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.     

State-to-state dynamics of the Cl + CH3OH --> HCl + CH2OH reaction

Molecular chlorine, methanol, and helium are co-expanded into a vacuum chamber using a custom designed "late-mixing" nozzle. The title reaction is initiated by photolysis of Cl2 at 355 nm, which generates monoenergetic Cl atoms that react with CH3OH at a collision energy of 1960 +/- 170 cm(-1) (0.24 +/- 0.02 eV). Rovibrational state distributions of the nascent HCl products are obtained via 2 + 1 resonance enhanced multiphoton ionization, center-of-mass scattering distributions are measured by the core-extraction technique, and the average internal energy of the CH3OH co-products is deduced by measuring the spatial anisotropy of the HCl products. The majority (84 +/- 7%) of the HCl reaction products are formed in HCl(v = 0) with an average rotational energy of [Erot] = 390 +/- 70 cm(-1). The remaining 16 +/- 7% are formed in HCl(v = 1) and have an average rotational energy of [Erot] = 190 +/- 30 cm(-1). The HCl(v = 1) products are primarily forward scattered, and they are formed in coincidence with CH2OH products that have little internal energy. In contrast, the HCl(v = 0) products are formed in coincidence with CH2OH products that have significant internal energy. These results indicate that two or more different mechanisms are responsible for the dynamics in the Cl + CH3OH reaction. We suggest that (1) the HCl(v = 1) products are formed primarily from collisions at high impact parameter via a stripping mechanism in which the CH2OH co-products act as spectators, and (2) the HCl(v = 0) products are formed from collisions over a wide range of impact parameters, resulting in both a stripping mechanism and a rebound mechanism in which the CH2OH co-products are active participants. In all cases, the reaction of fast Cl atoms with CH3OH is with the hydrogen atoms on the methyl group, not the hydrogen on the hydroxyl group.     

Dynamics of the CH3 + OH reaction

We study dynamics of the CH₃ + OH reaction over the temperature range of 300–2500 K using a quasiclassical method for the potential energy composed of explicit forms of short‐range and long‐range interactions. The explicit potential energy used in the study gives minimum energy paths on potential energy surfaces showing barrier heights, channel energies, and van der Waals well, which are consistent with ab initio calculations. Approximately, 20% of CH₃ + OH collisions undergo OH dissociation in a direct‐mode mechanism on a subpicosecond scale (<50 fs) with the rate coefficient as high as ～10^?10 cm³ molecule^?1 s^?1. Less than 10% leads to the formation of excited intermediates CH₃OH? with excess vibrational energies in CO and OH bonds. CH₃OH? stabilizes to CH₃OH, redissociates back to reactants, or forms one of various products after intramolecular energy redistribution via bond dissociation and formation on the time scale of 50–200 fs. The principal product is ¹CH₂ (k being ～10^?11), whereas ks for CH₂OH, CH₂O, and CH₃O are ～10^?12. The minor products are HCOH and CH₄ (k～10^?13). The total rate coefficient for CH₃ + OH → CH₃OH? → products is ～10^?11 and is weakly dependent on temperature. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 455–466, 2011     

Canonical variational transition-state theory study of the CF3CH2CH3 + OH reaction

Variational transition-state theory rate constants with multidimensional tunneling contributions using the small curvature method have been calculated for the CF3CH2CH3 (HFC-263fb) + OH reaction over a temperature range from 200 to 373 K. The mPW1B95-41.0 hybrid functional, parametrized by Albu and Swaminathan to generate theoretical rate constants nearly identical to the experimental values for the CH3F + OH reaction, has been used in conjunction with the 6-31+G** basis set to explore the potential energy surface of the title reaction. The good agreement found between theoretical predictions and the experimental data available suggests that the present approach is an excellent option to obtain high-quality results at low computational cost for direct dynamics studies of hydrogen abstraction reactions from complex hydrofluorocarbons. The reliability of the structure activity relationship used to estimate rate constant values for OH reactions with hydrofluorocarbons is also discussed in detail.     

