Variable Reaction Coordinate Transition State Research Articles

Theoretical and kinetic study of the reaction of C2H3 + HO2 on the C2H3O2H potential energy surface

We systematically investigate the C2H3 + HO2 reaction combined with conventional transition state theory, variable reaction coordinate transition state theory and Rice–Ramsberger–Kassel–Marcus/master-equation theory.

Recombination of aromatic radicals with molecular oxygen

The addition of molecular oxygen to hydrocarbon radicals yields peroxy radicals (ROO), which are crucial species in both atmospheric and combustion chemistry. For aromatic radicals there is little known about the recombination kinetics, especially for the high temperatures of relevance to combustion. Here, we have employed direct CASPT2 based variable reaction coordinate transition state theory to predict the high pressure recombination rates for four prototypical aromatic hydrocarbon radicals: phenyl, benzyl, 1-naphthyl, and 2-naphthyl. The variation in the predicted rates is discussed in relation to their molecular structure. The predicted rate coefficients are in reasonably satisfactory agreement with the limited experimental data and are expected to find utility in chemical modeling studies of PAH growth and oxidation.

Theoretical kinetics studies on the unimolecular degradation of trifluoromethyl sulfur pentafluoride, SF5CF3

Abstract Thermal gas-phase unimolecular decomposition of the atmospherically important greenhouse molecule, SF 5 CF 3 , is studied theoretically. The density functional theory method M06-2X is used to explore the potential energy surface of the unimolecular reaction of SF 5 CF 3 . Molecular energies are refined by single-point energy calculations by the combination CBS-Q electronic structure method. Next, statistical rate theories are employed to compute the thermal rate coefficients for important product channels as a function of temperature and pressure. Variable reaction coordinate-transition state theory (VRC-TST) is employed to calculate the rate constants for the reaction channels involving bond dissociations. The pressure dependence of the thermal rate coefficients is accounted for by a master equation formalism. The major reaction channel for the dissociation of SF 5 CF 3 is found to be C S bond cleavage producing SF 5 and CF 3 radicals with the Arrhenius equation as k 1 = 2.0 y 10 16 s ⿿1 exp (⿿265.4 kJ mol ⿿1 /RT). The present theoretical work may inspire experimental works regarding the unimolecular decomposition of SF 5 CF 3 .

Pressure-dependent kinetics of initial reactions in iso-octane pyrolysis.

This study focuses on the studies of the main pressure-dependent reaction types of iso-octane (iso-C8H18) pyrolysis, including initial C-C bond fission of iso-octane, isomerization, and β-scission reactions of the alkyl radicals produced by the C-C bond fission of iso-octane. For the C-C bond fission of iso-octane, the minimum energy potentials are calculated at the CASPT2(2e,2o)/6-31+G(d,p)//CAS(2e,2o)/6-31+G(d,p) level of theory. For the isomerization and the β-scission reactions of the alkyl radicals, the optimization of the geometries and the vibrational frequencies of the reactants, transition states, and products are performed at the B3LYP/CBSB7 level, and their single point energies are calculated by using the composite CBS-QB3 method. Variable reaction coordinate transition state theory (VRC-TST) is used for the high-pressure limit rate constant calculation and Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) is used to calculate the pressure-dependent rate constants of these channels with pressure varying from 0.01-100 atm. The rate constants obtained in this work are in good agreement with those available from literatures. We have updated the rate constants and thermodynamic parameters for species involved in these reactions into a current chemical kinetic mechanism and also have improved the concentration profiles of main products such as C3H6 and C4H6 in the shock tube pyrolysis of iso-octane. The results of this study provide insight into the pyrolysis of iso-octane and will be helpful in the future development of branched paraffin kinetic mechanisms.

