaluated to be 40+/-2 and 44+/-2 kcal mol(-1), respectively. The photoionization yield spectra indicate that the C2H3 and C2H2F radicals have ionization energies of 8.4+/-0.1 and 8.8+/-0.1 eV, respectively. Universal detection of photoproducts allowed us to determine the total branching ratios, distributions of kinetic energy, average kinetic energies, and fractions of translational energy release for all dissociation pathways of vinyl fluoride. In contrast, on optical excitation at 193 nm the C2H2 + HF channel dominates whereas the C2H3 + F channel is inactive. This reaction C2H3F --> C2H2 + HF occurs on the ground surface of potential energy after excitation at both wavelengths of 193 and 157 nm, indicating that internal conversion from the photoexcited state to the electronic ground state of vinyl fluoride is efficient. We computed the electronic energies of products and the ionization energies of fluorovinyl radicals.     

Gas-phase reactions of organic radicals and diradicals with ions

Reactions of polyatomic organic radicals with gas phase ions have been studied at thermal energy using a flowing afterglow-selected ion flow tube (FA-SIFT) instrument. A supersonic pyrolysis nozzle produces allyl radical (CH2CHCH2) and ortho-benzyne diradical (o-C6H4) for reaction with ions. We have observed: [CH2CHCH2 + H3O+ --> C3H6+ + H2O], [CH2CHCH2 + HO- --> no ion products], [o-C6H4 + H3O+ --> C6H5+ + H2O], and [o-C6H4 + HO- --> C6H3- + H2O]. The proton transfer reactions with H3O+ occur at nearly every collision (kII approximately with 10(-9) cm3 s(-1)). The exothermic proton abstraction for o-C6H4 + HO- is unexpectedly slow (kII approximately with 10(-10) cm3 s(-1)). This has been rationalized by competing associative detachment: o-C6H4 + HO- --> C6H5O + e-. The allyl + HO- reaction proceeds presumably via similar detachment pathways.     

Photodissociation of S atom containing amino acid chromophores

Photodissociation of 3-(methylthio)propylamine and cysteamine, the chromophores of S atom containing amino acid methionine and cysteine, respectively, was studied separately in a molecular beam at 193 nm using multimass ion imaging techniques. Four dissociation channels were observed for 3-(methylthio)propylamine, including (1) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)SCH(2)CH(2)CH(2)NH+H, (2) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)+SCH(2)CH(2)CH(2)NH(2), (3) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)S+CH(2)CH(2)CH(2)NH(2), and (4) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)SCH(2)+CH(2)CH(2)NH(2). Two dissociation channels were observed from cysteamine, including (5) HSCH(2)CH(2)NH(2)-->HS+CH(2)CH(2)NH(2) and (6) HSCH(2)CH(2)NH(2)-->HSCH(2)+CH(2)NH(2). The photofragment translational energy distributions suggest that reaction (1) and parts of the reactions (2), (3), (5) occur on the repulsive excited states. However, reaction (4), (6) occur only after the internal conversion to the electronic ground state. Since the dissociation from an excited state with a repulsive potential energy surface is very fast, it would not be quenched completely even in the condensed phase. Our results indicate that reactions following dissociation may play an important role in the UV photochemistry of S atom containing amino acid chromophores in the condensed phase. A comparison with the potential energy surface from ab initio calculations and branching ratios from RRKM calculations was made.     

A study of the hydrogen abstraction reactions of C2H radical with CH3CN, C2H5CN, and C3H7CN by dual-level generalized transition state theory

The hydrogen abstraction reactions C2H + CH3CN --> products (R1), C2H + CH3CH2CN --> products (R2), and C2H + CH3CH2CH2CN --> products (R3) have been investigated by dual-level generalized transition state theory. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) are performed at the BH&H-LYP and MP2 methods with the 6-311G(d, p) basis set, and the energy profiles are further refined at the MC-QCISD level of theory. The rate constants are evaluated using canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) over a wide temperature range 104-2000 K. The calculated CVT/SCT rate constants are in good agreement with the available experimental values. Our calculations show that for reaction R2, the alpha-hydrogen abstraction channel and beta-hydrogen abstraction channel are competitive over the whole temperature range. For reaction R3, the gamma-hydrogen abstraction channel is preferred at lower temperatures, while the contribution of beta-hydrogen abstraction will become more significant with a temperature increase. The branching ratio to the alpha-hydrogen abstraction channel is found negligible over the whole temperature range.     

