A shock tube study of the reaction of methyl radicals with hydroxyl radicals

The reaction of CH₃ with OH has been studied near 1200 K and 1 atmosphere pressure in shock tube experiments in which UV absorption was used to monitor [OH]. A rate coefficient of (1.1 ± 0.3) × 10¹³ cm³/mol-s was measured for removal of OH by CH₃. This measured value is compared with previous experimental data and calculations. Several possible reaction channels are discussed, and although products were not monitored, it seems probable, on the basis of other work and theoretical estimates, that the primary mechanism (?75%) for the removal of OH by CH₃ at these conditions is their combination to form CH₃OH. Rate coefficients of (5.3 ± 0.8) × 10¹² and (9.0 ± 1.4) × 10¹² cm³/mol-s were measured for the reactions of OH with acetone and ethane, respectively, at the same temperature and pressure.     

A computational study of the reaction of methane with methyl radical

The activation energy and optimized transition-state geometry for the abstraction of a hydrogen atom from methane by methyl radical have been calculated by the semiempirical methods MINDO /3 and MNDO . These results are compared with other semiempirical and ab initio results. The MINDO /3 method, based upon accuracy of the computed energy of activation, appears to be the computational method of greatest reliability. A method of locating the transition state on semiempirical surfaces is demonstrated.     

A computational study of the reaction of ground-state nitrogen atoms with chloromethyl radicals

A computational study of the N(4S) + CH2Cl reaction has been carried out. The first step of the reaction is the formation of an initial intermediate (NCH2Cl), which is relatively stable and does not involve any energy barrier. The two most exothermic products are those resulting from the release of a chlorine atom, H2C=N + Cl and trans-HC=NH + Cl. A kinetic study within the framework of the statistical theories suggests that the kinetically preferred product is also the most exothermic one. This is in contrast with the analogue reaction of nitrogen atoms with CH2F, where the preferred product from both thermodynamic and kinetic points of view is HFCN + H. Therefore, reactions of nitrogen atoms with chloromethyl radicals release chlorine atoms as major products. The rate coefficient for the title reaction is estimated to be about 3.09 x 10(-13) cm3 s(-1) molecule(-1) at 300 K, a value four times smaller than the rate coefficient for its fluorine analogue.     

A computational study of the thermochemistry of bromine- and iodine-containing methanes and methyl radicals

Bromo- and iodomethanes and the corresponding halogenated methyl radicals have been investigated by ab initio methods. Geometries and vibrational frequencies were derived with quadratic configuration interaction methods at the QCISD/6-311G(d,p) level of theory, and energies via QCISD(T)/6-311+G(3df,2p). Core electrons were represented with relativistic effective potentials. Anharmonicity of the out-of-plane bending modes in the methyl radicals was taken into account by numerical integration of the Schr?dinger equation with potentials derived from relaxed scans of these modes. The results are in good accord with experimental data where available. Thermochemistry derived via isodesmic reactions referenced to CH3, CH4, and monohalomethanes yields excellent accord with new experiments on dihalomethanes and provides recommendations for the more poorly characterized tri- and tetrahalomethanes and halomethyl radicals. For the methanes CH2Br2, CHBr3, CBr4, CH2I2, CHI3, CI4, CH2BrI, CHBr2I, and CHBrI2 we compute DeltafH degrees (298) values of 4.3, 51.6, 110.6, 108.1, 208.5, 321.3, 56.8, 104.8, and 157.1 kJ mol(-1), respectively. For the methyl radicals CH2Br, CHBr2, CBr3, CH2I, CHI2, CI3, CHBrI, CBr2I, and CBrI2 we compute DeltafH degrees (298) values of 166.6, 191.7, 224.0, 217.2, 290.4, 369.1, 241.6, 320.8, and 272.3 kJ mol(-1), respectively. Recommended confidence limits are +/-3 kJ mol(-1) per Br or I atom. Trends in these values and the corresponding C-H bond strengths are discussed and compared with prior experiments, empirical estimation schemes, and ab initio calculations.     