Pressure-dependent branching in the reaction of 1CH2 with C2H4 and other reactions on the C3H6 potential energy surface

A number of reactions on the C3H6 potential energy surface (PES) are of central importance to various combustion environments. High level theoretical methods were used to predict the temperature and pressure dependent kinetics for these reactions. The rovibrational properties of the key stationary points in the systems were determined with the CCSD(T)/cc-pVTZ method. High accuracy energies for these stationary points were obtained via the consideration of basis set, higher-order correlation, core–valence, anharmonic vibrational, relativistic, and diagonal Born–Oppenheimer corrections. Variable reaction coordinate transition state theory (VRC-TST) was employed to treat the barrierless channels on the PES, while, for channels possessing a distinct barrier, rate coefficients were instead obtained with conventional transition state theory employing rigid-rotor harmonic oscillator assumptions. For the VRC-TST calculations, the interaction energies required for the evaluation of the reactive flux were evaluated with the CASPT2/cc-pVTZ approach for the orientational sampling, coupled with one-dimensional corrections based on higher accuracy evaluations along the minimum energy path. A priori predictions for the temperature and pressure-dependent rate coefficients were obtained through master-equation calculations incorporating these TST predictions for the microcanonical rate coefficients coupled with a simple model for the collisional energy transfer rates. These theoretical predictions compare favorably with the limited experimental data. The analysis yields modified Arrhenius rate expressions for the 1CH2+C2H4, C2H3+CH3, CH2CHCH2+H, CH3CCH2+H, and CH3CHCH+H recombination reactions as well as for the dissociations and isomerizations of propene and cyclopropane.

Reaction of ketenyl radical with hydroxyl radical over C2H2O2 potential energy surface: A theoretical study

Abstract The potential energy surface of C 2 H 2 O 2 for HCCO + OH bimolecular reactions and the unimolecular decomposition of hydroxyketene (HOCH C O) are investigated employing high-level quantum chemical methods. Variable reaction coordinate transition state theory is used for the high-pressure limit rate constant calculation of the main reaction channels and RRKM-based multiwell master equation is used to calculate the pressure dependent rate constants and product branching ratios of these channels. The predicted rate constants are in good agreement with the limited experimental data available in the literature. The product distribution analysis shows that the association/decomposition of HCCO + OH to the formation of CO + 1 HCOH and CO + 3 HCOH channels are dominant in the whole temperature range of 500–2000 K below 1 atm, whereas at higher pressure and low temperature, the association reaction producing 3 CHCOOH becomes competitive. For the unimolecular decomposition of hydroxyketene, the formation of CO + 1 HCOH channel is found to be predominant over a wide range of temperatures and pressures. Rate constants of these reactions and thermodynamic parameters for species involved in these reactions are also provided.

Water binding energies of [Pb(amino acid-H)H2O]+ complexes determined by blackbody infrared radiative dissociation

The water binding energies (E(0)) of eight deprotonated Pb(2+)-amino acid (Aa) complexes of the form [Pb(Aa-H)H(2)O](+) (Aa = Gly, Ala, Val, Leu, Ile, Phe, Glu, and Lys) were determined using blackbody infrared radiative dissociation (BIRD). A Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer was used to trap ions generated by electrospray ionization (ESI) in a "zero"-pressure (~10(-10) torr) cell where dissociation can only occur by absorption of thermally generated photons. Since the [Pb(Aa-H)H(2)O](+) complexes have relatively few vibrational degrees of freedom (36-78) and are within the slow-exchange kinetic limit, the master equation was solved to extract meaningful threshold dissociation energies and thermal unimolecular dissociation rate constants (k(uni)). The master equation analysis uses variable reaction coordinate transition state theory (VRC-TST) to minimize the Rice-Ramsperger-Kassel-Marcus (RRKM) dissociation rate constants. The determined water binding energies range from 76.6 to 113.6 kJ mol(-1), and agree well with 0 K dissociation energies calculated using the B3LYP/6-31+G(d,p) and MP2(full)/6-311++G(2d,2p)//B3LYP/6-31+G(d,p) methods. The relative strengths of the binding energies reflect the known structural isomers (A-, B-, C-, and D-type) of these [Pb(Aa-H)H(2)O](+) complexes.

