Experimental and theoretical studies of rate coefficients for the reaction O(3P)+C2H5OH at high temperatures

Rate coefficients of the reaction O(3P)+C2H5OH in the temperature range 782-1410 K were determined using a diaphragmless shock tube. O atoms were generated by photolysis of SO2 at 193 nm with an ArF excimer laser; their concentrations were monitored via atomic resonance absorption. Our data in the range 886-1410 K are new. Combined with previous measurements at low temperature, rate coefficients determined for the temperature range 297-1410 K are represented by the following equation: k(T)=(2.89+/-0.09)x10(-16)T1.62 exp[-(1210+/-90)/T] cm3 molecule(-1) s(-1); listed errors represent one standard deviation in fitting. Theoretical calculations at the CCSD(T)/6-311+G(3df, 2p)//B3LYP/6-311+G(3df) level predict potential energies of various reaction paths. Rate coefficients are predicted with the canonical variational transition state (CVT) theory with the small curvature tunneling correction (SCT) method. Reaction paths associated with trans and gauche conformations are both identified. Predicted total rate coefficients, 1.60 x 10(-22)T3.50 exp(16/T) cm3 molecule(-1) s(-1) for the range 300-3000 K, agree satisfactorily with experimental observations. The branching ratios of three accessible reaction channels forming CH3CHOH+OH (1a), CH2CH2OH+OH (1b), and CH3CH2O+OH (1c) are predicted to vary distinctively with temperature. Below 500 K, reaction 1a is the predominant path; the branching ratios of reactions 1b,c become approximately 40% and approximately 11%, respectively, at 2000 K.     

Unimolecular decomposition of ethyl hydroperoxide: ab initio/Rice-Ramsperger-Kassel-Marcus theoretical prediction of rate constants

Alkyl hydroperoxides are found to be important intermediates in the combustion and oxidation processes of hydrocarbons. However, studies of ethyl hydroperoxide (CH(3)CH(2)OOH) are limited. In this work, kinetics and mechanisms for unimolecular decomposition of CH(3)CH(2)OOH have been investigated. The potential energy surface of decomposition reactions have first been predicted at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level. The results show that the formation of CH(3)CH(2)O + OH via O-O direct bond dissociation is dominant, the branching ratio of which is over 99% in the whole temperature range from 300 to 1000 K, and its rate constant can be expressed as k1 = 9.26 × 10(52)T(-11.91)exp(-26879/T) s(-1) at 1 atm. The rate constants of the reaction CH(3)CH(2)OOH → CH(3)CH(2)O + OH at different temperatures and pressures have been calculated, which can help us to comprehend the reactions of CH(3)CH(2)OOH at experimental conditions.     

Thermal decomposition of ethanol. III. A computational study of the kinetics and mechanism for the CH3+C2H5OH reaction

The mechanism for the CH3+C2H5OH reaction has been investigated by the modified Gaussian-2 method based on the geometric parameters of the stationary points optimized at the B3LYP/6-311+G(d,p) level of theory. Five transition states have been identified for the production of CH4+CH3CHOH (TS1), CH4+CH3CH2O (TS2), CH4+CH2CH2OH (TS3), CH3OH+CH3CH2 (TS4), and CH3CH2OCH3+H (TS5) with the corresponding barriers 12.0, 13.2, 16.0, 44.7, and 49.9 kcal/mol, respectively. The predicted rate constants and branching ratios for the three lower-energy H-abstraction reactions were calculated using the conventional and variational transition state theory with quantum-mechanical tunneling corrections for the temperature range 300-3000 K. The predicted total rate constant, kt=8.36 x 10(-76) T(20.00) exp(5258/T) cm3 mol(-1) s(-1) (300-600 K) and 6.10 x 10(-25) T(4.10)exp(-4058/T) cm3 mol(-1) s(-1) (600-3000 K), agrees closely with existing experimental data in the temperature range 403-523 K. Similarly, the predicted rate constants for CH3+CH3CD2OH and CD3+C2H5OD are also in reasonable agreement with available low temperature kinetic data.     