Photofragment translational spectroscopy of 1,3-butadiene and 1,3-butadiene-1,1,4,4-d(4) at 193 nm

The photodissociation dynamics of 1,3-butadiene at 193 nm have been investigated with photofragment translational spectroscopy coupled with product photoionization using tunable VUV synchrotron radiation. Five product channels are evident from this study: C(4)H(5) + H, C(3)H(3) + CH(3), C(2)H(3) + C(2)H(3), C(4)H(4) + H(2), and C(2)H(4) + C(2)H(2). The translational energy (P(E(T))) distributions suggest that these channels result from internal conversion to the ground electronic state followed by dissociation. To investigate the dissociation dynamics in more detail, further studies were carried out using 1,3-butadiene-1,1,4,4-d(4). Branching ratios were determined for the channels listed above, as well as relative branching ratios for the isotopomeric species produced from 1,3-butadiene-1,1,4,4-d(4) dissociation. C(3)H(3) + CH(3) is found to be the dominant channel, followed by C(4)H(5) + H and C(2)H(4) + C(2)H(2), for which the yields are approximately equal. The dominance of the C(3)H(3) + CH(3) channel shows that isomerization to 1,2-butadiene followed by dissociation is facile.     

Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.     

Low-energy paths for the unimolecular decomposition of CH3OH: a G2M/statistical theory study

The potential energy surface (PES) of the CH3OH system has been characterized by ab initio molecular orbital theory calculations at the G2M level of theory. The mechanisms for the decomposition of CH3OH and the related bimolecular reactions, CH3 + OH and 1CH2 + H2O, have been elucidated. The rate constants for these processes have been calculated using variational RRKM theory and compared with available experimental data. The total decomposition rate constants of CH3OH at the high- and low-pressure limits can be represented by k infinity = 1.56 x 10(16) exp(-44,310/T) s-1 and kAr0 = 1.60 x 10(36) T-12.2 exp(-48,140/T) cm3 molecule-1 s-1, respectively, covering the temperature range 1000-3000 K, in reasonable agreement with the experimental values. Our results indicate that the product branching ratios are strongly pressure dependent, with the production of CH3 + OH and 1CH2 + H2O dominant under high (P > 10(3) Torr) and low (P < 1 atm) pressures, respectively. For the bimolecular reaction of CH3 and OH, the total rate constant and the yields of 1CH2 + H2O and H2 + HCOH at lower pressures (P < 5 Torr) could be reasonably accounted for by the theory. For the reaction of 1CH2 with H2O, both the yield of CH3 + OH and the total rate constant could also be satisfactorily predicted theoretically. The production of 3CH2 + H2O by the singlet to triplet surface crossing, predicted to occur at 4.3 kcal mol-1 above the H2C...OH2 van der Waals complex (which lies 82.7 kcal mol-1 above CH3OH), was neglected in our calculations.     

High pressure pyrolysis of toluene. 1. Experiments and modeling of toluene decomposition

The pyrolysis of toluene, the simplest methyl-substituted aromatic molecule, has been studied behind reflected shock waves using a single pulse shock tube. Experiments were performed at nominal high pressures of 27 and 45 bar and spanning a wide temperature range from 1200 to 1900 K. A variety of stable species, ranging from small hydrocarbons to single ring aromatics (principal soot precursors such as phenylacetylene and indene) were sampled from the shock tube and analyzed using standard gas chromatographic techniques. A detailed chemical kinetic model with 262 reactions and 87 species was assembled to simulate the stable species profiles (specifically toluene, benzene and methane) from the current high-pressure pyrolysis data sets and shock tube-atomic resonance absorption spectrometry (ARAS) H atom profiles obtained from prior toluene pyrolysis experiments performed under similar high-temperature conditions and lower pressures from 1.5 to 8 bar. The primary steps in toluene pyrolysis represent the most sensitive and dominant reactions in the model. Consequently, in the absence of unambiguous direct experimental measurements, we have utilized recent high level theoretical estimates of the barrierless association rate coefficients for these primary reactions, C6H5 + CH3 --> C6H5CH3 (1a) and C6H5CH2 + H --> C6H5CH3 (1b) in the detailed chemical kinetic model. The available data sets can be successfully reconciled with revised values for deltaH0f(298K)(C6H5CH2) = 51.5 +/- 1.0 kcal/mol and deltaH0f(298K)(C6H5) = 78.6 +/- 1.0 kcal/mol that translate to primary dissociation rate constants, reverse of 1a and 1b, represented by k(-1a,infinity) = (4.62 x 10(25))T(-2.53)exp[-104.5 x 10(3)/RT] s(-1) and k(-1b,infinity) = (1.524 x 10(16))T(-0.04)exp[-93.5 x 10(3)/RT] s(-1) (R in units of cal/(mol K)). These high-pressure limiting rate constants suggest high-temperature branching ratios for the primary steps that vary from 0.39 to 0.52 over the temperature range 1200-1800 K.     