Statistical Theory for the Kinetics and Dynamics of Roaming Reactions

We present a statistical theory for the effect of roaming pathways on product branching fractions in both unimolecular and bimolecular reactions. The analysis employs a separation into three distinct steps: (i) the formation of weakly interacting fragments in the long-range/van der Waals region of the potential via either partial decomposition (for unimolecular reactants) or partial association (for bimolecular reactants), (ii) the roaming step, which involves the reorientation of the fragments from one region of the long-range potential to another, and (iii) the abstraction, addition, and/or decomposition from the long-range region to yield final products. The branching between the roaming induced channel(s) and other channels is obtained from a steady-state kinetic analysis for the two (or more) intermediates in the long-range region of the potential. This statistical theory for the roaming-induced product branching is illustrated through explicit comparisons with reduced dimension trajectory simulations for the decompositions of H(2)CO, CH(3)CHO, CH(3)OOH, and CH(3)CCH. These calculations employ high-accuracy analytic potentials obtained from fits to wide-ranging CASPT2 ab initio electronic structure calculations. The transition-state fluxes for the statistical theory calculations are obtained from generalizations of the variable reaction coordinate transition state theory approach. In each instance, at low energy the statistical analysis accurately reproduces the branching obtained from the trajectory simulations. At higher energies, e.g., above 1 kcal/mol, increasingly large discrepancies arise, apparently due to a dynamical biasing toward continued decomposition of the incipient molecular fragments (for unimolecular reactions). Overall, the statistical theory based kinetic analysis is found to provide a useful framework for interpreting the factors that determine the significance of roaming pathways in varying chemical environments.

Ab initio and kinetic study of the reaction of ketones with ȮH for T = 500–2000 K. Part I: hydrogen-abstraction from H3CC(O)CH3–x(CH3)x, x = 0 ↦ 2

A theoretical study is presented of the mechanism and kinetics of the reactions of the hydroxyl radical with three ketones: dimethyl (DMK), ethylmethyl (EMK) and iso-propylmethyl (iPMK) ketones. CCSD(T) values extrapolated to the basis set limit are used to benchmark the computationally less expensive methods G3 and G3MP2BH&H, for the DMK + OH reaction system. These latter methods are then used in computations involving the reactions of the larger ketones. All possible abstraction channels have been modeled. A stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel has been recognized in part of the abstracting processes. High-pressure limit rate constants of the title reactions have been calculated in the temperature range of 500-2000 K using the Variflex code including Eckart tunneling corrections. Variable reaction coordinate transition state theory (VRC-TST) has been used for the rate constants of the barrier-less entrance channel. Calculated total rate constants (cm(3) mol(-1) s(-1)) are reported as follows: k(DMK) = 1.32 × 10(2)×T(3.30)exp(503/T), k(EMK) = 3.84 × 10(1)×T(3.51)exp(1515/T), k(iPMK) = 2.08 × 10(1)×T(3.58)exp(2161/T). Group rate constants (on a per H atom basis) for different carbon sites in title reactions have also been provided.