Ab initio study on the oxidation of NCN by O (3P): prediction of the total rate constant and product branching ratios

The reaction of NCN with O is relevant to the formation of prompt NO according to the new mechanism, CH+N2-->cyclic-C(H)NN- -->HNCN-->H+NCN. The reaction has been investigated by ab initio molecular orbital and transition state theory calculations. The mechanisms for formation of possible product channels involved in the singlet and triplet potential energy surfaces have been predicted at the highest level of the modified GAUSSIAN-2 (G2M) method, G2M (CC1). The barrierless association/dissociation processes on the singlet surface were also examined with the third-order Rayleigh-Schr?dinger perturbation (CASPT3) and the multireference configuration interaction methods including Davidson's correction for higher excitations (MRCI+Q) at the CASPT3(6,6)/6-311+G(3df)//UB3LYP/6-311G(d) and MRCI+Q(6,6)/6-311+G(3df)//UB3LYP/6-311G(d) levels. The rate constants for the low-energy channels producing CO+N2, CN+NO, and N(4S)+NCO have been calculated in the temperature range of 200-3000 K. The results show that the formation of CN+NO is dominant and its branching ratio is over 99% in the whole temperature range; no pressure dependence was noted at pressures below 100 atm. The total rate constant can be expressed by: kt=4.23x10(-11) T0.15 exp(17/T) cm3 molecule(-1) s(-1).     

Experimental and theoretical investigations of rate coefficients of the reaction S(3P)+O2 in the temperature range 298-878 K

Rate coefficients of the reaction S+O(2) with Ar under 50 Torr in the temperature range 298-878 K were determined with the laser photolysis technique. S atoms were generated by photolysis of OCS with a KrF excimer laser at 248 nm; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO(2). Our measurements show that k(298 K)=(1.92+/-0.29)x10(-12) cm(3) molecule(-1) s(-1), in satisfactory agreement with previous reports. New data determined for 505-878 K show non-Arrhenius behavior; combining our results with data reported at high temperatures, we derive an expression k(T)=(9.02+/-0.27)x10(-19)T(2.11+/-0.15) exp[(730+/-120)/T] cm(3) molecule(-1) s(-1) for 298< or =T< or =3460 K. Theoretical calculations at the G2M (RCC2) level, using geometries optimized with the B3LYP/6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel RRKM calculations agree satisfactorily with experimental observations; the reaction channel via SOO(1A') dominates at T<500 K, whereas channels involving formation of SOO(3A") followed by isomerization to SO(2) before dissociation, and formation of SOO(1A") followed by direct dissociation, become important at high temperatures, accounting for the observed rapid increase in rate coefficient.     

Theoretical studies on the unimolecular decomposition of ethylene glycol

The unimolecular decomposition processes of ethylene glycol have been investigated with the QCISD(T) method with geometries optimized at the B3LYP/6-311++G(d,p) level. Among the decomposition channels identified, the H(2)O-elimination channels have the lowest barriers, and the C-C bond dissociation is the lowest-energy dissociation channel among the barrierless reactions (the direct bond cleavage reactions). The temperature and pressure dependent rate constant calculations show that the H(2)O-elimination reactions are predominant at low temperature, whereas at high temperature, the direct C-C bond dissociation reaction is dominant. At 1 atm, in the temperature range 500-2000 K, the calculated rate constant is expressed to be 7.63 × 10(47)T(-10.38) exp(-42262/T) for the channel CH(2)OHCH(2)OH → CH(2)CHOH + H(2)O, and 2.48 × 10(51)T(-11.58) exp(-43593/T) for the channel CH(2)OHCH(2)OH → CH(3)CHO + H(2)O, whereas for the direct bond dissociation reaction CH(2)OHCH(2)OH → CH(2)OH + CH(2)OH the rate constant expression is 1.04 × 10(71)T(-16.16) exp(-52414/T).     