Analysis of the reaction force for a gas phase S(N)2 process: CH3Cl + H2O --> CH3OH + HCl

The "reaction force" F(R(c)) is the negative derivative of a system's potential energy V(R(c)) along the intrinsic reaction coordinate of a process. If V(R(c)) goes through a maximum, as is commonly the case, then F(R(c)) has a characteristic profile: a negative minimum followed by zero at the transition state and then a positive maximum. These features reflect four phases of the reaction: an initial one of reactant preparation, followed by two of transition to products, and then relaxation of the latter. In this study, we have analyzed, in these terms, a gas-phase S(N)2 substitution, selected to be CH3Cl + H2O --> CH3OH + HCl. We examine, at the B3LYP/6-31G level, the geometries, energetics, and molecular surface electrostatic potentials, local ionization energies, and internal charge separation.     

Product branching from the CH2CH2OH radical intermediate of the OH + ethene reaction

Using a crossed laser-molecular beam scattering apparatus and tunable photoionization detection, these experiments determine the branching to the product channels accessible from the 2-hydroxyethyl radical, the first radical intermediate in the addition reaction of OH with ethene. Photodissociation of 2-bromoethanol at 193 nm forms 2-hydroxyethyl radicals with a range of vibrational energies, which was characterized in our first study of this system ( J. Phys. Chem. A 2010 , 114 , 4934 ). In this second study, we measure the relative signal intensities of ethene (at m/e = 28), vinyl (at m/e = 27), ethenol (at m/e = 44), formaldehyde (at m/e = 30), and acetaldehyde (at m/e = 44) products and correct for the photoionization cross sections and kinematic factors to determine a 0.765:0.145:0.026:0.063:<0.01 branching to the OH + C(2)H(4), H(2)O + C(2)H(3), CH(2)CHOH + H, H(2)CO + CH(3), and CH(3)CHO + H product asymptotes. The detection of the H(2)O + vinyl product channel is surprising when starting from the CH(2)CH(2)OH radical adduct; prior studies had assumed that the H(2)O + vinyl products were solely from the direct abstraction channel in the bimolecular collision of OH and ethene. We suggest that these products may result from a frustrated dissociation of the CH(2)CH(2)OH radical to OH + ethene in which the C-O bond begins to stretch, but the leaving OH moiety abstracts an H atom to form H(2)O + vinyl. We compare our experimental branching ratio to that predicted from statistical microcanonical rate constants averaged over the vibrational energy distribution of our CH(2)CH(2)OH radicals. The comparison suggests that a statistical prediction using 1-D Eckart tunneling underestimates the rate constants for the branching to the product channels of OH + ethene, and that the mechanism for the branching to the H(2)O + vinyl channel is not adequately treated in such theories.     

Ab initio studies on reaction H2C=C(OH)Li+CH3 + (CH3 -)

The possible structures and isomerizations of H₂C=C(OH)Li are studied theoretically by the gradient analytical method at RHF/6-31+G level. According to these results, reactions of H₂C=C(OH)Li with CH₃ ⁺ and CH ₃ ^- are investigated thoroughly. When H₂C=C(OH)Li reacts with CH ₃ ⁺ , H_zC=C(OH)Li firstly changes from structure1 to structure4, and then combines with CH₃ ⁺. In this reaction, the configuration of central carbon is retained. When H₂C=C(OH)Li reacts with CH ₃ ^- , structure1 firstly breaks its C-O bond to give contact ion-pair. Then through transition state16 which is similar to structure2, the attack of CH ₃ ^- from the opposite side of^-OH replaces^-OH group and inverts the configuration of carbenoid carbon atom. All the results show that the ambident reactivity of carbenoid has close relationship with the stability of special structures. Project supported by the National Natural Science Foundation of China (Grant No. 29773025).     

Investigation of the radical product channel of the CH3COO2 + HO2 reaction in the gas phase