Rate constants for hydrogen-abstraction by [formula omitted] from n-butanol

Rate constants of hydrogen-atom abstraction from n-butanol by the OH radical have been calculated using variational transition state theory and conventional transition state theory (the latter includes Eckart tunneling corrections) as implemented in Variflex. A stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel has been recognized in part of the abstracting processes. A two-transition-state model is used in calculating the individual rate constant for the channels with a barrier-less process in the entrance channel. Variable reaction coordinate transition state theory (VRC-TST) has been used to calculate the rate constants of the barrier-less process. Computed total and individual rate constants, based on a G3 potential energy surface, in the temperature range of 500–2000 K for n - butanol + O ˙ H are reported as follows (cm 3 mol −1 s −1): k t = ( 4.03 ± 3.33 ) × 10 1 × T 3.57 ± 0.10 exp [ ( 2128 ± 98 ) / T ] k α = ( 4.56 ± 2.65 ) × 10 3 × T 2.81 ± 0.07 exp [ ( 1855 ± 69 ) / T ] k β = ( 8.29 ± 8.94 ) × 10 - 1 × T 3.74 ± 0.13 exp [ ( 2050 ± 128 ) / T ] k γ = ( 5.09 ± 3.66 ) × 10 1 × T 3.35 ± 0.09 exp [ ( 2193 ± 85 ) / T ] k δ = ( 2.15 ± 0.93 ) × 10 4 × T 2.77 ± 0.05 exp [ ( − 93.3 ± 51.6 ) / T ] k oh = ( 5.88 ± 1.85 ) × 10 2 × T 2.82 ± 0.04 exp [ ( 294.2 ± 37.5 ) / T ] The computed individual and total rate constants based on the CCSD(T) potential energy surface over the same temperature range can be similarly expressed as: k t = ( 1.74 ± 2.20 ) × 10 1 × T 3.69 ± 0.16 exp [ ( 1703 ± 150 ) / T ] k α = ( 1.26 ± 1.49 ) × 10 3 × T 2.97 ± 0.15 exp [ ( 1339 ± 141 ) / T ] k β = ( 6.80 ± 7.27 ) × 10 - 1 × T 3.77 ± 0.13 exp [ ( 1848 ± 127 ) / T ] k γ = ( 1.19 ± 1.18 ) × T 3.79 ± 0.12 exp [ ( 2293 ± 118 ) / T ] k δ = ( 1.75 ± 0.77 ) × 10 4 × T 2.79 ± 0.06 exp [ ( − 220 ± 52 ) / T ] k oh = ( 5.22 ± 1.77 ) × 10 2 × T 2.84 ± 0.04 exp [ ( 317 ± 40 ) / T ] The computed total rate constants straddle the very recent shock tube measurements of the total reaction rate at 1017–1221 K, with the G3 results in particularly good agreement with experiment. Abstraction from the terminal methyl group is insignificant for temperatures below 500 K but becomes increasingly dominant at higher temperatures. Abstraction from the alcoholic group is never important.

Theoretical rate coefficients for allyl + HO2 and allyloxy decomposition

The kinetics of the allyl+HO2 bimolecular reaction, the thermal decomposition of C3H5OOH, and the unimolecular reactions of C3H5O are studied theoretically. High-level ab initio calculations of the C3H5OOH and C3H5O potential energy surfaces are coupled with RRKM master equation methods to compute the temperature- and pressure-dependence of the rate coefficients. Variable reaction coordinate transition state theory is used to characterize the barrierless transition states for the allyl+HO2 and C3H5O+OH reactions. The predicted rate coefficients for allyl+HO2→C3H5OOH→products are in good agreement with experimental values. The calculations for allyl+HO2→C3H6+O2 underpredict the observed rate. The new rate coefficients suggest that the reaction of allyl+HO2 will promote chain-branching significantly more than previous models suggest.

Ab initio kinetics for the decomposition of monomethylhydrazine (CH3NHNH2)

The decomposition kinetics of CH3NHNH2 (monomethylhydrazine) is studied with ab initio transition state theory-based master equation analyses. The simple NN and CN bond fissions to produce the radicals CH3NH+NH2 or CH3+NHNH2 are expected to dominate the decomposition kinetics. The transition states for these two bond fissions are studied with variable reaction coordinate transition state theory employing directly determined CASPT2/aug-cc-pVDZ interaction energies. Orientation independent corrections for limitations in the basis set and for the effects of conserved mode geometry relaxation are included. The bond dissociation energies are evaluated at the QCISD(T)/CBS//B3LYP/6-311++G(d,p) level. The transition state theory analysis directly provides high pressure dissociation and recombination rate coefficients. Predictions for the pressure dependence and product branching in the dissociation of CH3NHNH2 are obtained by solving the master equation.

Rate Constants for the Thermal Decomposition of Ethanol and Its Bimolecular Reactions with OH and D: Reflected Shock Tube and Theoretical Studies