Ab initio chemical kinetics for the OH+HNCN reaction

The kinetics and mechanism of the reaction of the cyanomidyl radical (HNCN) with the hydroxyl radical (OH) have been investigated by ab initio calculations with rate constants prediction. The single and triplet potential energy surfaces of this reaction have been calculated by single-point calculations at the CCSD(T)/6-311+G(3df,2p) level based on geometries optimized at the B3LYP/6-311+G(3df,2p) and CCSD/6-311++G(d,p) levels. The rate constants for various product channels in the temperature range of 300-3000 K are predicted by variational transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories. The predicted total rate constants can be represented by the expressions ktotal=2.66 x 10(+2)xT-4.50 exp(-239/T) in which T=300-1000 K and 1.38x10(-20)xT2.78 exp(1578/T) cm3 molecule(-1) s(-1) where T=1000-3000 K. The branching ratios of primary channels are predicted: k1 for forming singlet HON(H)CN accounts for 0.32-0.28, and k4 for forming singlet HONCNH accounts for 0.68-0.17 in the temperature range of 300-800 K. k2+k7 for producing H2O+NCN accounts for 0.55-0.99 in the high-temperature range of 800-3000 K. The branching ratios of k3 for producing HCN+HNO, k6 for producing H2N+NCO, k8 for forming 3HN(OH)CN, k9 for producing CNOH+3NH, and k5+k10 for producing NH2+NCO are negligible. The rate constants for key individual product channels are provided in a table for different temperature and pressure conditions.     

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.     

Reflected shock tube studies of high-temperature rate constants for OH + CH4 --> CH3 + H2O and CH3 + NO2 --> CH3O + NO

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm has been used to study the reactions OH + CH(4) --> CH(3) + H(2)O and CH(3) + NO(2) --> CH(3)O + NO. Over the temperature range 840-2025 K, the rate constants for the first reaction can be represented by the Arrhenius expression k = (9.52 +/- 1.62) x 10(-11) exp[(-4134 +/- 222 K)/T] cm(3) molecule(-1) s(-1). Since this reaction is important in both combustion and atmospheric chemistry, there have been many prior investigations with a variety of techniques. The present results extend the temperature range by 500 K and have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range 195-2025 K. A three-parameter expression describes the rate behavior over this temperature range, k = (1.66 x 10(-18))T(2.182) exp[(-1231 K)/T] cm(3) molecule(-1) s(-1). Previous theoretical studies are discussed, and the present evaluation is compared to earlier theoretical estimates. Since CH(3) radicals are a product of the reaction and could cause secondary perturbations in rate constant determinations, the second reaction was studied by OH radical production from the fast reactions CH(3)O --> CH(2)O + H and H + NO(2) --> OH + NO. The measured rate constant is 2.26 x 10(-11) cm(3) molecule(-1) s(-1) and is not dependent on temperature from 233 to 1700 K within experimental error.     

Theoretical study on the OH + CH3NHCOOCH3 reaction

The multiple-channel reactions OH + CH3NHC(O)OCH3 --> products are investigated by direct dynamics method. The optimized geometries, frequencies, and minimum energy path are all obtained at the MP2/6-311+G(d,p) level, and energetic information is further refined by the BMC-CCSD (single-point) method. The rate constants for every reaction channels, R1, R2, R3, and R4, are calculated by canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200-1000 K. The total rate constants are in good agreement with the available experimental data and the two-parameter expression k(T) = 3.95 x 10(-12) exp(15.41/T) cm3 molecule(-1) s(-1) over the temperature range 200-1000 K is given. Our calculations indicate that hydrogen abstraction channels R1 and R2 are the major channels due to the smaller barrier height among four channels considered, and the other two channels to yield CH3NC(O)OCH3 + H2O and CH3NHC(O)(OH)OCH3 + H2O are minor channels over the whole temperature range.     