The reaction of CH(3)C(O)O(2) with HO(2) has been investigated at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl(2)/CH(3)CHO/CH(3)OH/air mixtures. The branching ratio for the reaction channel forming CH(3)C(O)O, OH and O(2) (reaction ) has been determined from experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The dependence of the phenol yield on benzene concentration was found to be consistent with its formation from the OH-initiated oxidation of benzene, thereby confirming the presence of OH radicals in the system. The dependence of the phenol yield on the initial peroxy radical precursor reagent concentration ratio, [CH(3)OH](0)/[CH(3)CHO](0), is consistent with OH formation resulting mainly from the reaction of CH(3)C(O)O(2) with HO(2) in the early stages of the experiments, such that the limiting yield of phenol at high benzene concentrations is well-correlated with that of CH(3)C(O)OOH, a well-established product of the CH(3)C(O)O(2) + HO(2) reaction (via channel (3a)). However, a delayed source of phenol was also identified, which is attributed mainly to an analogous OH-forming channel of the reaction of HO(2) with HOCH(2)O(2) (reaction ), formed from the reaction of HO(2) with product HCHO. This was investigated in additional series of experiments in which Cl(2)/CH(3)OH/benzene/air and Cl(2)/HCHO/benzene/air mixtures were photolysed. The various reaction systems were fully characterised by simulations using a detailed chemical mechanism. This allowed the following branching ratios to be determined: CH(3)C(O)O(2) + HO(2)--> CH(3)C(O)OOH + O(2), k(3a)/k(3) = 0.38 +/- 0.13; --> CH(3)C(O)OH + O(3), k(3b)/k(3) = 0.12 +/- 0.04; --> CH(3)C(O)O + OH + O(2), k(3c)/k(3) = 0.43 +/- 0.10: HOCH(2)O(2) + HO(2)--> HCOOH + H(2)O + O(2), k(17b)/k(17) = 0.30 +/- 0.06; --> HOCH(2)O + OH + O(2), k(17c)/k(17) = 0.20 +/- 0.05. The results therefore provide strong evidence for significant participation of the radical-forming channels of these reactions, with the branching ratio for the title reaction being in good agreement with the value reported in one previous study. As part of this work, the kinetics of the reaction of Cl atoms with phenol (reaction (14)) have also been investigated. The rate coefficient was determined relative to the rate coefficient for the reaction of Cl with CH(3)OH, during the photolysis of mixtures of Cl(2), phenol and CH(3)OH, in either N(2) or air at 296 K and 760 Torr. A value of k(14) = (1.92 +/- 0.17) x 10(-10) cm(3) molecule(-1) s(-1) was determined from the experiments in N(2), in agreement with the literature. In air, the apparent rate coefficient was about a factor of two lower, which is interpreted in terms of regeneration of phenol from the product phenoxy radical, C(6)H(5)O, possibly via its reaction with HO(2).     

Product pair correlation in CH3OH photodissociation at 157 nm: the OH + CH3 channel

The OH + CH(3) product channel for the photodissociation of CH(3)OH at 157 nm was investigated using the velocity map imaging technique with the detection of CH(3) radical products via (2+1) resonance-enhanced multiphoton ionization (REMPI). Images were measured for the CH(3) formed in the ground and excited states (v(2) = 0, 1, 2, and 3) of the umbrella vibrational mode and correlated OH vibrational state distributions were also determined. We find that the vibrational distribution of the OH fragment in the OH + CH(3) channel is clearly inverted. Anisotropic distributions for the CH(3) (v(2) = 0, 1, 2, and 3) products were also determined, which is indicative of a fast dissociation process for the C-O bond cleavage. A slower CH(3) product channel was also observed, that is assigned to a two-step photodissociation process, in which the first step is the production of a CH(3)O(X (2)E) radical via the cleavage of the O-H bond in CH(3)OH, followed by probe laser photodissociation of the nascent CH(3)O radicals yielding CH(3)(X (2)A(1), v = 0) products.     

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.     

Accurate potential energy surface and quantum reaction rate calculations for the H+CH4-->H2+CH3 reaction

Calculations for the cumulative reaction probability N(E) (for J=0) and the thermal rate constant k(T) of the H+CH(4)-->H(2)+CH(3) reaction are presented. Accurate electronic structure calculations and a converged Shepard-interpolation approach are used to construct a potential energy surface which is specifically designed to allow the precise calculation of k(T) and N(E). Accurate quantum dynamics calculations employing flux correlation functions and multiconfigurational time-dependent Hartree wave packet propagation compute N(E) and k(T) based on this potential energy surface. The present work describes in detail the various convergence test performed to investigate the accuracy of the calculations at each step. These tests demonstrate the predictive power of the present calculations. In addition, approximate approaches for reaction rate calculations are discussed. A quite accurate approximation can be obtained from a potential energy surface which includes only interpolation points on the minimum energy path.     

Theoretical study of the complex-forming CH + H2 --> CH2 + H reaction

The complex-forming CH + H2 --> CH2 + H reaction is studied employing a recently developed global potential energy function. The reaction probability in the total angular momentum J = 0 limit is estimated with a four-atom quantum wave packet method and compared with classical trajectory and statistical theory results. The formation of complexes from different reactant internal states is also determined with wave packet calculations. While there is no barrier to reaction along the minimum energy path, we find that there are angular constraints to complex formation. Trajectory-based estimates of the low-pressure rate constants are made and compared with experimental results. We find that zero-point energy violation in the trajectories is a particularly severe problem for this reaction.     