The thermal decomposition of ethanol and its reactions with OH and D have been studied with both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for ethanol have been measured at high T in reflected shock waves using OH optical absorption and high-sensitivity H-atom ARAS detection. The three dissociation processes that are dominant at high T are C2H5OH--> C2H4+H2O (A) -->CH3+CH2OH (B) -->C2H5+OH (C).The rate coefficient for reaction C was measured directly with high sensitivity at 308 nm using a multipass optical White cell. Meanwhile, H-atom ARAS measurements yield the overall rate coefficient and that for the sum of reactions B and C , since H-atoms are instantaneously formed from the decompositions of CH(2)OH and C(2)H(5) into CH(2)O + H and C(2)H(4) + H, respectively. By difference, rate constants for reaction 1 could be obtained. One potential complication is the scavenging of OH by unreacted ethanol in the OH experiments, and therefore, rate constants for OH+C2H5OH-->products (D)were measured using tert-butyl hydroperoxide (tBH) as the thermal source for OH. The present experiments can be represented by the Arrhenius expression k=(2.5+/-0.43) x 10(-11) exp(-911+/-191 K/T) cm3 molecule(-1) s(-1) over the T range 857-1297 K. For completeness, we have also measured the rate coefficient for the reaction of D atoms with ethanol D+C2H5OH-->products (E) whose H analogue is another key reaction in the combustion of ethanol. Over the T range 1054-1359 K, the rate constants from the present experiments can be represented by the Arrhenius expression, k=(3.98+/-0.76) x10(-10) exp(-4494+/-235 K/T) cm3 molecule(-1) s(-1). The high-pressure rate coefficients for reactions B and C were studied with variable reaction coordinate transition state theory employing directly determined CASPT2/cc-pvdz interaction energies. Reactions A , D , and E were studied with conventional transition state theory employing QCISD(T)/CBS energies. For the saddle point in reaction A , additional high-level corrections are evaluated. The predicted reaction exo- and endothermicities are in good agreement with the current Active Thermochemical Tables values. The transition state theory predictions for the microcanonical rate coefficients in ethanol decomposition are incorporated in master equation calculations to yield predictions for the temperature and pressure dependences of reactions A - C . With modest adjustments (<1 kcal/mol) to a few key barrier heights, the present experimental and adjusted theoretical results yield a consistent description of both the decomposition (1-3) and abstraction kinetics (4 and 5). The present results are compared with earlier experimental and theoretical work.

Uncertainty driven theoretical kinetics studies for CH3OH ignition: HO2 + CH3OH and O2 + CH3OH

A combination of global uncertainty screening and ab initio theoretical chemical kinetics is used to iteratively improve the mechanism of Li et al. [3] for the ignition of methanol at high pressure. The initial application of the screening analysis indicates that the CH3OH+HO2 reaction dominates the uncertainty in the predicted ignition delay for stoichiometric CH3OH combustion at 1100K and 20bar. The rate coefficients for both product channels (CH2OH+H2O2 and CH3O+H2O2) in this reaction are predicted with ab initio transition state theory employing barriers and rovibrational properties obtained at the CCSD(T)/CBS//CASPT2/cc-pvtz level. The estimated uncertainty in these predictions is a factor of 2. The second iteration of the screening analysis indicates that the CH3OH+O2 reaction next dominates the uncertainty in the ignition delay at high pressure. The associated rate coefficient is updated using a two transition state model that employs CCSD(T)/CBS//CASPT2/cc-pvtz properties for the tight transition state and direct CASPT2/aug-cc-pvdz based variable reaction coordinate transition state theory for the barrierless formation of the long-range CH2OH…HO2 complex. The final predictions for the ignition delay are a factor of 4 greater than those with the original model and the width of the distributions of ignition delay relative to its peak value decreases by a factor of 3. Further reduction in the uncertainty will require more accurate predictions for the CH3OH+HO2 reaction and new predictions for the HO2+HO2 reaction. The predictions for the CH3OH+HO2→CH2OH+H2O2, CH3OH+HO2→CH3O+H2O2, and CH3OH+O2→CH2OH+HO2 rate constants are well represented over the 400–2500K temperature range, by the expressions 3.78×10−29T5.06exp(−5140/T), 5.54×10−26T4.12exp(−8170/T) and 5.95×10−19T2.27exp(−21520/T)cm3molecule−1s−1, respectively, where T is in K.