Thermal decomposition of ethanol. 4. Ab initio chemical kinetics for reactions of H atoms with CH3CH2O and CH3CHOH radicals

The potential energy surfaces of H-atom reactions with CH(3)CH(2)O and CH(3)CHOH, two major radicals in the decomposition and oxidation of ethanol, have been studied at the CCSD(T)/6-311+G(3df,2p) level of theory with geometric optimization carried out at the BH&HLYP/6-311+G(3df,2p) level. The direct hydrogen abstraction channels and the indirect association/decomposition channels from the chemically activated ethanol molecule have been considered for both reactions. The rate constants for both reactions have been calculated at 100-3000 K and 10(-4) Torr to 10(3) atm Ar pressure by microcanonical VTST/RRKM theory with master equation solution for all accessible product channels. The results show that the major product channel of the CH(3)CH(2)O + H reaction is CH(3) + CH(2)OH under atmospheric pressure conditions. Only at high pressure and low temperature, the rate constant for CH(3)CH(2)OH formation by collisonal deactivation becomes dominant. For CH(3)CHOH + H, there are three major product channels; at high temperatures, CH(3)+CH(2)OH production predominates at low pressures (P < 100 Torr), while the formation of CH(3)CH(2)OH by collisional deactivation becomes competitive at high pressures and low temperatures (T < 500 K). At high temperatures, the direct hydrogen abstraction reaction producing CH(2)CHOH + H(2) becomes dominant. Rate constants for all accessible product channels in both systems have been predicted and tabulated for modeling applications. The predicted value for CH(3)CHOH + H at 295 K and 1 Torr pressure agrees closely with available experimental data. For practical modeling applications, the rate constants for the thermal unimolecular decomposition of ethanol giving key accessible products have been predicted; those for the two major product channels taking place by dehydration and C-C breaking agree closely with available literature data.     

Kinetics of the multichannel reaction of methanethiyl radical (CH3S*) with 3O2

The CH3S* + O2 reaction system is considered an important process in atmospheric chemistry and in combustion as a pathway for the exothermic conversion of methane-thiyl radical, CH3S*. Several density functional and ab initio computational methods are used in this study to determine thermochemical parameters, reaction paths, and kinetic barriers in the CH3S* + O2 reaction system. The data are also used to evaluate feasibility of the DFT methods for higher molecular weight oxy-sulfur hydrocarbons, where sulfur presents added complexity from its many valence states. The methods include: B3LYP/6-311++G(d,p), B3LYP/6-311++G(3df,2p), CCSD(T)/6-311G(d,p)//MP2/6-31G(d,p), B3P86/6-311G(2d,2p)//B3P86/6-31G(d), B3PW91/6-311++G(3df,2p), G3MP2, and CBS-QB3. The well depth for the CH3S* + 3O2 reaction to the syn-CH3SOO* adduct is found to be 9.7 kcal/mol. Low barrier exit channels from the syn-CH3SOO* adduct include: CH2S + HO2, (TS6, E(a) is 12.5 kcal/mol), CH3 + SO2 via CH3SO2 (TS2', E(a) is 17.8) and CH3SO + O (TS17, E(a) is 24.7) where the activation energy is relative to the syn-CH3SOO* stabilized adduct. The transition state (TS5) for formation of the CH3SOO adduct from CH3S* + O2 and the reverse dissociation of CH3SOO to CH3S* + O2 is relatively tight compared to typical association and simple bond dissociation reactions; this is a result of the very weak interaction. Reverse reaction is the dominant dissociation path due to enthalpy and entropy considerations. The rate constants from the chemical activation reaction and from the stabilized adduct to these products are estimated as functions of temperature and pressure. Our forward rate constant and CH3S loss profile are in agreement with the experiments under similar conditions. Of the methods above, the G3MP2 and CBS-QB3 composite methods are recommended for thermochemical determinations on these carbon-sulfur-oxygen systems, when they are feasible.     