A computational study of the kinetics and mechanism for the reaction of HCO with HNO

The mechanism for the reaction of HCO with HNO has been studied at the G2M level of theory, based on the geometric parameters optimized by the BH&HLYP/6‐311G(d, p) method. There are three direct hydrogen abstraction channels producing (1) H₂CO + NO, (2) H₂NO + CO, and (3) HNOH + CO with barriers of 3.7, 3.9, and 10.4 kcal/mol, respectively. Another important reaction channel, (4), involves an association process forming HN(O)CHO (LM1) with a very small barrier and the subsequent isomerization and decomposition of LM1 producing HNOH + CO as major products. The rate constants of the dominant reaction channels (1), (2), and (4) in the temperature range 200–3000 K have been predicted by the microcanonical RRKM and transition state theory calculations with Eckart tunneling corrections. The theoretical result shows that in the high temperature range ( T > 1500 K), k₁ (H₂CO + NO) and k₂(H₂NO + CO) are preponderant, while in the low temperature range, both k₄(LM1) and k₄(HNOH + CO) appear to be dominant at high and low pressures, respectively. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 205–215, 2004     

The rate of reaction of methyl radicals with ozone

The absolute rate constant for the reaction of methyl radicals with ozone has been measured as a function of temperature. Small concentrations of CH₃ were generated by flash photolyzing CH₃NO₂ at 193 nm with an ArF laser. A photoionization mass spectrometer was used to follow the rate of decay of CH₃ at various ozone concentrations. The resulting rate constants could be fit by the expressions over the temperature range of 243–384 K. These rate constants can be modeled by simple transition state theory using reasonable parameters for the activated complex. Use of this rate constant shows that less than 1% of the methyl radicals formed in the stratosphere react with ozone.     

Shock-initiated decomposition of tetramethylgermane and the reaction of methyl radicals with toluene

The shock-initiated decomposition of tetramethylgermane (1078–1242 K) has been found to involve successive elimination of methyl radicals with the rate constant k₁ for the first step given by In the presence of excess toluene the products were CH₄ (major), C₂H₄, and C₂H₆. Results relevant to the reaction of methyl radicals with toluene compared to methyl radical recombination are discussed.     

Kinetics of the reaction of OH radicals with t-amyl methyl ether revisited

The kinetics of the reaction of OH radicals with t-amyl methyl ether (TAME) have been reinvestigated using both absolute (flash photolysis resonance fluorescence) and relative rate techniques. Relative rate experiments were conducted at 295 K in 99 kPa (740 torr) of synthetic air using ethyl t-butyl ether, cyclohexane, and di-isopropyl ether as reference compounds. Absolute rate experiments were performed over the temperature range 240–400 K at a total pressure of 4.7 kPa (35 torr) of argon. Rate constant determinations from both techniques are in good agreement and can be represented by k₁=(6.32 ± 0.72) × 10^?12 exp[(?40 ± 70)/T] cm³ molecule^?1 s^?1. Quoted errors represent 2σ from the least squares analysis and do not include any estimate of systematic errors. We show that results from the previous kinetic study of reaction (1) are in error due to the presence of a reactive impurity. Results are discussed in terms of the atmospheric chemistry of TAME. © 1993 John Wiley & Sons, Inc.     

Spectroscopy and reaction kinetics of HS radicals

A laser-induced fluorescence technique is used to monitor the HS radical concentration. The rate constants at room temperature have been determined for the reaction of HS radical with various added scavengers.     

A computational study of the addition reaction of cyclopropenylidene with methyleneimine

The reaction mechanism between cyclopropenylidene and methyleneimine has been systematically investigated at the MP2/6–31+G* level of theory, including geometry optimization, vibrational analysis, and energy property for the involved stationary points on the potential energy surface. The energies of the different species are calculated by the single point energy calculations of CCSD(T)/6-31+G*//MP2/6-31+G* level. It was found that an important initial intermediate (INTA) characterized by spiro-compound structure has been located along the three pathways (1), (2R), and (2L) firstly. After that, another common intermediate (INTB) has been formed via TSB. At last, three different products possessing three- and four-membered ring characters have been obtained through corresponding reaction pathways. In the first reaction pathway (1), a three-membered ring alkyne compound has been obtained. As for the other two reaction pathways (2R) and (2L), the four-membered ring conjugated diene compound has been produced. As a result, the energy barrier of the rate-determining step of the pathway (1) is lower than that of the pathway (2R) and (2L), and the ultima product of pathway (2R) and (2L) is more stable than that of the pathway (1).     