Theoretical rate coefficients for the reaction of methyl radical with hydroperoxyl radical and for methylhydroperoxide decomposition

The kinetics of the CH3+HO2 bimolecular reaction and the thermal decomposition of CH3OOH are studied theoretically. Direct variable reaction coordinate transition state theory (VRC-TST), coupled with high level multireference electronic structure calculations, is used to compute capture rates for the CH3+HO2 reaction and to characterize the transition state of the barrierless CH3O+OH product channel. The CH2O+H2O product channel and the CH3+HO2→CH4+O2 reaction are treated using variational transition state theory and the harmonic oscillator and rigid rotor approximations. Pressure dependence and product branching in the bimolecular and decomposition reactions are modeled using master equation simulations. The predicted rate coefficients for the major products channels of the bimolecular reaction, CH3O+OH and CH4+O2, are found to be in excellent agreement with values obtained in two recent modeling studies. The present calculations are also used to obtain rate coefficients for the CH3O+OH association/decomposition reaction.

A shock-tube and theory study of the dissociation of acetone and subsequent recombination of methyl radicals

The dissociation of acetone: CH3COCH3→CH3CO+CH3, quickly followed by CH3CO→CH3+CO, has been examined with Laser-Schlieren measurements in incident shock waves over 32–717Torr and 1429–1936K using 5% acetone dilute in krypton. A few very low pressure experiments (∼10Torr) were used in a marginal effort to resolve the extremely fast vibrational relaxation of this molecule. This effort was partly motivated as a test for molecular, “roaming methyl” reactions, and also as a source of methyl radicals to test the application of a recent high-temperature mechanism for ethane decomposition [J.H. Kiefer, S. Santhanam, N.K. Srinivasan, R.S. Tranter, S.J. Klippenstein, M.A. Oehlschlaeger, Proc. Combust. Inst. 30 (2005) 1129–1135] on the reverse methyl combination. The gradient profiles show strong initial positive gradients and following negative values fully consistent with methyl radical formation and its following recombination. Thus C–C fission is certainly a large part of the process and molecular channels cannot be responsible for more than 30% of the dissociation. Rates obtained for the C–C fission show strong falloff well fit by variable reaction coordinate transition state theory when combined with a master equation. The calculated barrier is 82.8kcal/mol, the fitted 〈ΔE〉down=400(T/298)cm−1, similar to what was found in a recent study of C–C fission in acetaldehyde, and the extrapolated k∞=1025.86T−2.72exp(−87.7(kcal/mol)/RT), which agrees with the literature rate for CH3+CH3CO. Large negative (exothermic) gradients appearing late from methyl combination are accurately fit in both time of onset and magnitude by the earlier ethane dissociation mechanism. The measured dissociation rates are in close accord with one earlier shock-tube study [K. Sato, Y. Hidaka, Combust. Flame 122 (2000) 291–311], but show much less falloff than the high pressure experiments of Ernst et al. [J. Ernst, K. Spindler, H.Gg. Wagner, Ber. Bunsenges. Phys. Chem. 80 (1976) 645–650].

Kinetics and Product Branching Ratios of the Reaction of 1CH2 with H2 and D2

The reactions of singlet methylene (a(1)A1 (1)CH2) with hydrogen and deuterium have been studied by experimental and theoretical techniques. The rate coefficients for the removal of singlet methylene with H2 (k1) and D2 (k2) have been measured from 195 to 798 K and are essentially temperature-independent with values of k1 = (10.48 +/- 0.32) x 10(-11) cm(3) molecule(-1) s(-1) and k2 = (5.98 +/- 0.34) x 10(-11) cm(3) molecule(-1) s(-1), where the errors represent 2sigma, giving a ratio of k1/k2 = 1.75 +/- 0.11. In the reaction with H2, singlet methylene can be removed by reaction giving CH3 + H or deactivated to ground-state triplet methylene. Direct measurement of the H atom product showed that the fraction of relaxation decreased from 0.3 at 195 K to essentially zero at 398 K. For the reaction with deuterium, either H or D may be eliminated. Experimentally, the H:D ratio was determined to be 1.8 +/- 0.5 over the range 195-398 K. Theoretically, the reaction kinetics has been predicted with variable reaction coordinate transition state theory and with rigid-body trajectory simulations employing various high-level, ab initio-determined potential energy surfaces. The magnitudes of the calculated rate coefficients are in agreement with experiment, but the calculations show a significant negative temperature dependence that is not observed in the experimental results. The calculated and experimental H to D ratios from the reaction of singlet methylene with D2 are in good agreement, suggesting that the reaction proceeds entirely through the formation of a long-lived methane intermediate with a statistical distribution of energy.