Theoretical study on the mechanism of the (3)CH(2) + NO(2) reaction

The complex doublet potential energy surface of the CH(2)NO(2) system is investigated at the B3LYP/6-31G(d,p) and QCISD(T)/6-311G(d,p) (single-point) levels to explore the possible reaction mechanism of the triplet CH(2) radical with NO(2). Forty minimum isomers and 92 transition states are located. For the most relevant reaction pathways, the high-level QCISD(T)/6-311 + G(2df,2p) calculations are performed at the B3LYP/6-31G(d,p) geometries to accurately determine the energetics. It is found that the top attack of the (3)CH(2) radical at the N-atom of NO(2) first forms the branched open-chain H(2)CNO(2) a with no barrier followed by ring closure to give the three-membered ring isomer cC(H(2))ON-O b that will almost barrierlessly dissociate to product P(1) H(2)CO + NO. The lesser followed competitive channel is the 1,3-H-shift of a to isomer HCN(O)OH c, which will take subsequent cis-trans conversion and dissociation to P(2) OH + HCNO. The direct O-extrusion of a to product P(3) (3)O + H(2)CNO is even much less feasible. Because the intermediates and transition states involved in the above three channels are all lower than the reactants in energy, the title reaction is expected to be rapid, as is consistent with the measured large rate constant at room temperature. Formation of the other very low-lying dissociation products such as NH(2) + CO(2), OH + HNCO and H(2)O + NCO seems unlikely due to kinetic hindrance. Moreover, the (3)CH(2) attack at the end-O of NO(2) is a barrier-consumed process, and thus may only be of significance at very high temperatures. The reaction of the singlet CH(2) with NO(2) is also briefly discussed. Our calculated results may assist in future laboratory identification of the products of the title reaction.     

Experimental and theoretical investigation of rate coefficients of the reaction S(3P)+OCS in the temperature range of 298-985 K

The reaction S(3P)+OCS in Ar was investigated over the pressure range of 50-710 Torr and the temperature range of 298-985 K with the laser photolysis technique. S atoms were generated by photolysis of OCS with light at 248 nm from a KrF excimer laser; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO2. At pressures less than 250 Torr, our measurements give k(298 K)=(2.7+/-0.5)x10(-15) cm3 molecule-1 s-1, in satisfactory agreement with a previous report by Klemm and Davis [J. Phys. Chem. 78, 1137 (1974)]. New data determined for 407-985 K connect rate coefficients reported previously for T>or=860 and Tor=500 Torr, the reaction rate was enhanced. Theoretical calculations at the G2M(CC2) level, using geometries optimized with the B3LYP6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) calculations agree satisfactorily with experimental observations. According to our calculations, the singlet channel involving formation of SSCO followed by direct dissociation into S2(a 1Deltag)+CO dominates below 2000 K; SSCO is formed via intersystem crossing from the triplet surface. At low temperature and under high pressure the stabilization of OCS2, formed via isomerization of SSCO, becomes important; its formation and further reaction with S atoms partially account for the observed increase in the rate coefficient under such conditions.     

Experimental and ab initio study of the HO2.CH3OH complex: thermodynamics and kinetics of formation

Near-infrared spectroscopy was used to monitor HO2 formed by pulsed laser photolysis of Cl2-O2-CH3OH-N2 mixtures. On the microsecond time scale, [HO2] exhibited a time dependence consistent with a mechanism in which [HO2] approached equilibrium via HO2 + HO2.CH3OH (3, -3). The equilibrium constant for reaction 3, K(p), was measured between 231 and 261 K at 50 and 100 Torr, leading to standard reaction enthalpy and entropy values (1 sigma) of delta(r) = -37.4 +/- 4.8 kJ mol(-1) and delta(r) = -100 +/- 19 J mol(-1) K(-1). The effective bimolecular rate constant, k3, for formation of the HO2.CH3OH complex is .10(-15).exp[(1800 +/- 500)/T] cm3 molecule(-1) s(-1) at 100 Torr (1 sigma). Ab initio calculations of the optimized structure and energetics of the HO2.CH3OH complex were performed at the CCSD(T)/6-311++G(3df,3pd)//MP2(full)/6-311++G(2df,2pd) level. The complex was found to have a strong hydrogen bond (D(e) = 43.9 kJ mol(-1)) with the hydrogen in HO2 binding to the oxygen in CH3OH. The calculated enthalpy for association is delta(r) = -36.8 kJ mol(-1). The potentials for the torsion about the O2-H bond and for the hydrogen-bond stretch were computed and 1D vibrational levels determined. After explicitly accounting for these degrees of freedom, the calculated Third Law entropy of association is delta(r) = -106 J mol(-1) K(-1). Both the calculated enthalpy and entropy of association are in reasonably good agreement with experiment. When combined with results from our previous study (Christensen et al. Geophys. Res. Lett. 2002, 29; doi:10.1029/2001GL014525), the rate coefficient for the reaction of HO2 with the complex, HO2 + HO2.CH3OH, is determined to be (2.1 +/- 0.7) x 10(-11) cm3 molecule(-1) s(-1). The results of the present work argue for a reinterpretation of the recent measurement of the HO2 self-reaction rate constant by Stone and Rowley (Phys. Chem. Chem. Phys. 2005, 7, 2156). Significant complex concentrations are present at the high methanol concentrations used in that work and lead to a nonlinear methanol dependence of the apparent rate constant. This nonlinearity introduces substantial uncertainty in the extrapolation to zero methanol.     