Thermochemistry and kinetics of the reaction of methyl mercaptan with iodine

The gas-phase reaction CH₃SH + I₂ has been studied spectrophotometrically over the temperature range of 476–604 K. It was found that the reaction undergoes H abstraction by I at ≤575 K, leading to the formation of MeSI and followed by a secondary reaction which leads to the formation of MeSSMe: Taking into consideration the effect of reaction (2), the equilibrium constant K₁ (554 K) has been evaluated to be 0.025 ± 0.004. This value was combined with the estimated values S (CH₃SI, g) = 73.7 ± 1.0 eu and 〈ΔC〉 = 0.87 ± 0.3 eu to obtain ΔH = 4.03 ± 0.73 kcal/mol. This yields ΔH (CH₃SI, g) = 7.16 ± 0.73 kcal/mol when combined with known thermochemical values for CH₃SH, HI, and I₂. A kinetic study was vitiated by the concurrent heterogeneous reaction of MeSH and I₂ at lower temperatures and the rather complicated chemistry occurring at elevated temperatures. However, attempts at measuring rate constants at 554 K lead to a lower limit of ΔH (CH₃S·, g) ≥ 29.5 ± 2 kcal/mol when an estimated value of A = 10^{10.8 ± 0.2} L/mol·s for the reactionc is used. DH (CH₃S–I) is estimated to be 49.3 ± 1.7 kcal/mol. The bond strengths of some divalent sulfurs and the reaction mechanisms are discussed. A crude estimate of DH⁰(H–CH₂SH) = 96 ± 1 kcal has been obtained from the kinetic data.     

Kinetics into the steady state. I. study of the reaction of oxygen atoms with methyl radicals

The exponential relaxation of CH₃, produced by the reaction O + C₂H₄ → CH₃ + HCO, to its steady-state concentration was quantitatively monitored after the reactants were mixed. The relaxation profiles yield the rate constant of the reaction O + CH₃ → H₂CO + H equal to (1.85 ± 0.28) × 10^-10 cm³/molecule-sec at 300°K. Ancillary experiments yielded values for the rate constant for the reaction of O atoms with C₂H₄ at 300°K, the average of which is 7.7 × 10^-12 cm³/molecule-sec. The experimental technique, which employs a fast-flow reactor coupled to a photoionization mass spectrometer, is described in detail and its potential discussed.     

A kinetics study of the reaction of Cl with NO2

The third order rate coefficients for the addition reaction of Cl with NO₂, Cl + NO₂ + M → ClNO₂ (ClONO) + M; k₁, were measured to be k₁(He) = (7.5 ± 1.1) × 10^?31 cm⁶ molecule^?2 s^?1 and k₁(N₂) = (16.6 ± 3.0) × 10^?31 cm⁶ molecule^?2 s^?1 at 298 K using the flash photolysis-resonance fluorescence method. The pressure range of the study was 15 to 500 torr He and 19 to 200 torr N₂. The temperature dependence of the third order rate coefficients were also measured between 240 and 350 K. The 298 K results are compared with those from previous low pressure studies.     

Ab initio reaction kinetics of hydrogen abstraction from methyl formate by hydrogen, methyl, oxygen, hydroxyl, and hydroperoxy radicals

Combustion of renewable biofuels, including energy-dense biodiesel, is expected to contribute significantly toward meeting future energy demands in the transportation sector. Elucidating detailed reaction mechanisms will be crucial to understanding biodiesel combustion, and hydrogen abstraction reactions are expected to dominate biodiesel combustion during ignition. In this work, we investigate hydrogen abstraction by the radicals H·, CH(3)·, O·, HO(2)·, and OH· from methyl formate, the simplest surrogate for complex biodiesels. We evaluate the H abstraction barrier heights and reaction enthalpies, using multireference correlated wave function methods including size-extensivity corrections and extrapolation to the complete basis set limit. The barrier heights predicted for abstraction by H·, CH(3)·, and O· are in excellent agreement with derived experimental values, with errors ≤1 kcal/mol. We also predict the reaction energetics for forming reactant complexes, transition states, and product complexes for reactions involving HO(2)· and OH·. High-pressure-limit rate constants are computed using transition state theory within the separable-hindered-rotor approximation for torsions and the harmonic oscillator approximation for other vibrational modes. The predicted rate constants differ significantly from those appearing in the latest combustion kinetics models of these reactions.     