Mutual sensitization of the oxidation of nitric oxide and a natural gas blend in a JSR at elevated pressure: experimental and detailed kinetic modeling study

The mutual sensitization of the oxidation of NO and a natural gas blend (methane-ethane 10:1) was studied experimentally in a fused silica jet-stirred reactor operating at 10 atm, over the temperature range 800-1160 K, from fuel-lean to fuel-rich conditions. Sonic quartz probe sampling followed by on-line FTIR analyses and off-line GC-TCD/FID analyses were used to measure the concentration profiles of the reactants, the stable intermediates, and the final products. A detailed chemical kinetic modeling of the present experiments was performed yielding an overall good agreement between the present data and this modeling. According to the proposed kinetic scheme, the mutual sensitization of the oxidation of this natural gas blend and NO proceeds through the NO to NO2 conversion by HO2, CH3O2, and C2H5O2. The detailed kinetic modeling showed that the conversion of NO to NO2 by CH3O2 and C2H5O2 is more important at low temperatures (ca. 820 K) than at higher temperatures where the reaction of NO with HO2 controls the NO to NO2 conversion. The production of OH resulting from the oxidation of NO by HO2, and the production of alkoxy radicals via RO2 + NO reactions promotes the oxidation of the fuel. A simplified reaction scheme was delineated: NO + HO2 --> NO2 + OH followed by OH + CH4 --> CH3 + H2O and OH + C2H6 --> C2H5 + H2O. At low-temperature, the reaction also proceeds via CH3 + O2 (+ M) --> CH3O2 (+ M); CH3O2 + NO --> CH3O + NO2 and C2H5 + O2 --> C2H5O2; C2H5O2 + NO --> C2H5O + NO2. At higher temperature, methoxy radicals are produced via the following mechanism: CH3 + NO2 --> CH3O + NO. The further reactions CH3O --> CH2O + H; CH2O + OH --> HCO + H2O; HCO + O2 --> HO2 + CO; and H + O2 + M --> HO2 + M complete the sequence. The proposed model indicates that the well-recognized difference of reactivity between methane and a natural gas blend is significantly reduced by addition of NO. The kinetic analyses indicate that in the NO-seeded conditions, the main production of OH proceeds via the same route, NO + HO2 --> NO2 + OH. Therefore, a significant reduction of the impact of the fuel composition on the kinetics of oxidation occurs.     

Theoretical and experimental study of the product branching in the reaction of acetic acid with OH radicals

The product distribution of the reaction of acetic acid, CH(3)COOH, with hydroxyl radicals, OH, was studied experimentally and theoretically. Mass-spectrometric measurements at 290 K and 2 Torr of He of the CO(2) yield versus the loss of acetic acid yielded a branching fraction of 64 +/- 14% for the abstraction of the acidic hydrogen as follows: CH(3)COOH + OH --> CH(3)COO + H(2)O --> CH(3) + CO(2) + H(2)O. A quantum chemical and theoretical kinetic analysis showed that the abstraction of the acidic hydrogen is enhanced relative to the abstraction of -CH(3) hydrogens because of the formation of a strong pre-reactive H-bonded complex, where the H-bonds are retained in the H-abstraction transition state. The potential energy surface of the reaction is explored in detail, and the reaction products of the individual channels are identified. The theoretical product branching is found to be critically dependent on the energetic and rovibrational differences between the H-abstraction transition states.