Theoretical Study of Isomerization and Decomposition Reactions for Methyl-nitramine

The complex potential energy surface and reaction mechanisms for the unimolecular isomerization and decomposition of methyl-nitramine (CH3NHNO2) were theoretically probed at the QCISD(T)/6-311+G*//B3LYP/6-311+G* level of theory. The results demonstrated that there are four low-lying energy channels: (i) the NN bond fission pathway; (ii) a sequence of isomerization reactions via CH3NN(OH)O; (IS2a); (iii) the HONO elimination pathway; (iv) the isomerization and the dissociation reactions via CH3NHONO (IS3). The rate constants of each initial step (rate-determining step) for these channels were calculated using the canonical transition state theory. The Arrhenius expressions of the channels over the temperature range 298-2000 K are k6(T)=1014:8e-46:0=RT , k7(T)=1013:7e-42:1=RT , k8(T)=1013:6e-51:8=RT and k9(T)=1015:6e-54:3=RT s-1, respectively. The calculated overall rate constants is 6.9￡10-4 at 543 K, which is in good agreement with the experimental data. Based on the analysis of the rate constants, the dominant pathway is the isomerization reaction to form CH3NN(OH)O at low temperatures, while the NN bond fission and the isomerization reaction to produce CH3NHONO are expected to be competitive with the isomerization reaction to form CH3NN(OH)O at high temperatures.     

Absolute rate coefficient of the OH + CH(3)C(O)OH reaction at T = 287-802 K. The two faces of pre-reactive H-bonding

The rate constants for the reaction OH + CH3C(O)OH --> products (1) were determined over the temperature range 287-802 K at 50 and 100 Torr of Ar or N2 bath gas using pulsed laser photolysis generation of OH by CH3C(O)OH photolysis at 193 nm coupled with OH detection by pulsed laser-induced fluorescence. The rate coefficient displays a complex temperature dependence with a sharp minimum at 530 K, indicating the competition between a reaction proceeding through a pre-reactive H-bonded complex to form CH3C(O)O + H2O, expected to prevail at low temperatures, and a direct methyl-H abstraction channel leading to CH2C(O)OH + H2O, which should dominate at high temperatures. The temperature dependence of the rate constant can be described adequately by k1(287-802 K) = 2.9 x 10(-9) exp{-6030 K/T} + 1.50 x 10(-13) exp{515 K/T} cm3 molecule(-1)(s-1), with a value of (8.5 +/- 0.9) x 10-13 cm3 molecule(-1)(s-1) at 298 K. The steep increase in rate constant in the range 550-800 K, which is reported for the first time, implies that direct abstraction of a methyl-H becomes the dominant pathway at temperatures greater than 550 K. However, the data indicates that up to about 800 K direct methyl-H abstraction remains adversely affected by the long-range H-bonding attraction between the approaching OH radical and the carboxyl -C(O)OH functionality.     