MINDO/2 calculations on the reaction of methyl radicals with ethylene and butadiene

Semiempirical calculations using the MINDO/2 procedure have been carried out on the potential surface for the reaction of a methyl radical with ethylene and trans-butadiene. The transition state is predicted to be reactant-like in character and no evidence of resonance stabilization of the activated complex is found for butadiene. It is conjectured that the experimentally observed lowering of the activation energy for butadiene relative to ethylene may be attributed to differential correlation effects.     

A computational mechanistic study of the deamination reaction of melamine

A detailed computational study of the deamination reaction of melamine by OH^–, n H₂O/OH^–, n H₂O (where n = 1, 2, 3), and protonated melamine with H₂O, has been carried out using density functional theory and ab initio calculations. All structures were optimized at M06/6‐31G(d) level of theory, as well as with the B3LYP functional with each of the basis sets: 6‐31G(d), 6‐31 + G(d), 6‐31G(2df,p), and 6‐311++G(3df,3pd). B3LYP, M06, and ω B97XD calculations with 6‐31 + G(d,p) have also been performed. All structures were optimized at B3LYP/6‐31 + G(d,p) level of theory for deamination simulations in an aqueous medium, using both the polarizable continuum solvation model and the solvation model based on solute electron density. Composite method calculations have been conducted at G4MP2 and CBS‐QB3. Fifteen different mechanistic pathways were explored. Most pathways consisted of two key steps: formation of a tetrahedral intermediate and in the final step, an intermediate that dissociates to products via a 1,3‐proton shift. The lowest overall activation energy, 111 kJ mol^?1 at G4MP2, was obtained for the deamination of melamine with 3H₂O/OH^?.     

Kinetic study of the reaction of BrO radicals with dimethylsulfide

The kinetics and mechanism of the reaction of BrO with dimethylsulfide (DMS) have been studied by the mass spectrometric discharge-flow method in the temperature range (233–320) K and at a total pressure around 1 torr. The temperature dependence of the reaction rate constant k₁ = (1.5 ± 0.4) × 10⁻¹⁴ exp [(845 ± 175)/T] cm³ molecule⁻¹s⁻¹ has been determined under pseudo-first-order conditions in excess of DMS over BrO radicals. Mass spectrometric calibration of the reaction product dimethylsulfoxide (DMSO) allowed for a determination of the branching ratio of (0.94 ± 0.11) for the DMSO forming channel. These data indicate that the reaction is likely to proceed through a channel involving a long-lived intermediate: BrO + CH₃SCH₃ →[CH₃S(OBr)CH₃]* → CH₃S(O)CH₃ + Br. The atmospheric application of the data is briefly discussed. © 1996 John Wiley & Sons, Inc.     

The reaction of anthracene with OH radicals: an experimental study of the kinetics between 58 and 470 K

The first direct measurement of the reaction rate constant of a polycyclic aromatic hydrocarbon in the gas phase in the temperature range 58-470 K is reported. The reaction is OH+ anthracene and the experiment has been performed in a continuous flow Cinetique de Reaction en Ecoulement Supersonique Uniforme apparatus, which had to be modified for this purpose. Pulsed laser photolysis of H(2)O(2) has been used to generate OH radicals and laser-induced fluorescence to observe the kinetic decay of the radicals and hence determine the rate coefficients. The reaction is found to be fast, and the rate constant increases monotonically as the temperature is lowered. The rate coefficients match the expression k(cm(3) molecules(-1) s(-1))=1.12 x 10(-10)(T/300)(-0.46).