Synchrotron Photoionization Mass Spectrometry Measurements of Kinetics and Product Formation in the Allyl Radical (H2CCHCH2) Self-Reaction

Product channels for the self-reaction of the resonance-stabilized allyl radical, C3H5 + C3H5, have been studied with isomeric specificity at temperatures from 300-600 K and pressures from 1-6 Torr using time-resolved multiplexed photoionization mass spectrometry. Under these conditions 1,5-hexadiene was the only C6H10 product isomer detected. The lack of isomerization of the C6H10 product is in marked contrast to the C6H6 product in the related C3H3 + C3H3 reaction, and is due to the more saturated electronic structure of the C6H10 system. The disproportionation product channel, yielding allene + propene, was also detected, with an upper limit on the branching fraction relative to recombination of 0.03. Analysis of the allyl radical decay at 298 K yielded a total rate coefficient of (2.7 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), in good agreement with previous experimental measurements using ultraviolet kinetic absorption spectroscopy and a recent theoretical determination using variable reaction coordinate transition state theory. This result provides independent indirect support for the literature value of the allyl radical ultraviolet absorption cross-section near 223 nm.

The Temperature and Pressure Dependence of the Reactions H + O2 (+M) → HO2 (+M) and H + OH (+M) → H2O (+M)

The reactions H + O2 (+M) --> HO2 (+M) and H + OH (+M) --> H2O (+M) have been studied using high-level quantum chemistry methods. On the basis of potential energy hypersurfaces obtained at the CASPT2/aug-cc-pVTZ level of theory, high-pressure limiting rate coefficients have been calculated using variable reaction coordinate transition state theory. Over the temperature range 300-3000 K, the following expressions were obtained in units of cm(3) molecule(-1) s(-1): k(infinity)(H + O2) = (25T(-0.367) + (7.5 x 10(-2)) T(0.702)) x 10(-11) and k(infinity)(H + OH) = (4.17 x 10(-11)) T(0.234)exp (57.5/T). Available experimental data on the pressure dependence of the reactions were analyzed using a two-dimensional master equation. The following low-pressure limiting rate coefficients were obtained over the temperature range 300-3000 K in units of cm(6) molecule(-2) s(-1): k0(H + O2 + Ar) = (9.1x10(-29)) T(-1.404)exp (-134/T), k0(H + O2 + N2) = (2.0x10(-27)) T(-1.73)exp (-270/T), k0(H + OH + Ar) = (8.6x10(-28)) T(-1.527)exp (-185/T), and k0(H + OH + N2) = (1.25x10(-26)) T(-1.81)exp (-251/T). For the H + O2 reaction system, F(cent)(Ar) = 0.67 and F(cent)(N2) = 0.72 were obtained as center broadening factors, whereas F(cent)(Ar) = 0.72 and F(cent)(N2) = 0.73 were obtained for the H + OH reaction system. The calculations provide a good description of most of the experimental data, except the room temperature measurements on the H + OH (+M) --> H2O (+M) reaction.

Kinetics of CH + N2 Revisited with Multireference Methods

The potential energy surface for the CH + N2 reaction was reexamined with multireference ab initio electronic structure methods employing basis sets up to aug-cc-pvqz. Comparisons with related CCSD(T) calculations were also made. The multireference ab initio calculations indicate significant shortcomings in single reference based methods for two key rate-limiting transition states. Transition state theory calculations incorporating the revised best estimates for the transition state properties provide order of magnitude changes in the predicted rate coefficient in the temperature range of importance to the mechanism for prompt NO formation. At higher temperatures, two distinct pathways make a significant contribution to the kinetics. A key part of the transition state analysis involves a variable reaction coordinate transition state theory treatment for the formation of H + NCN from HNCN. The present predictions for the rate coefficients resolve the discrepancy between prior theory and very recent experimental measurements.