High-temperature rate constants for CH3OH + Kr --> products, OH + CH3OH --> products, OH + (CH3)(2)CO --> CH2COCH3 + H2O, and OH + CH3 --> CH) + H2O

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the dissociation of methanol between 1591 and 2865 K. Rate constants for two product channels [CH3OH + Kr --> CH3 + OH + Kr (1) and CH3OH + Kr --> 1CH2 + H2O + Kr (2)] were determined. During the course of the study, it was necessary to determine several other rate constants that contributed to the profile fits. These include OH + CH3OH --> products, OH + (CH3)2CO --> CH2COCH3 + H2O, and OH + CH3 --> 1,3CH2 + H2O. The derived expressions, in units of cm(3) molecule(-1) s(-1), are k(1) = 9.33 x 10(-9) exp(-30857 K/T) for 1591-2287 K, k(2) = 3.27 x 10(-10) exp(-25946 K/T) for 1734-2287 K, kOH+CH3OH = 2.96 x 10-16T1.4434 exp(-57 K/T) for 210-1710 K, k(OH+(CH3)(2)CO) = (7.3 +/- 0.7) x 10(-12) for 1178-1299 K and k(OH+CH3) = (1.3 +/- 0.2) x 10(-11) for 1000-1200 K. With these values along with other well-established rate constants, a mechanism was used to obtain profile fits that agreed with experiment to within <+/-10%. The values obtained for reactions 1 and 2 are compared with earlier determinations and also with new theoretical calculations that are presented in the preceding article in this issue. These new calculations are in good agreement with the present data for both (1) and (2) and also for OH + CH3 --> products.     

Reaction of CH3 with NO2

The elementary reaction of the CH3 radical with NO2 was investigated by time-resolved FTIR spectroscopy and quantum chemical calculations. The CH3 radical was produced by laser photolysis of CH3Br or CH3I at 248 nm. Vibrationally excited products OH, HNO and CO2 were observed by the time-resolved spectroscopy for the first time. The formation of another product NO was also verified. According to these observations, the product channels leading to CH3O+NO, CH2NO+OH and HNO+H2CO were identified. The channel of CH3O+NO was the major one. The reaction mechanisms of the above channels were studied by quantum chemical calculations at CCSD(T)/6-311++G(df,p)//MP2/6-311G(d,p) level. The calculated results fit with the experimental observations well.     

Ab initio studies of CIO, reactions: prediction of the rate constants of CIO + NO for the forward and reverse processes.

The mechanisms for ClO+NO and its reverse reactions were investigated by means of ab initio molecular orbital and statistical theory calculations. The species involved were optimized at the B3LYP/6-311 +G(3df) level, and their energies were refined at the CCSD(T)/6-311+ G(3df)//B3LYP/6-311 + G(3df) level. Five isomers and the transition states among them were located. The relative stability of these isomers is ClNO2 > cis-ClONO > trans-ClONO > cis-OClNO>trans-OClNO. The heats of formation of the three most-stable isomers were predicted using isodesmic reactions by different methods. The predicted bimolecular reaction rate constant shows that, below 100 atm, the formation of Cl+NO2 is dominant and pressure-independent. The total rate constant can be expressed as: k(ClO+NO)= 1.43 x 10(-9)T(-083)exp(92/ T) cm3 molecule(-1)s(-1) in the temperature range of 200-1000 K, in close agreement with experimental data. For the reverse reaction, Cl+NO2-->ClNO2 and ClONO (cis and trans isomers), the sum of the predicted rate constants for the formation of the three isomers and their relative yields also reproduce the experimental data well. The predicted total third-order rate constants in the temperature range of 200-1000 K can be represented by: k0(He) = 4.89 x 10(-6)T(-5.85) exp(-796/T) cm6 molecule(-1)s(-1) and k0(N2) =5.72 x 10(-15)T(-5.80) exp(-814/T) cm6 molecule(-1)s(-1). The predicted high- and low-pressure limit decomposition rates of CINO2 in Ar in the temperature range 400-1500 K can be expressed, respectively, by: k-(ClNO2) = 7.25 x 10(19)T(-1.89) exp(-16875/T) s(-1) and kd(ClNO2) = 2.51 x 10(38)T(-6.8) exp(-18409/T) cm3 molecule(-1) s(-1). The value of k0(ClNO2) is also in reasonable agreement with available experimental data.