CH3NHNH2 + OH reaction: Mechanism and dynamics studies

A direct dynamics study was carried out for the multichannel reaction of CH₃NHNH₂ with OH radical. Two stable Conformers (I, II) of CH₃NHNH₂ are identified by the rotation of the ? CH₃ group. For each conformer, five hydrogen‐abstraction channels are found. The reaction mechanisms of product radicals (CH₃NNH₂ and CH₃NHNH) with OH radical are also investigated theoretically. The electronic structure information on the potential energy surface is obtained at the B3LYP/6‐311G(d,p) level and the energetics along the reaction path is refined by the BMC‐CCSD method. Hydrogen‐bonded complexes are presented at both the reactant and product sides of the five channels, indicating that the reaction may proceed via an indirect mechanism. The influence of the basis set superposition error (BSSE) on the energies of all the complexes is discussed by means of the CBS‐QB3 method. The rate constants of CH₃NHNH₂ + OH are calculated using canonical variational transition‐state theory with the small‐curvature tunneling correction (CVT/SCT) in the temperature range of 200–1000 K. Slightly negative temperature dependence of rate constant is found in the temperature range from 200 to 345 K. The agreement between the theoretical and experimental results is good. It is shown that for Conformer I, hydrogen‐abstraction from ? NH? position is the primary pathway at low temperature; the hydrogen‐abstraction from ? NH₂ is a competitive pathway as the temperature increases. A similar case can be concluded for Conformer II. The overall rate constant is evaluated by considering the weight factors of each conformer from the Boltzmann distribution function, and the three‐term Arrhenius expressions are fitted to be k_T = 1.6 × 10^?24T^4.03exp (1411.5/T) cm³ molecule^?1 s^?1 between 200–1000 K. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Secondary kinetics of methanol decomposition: theoretical rate coefficients for 3CH2 + OH, 3CH2 + 3CH2, and 3CH2 + CH3

Direct variable reaction coordinate transition state theory (VRC-TST) rate coefficients are reported for the (3)CH(2) + OH, (3)CH(2) + (3)CH(2), and (3)CH(2) + CH(3) barrierless association reactions. The predicted rate coefficient for the (3)CH(2) + OH reaction (approximately 1.2 x 10(-10) cm(3) molecule(-1) s(-1) for 300-2500 K) is 4-5 times larger than previous estimates, indicating that this reaction may be an important sink for OH in many combustion systems. The predicted rate coefficients for the (3)CH(2) + CH(3) and (3)CH(2) + (3)CH(2) reactions are found to be in good agreement with the range of available experimental measurements. Product branching in the self-reaction of methylene is discussed, and the C(2)H(2) + 2H and C(2)H(2) + H2 products are predicted in a ratio of 4:1. The effect of the present set of rate coefficients on modeling the secondary kinetics of methanol decomposition is briefly considered. Finally, the present set of rate coefficients, along with previous VRC-TST determinations of the rate coefficients for the self-reactions of CH(3) and OH and for the CH(3) + OH reaction, are used to test the geometric mean rule for the CH(3), (3)CH(2), and OH fragments. The geometric mean rule is found to predict the cross-combination rate coefficients for the (3)CH(2) + OH and (3)CH(2) + CH(3) reactions to better than 20%, with a larger (up to 50%) error for the CH(3) + OH reaction.     

Quantum Monte Carlo study of the reaction: Cl+CH3OH-->CH2OH+HCl

A theoretical study is reported of the Cl+CH3OH-->CH2OH+HCl reaction based on the diffusion Monte Carlo (DMC) variant of the quantum Monte Carlo method. Using a DMC trial function constructed as a product of Hartree-Fock and correlation functions, we have computed the barrier height, heat of reaction, atomization energies, and heats of formation of reagents and products. The DMC heat of reaction, atomization energies, and heats of formation are found to agree with experiment to within the error bounds of computation and experiment. M?ller-Plesset second order perturbation theory (MP2) and density functional theory, the latter in the B3LYP generalized gradient approximation, are found to overestimate the experimental heat of reaction. Intrinsic reaction coordinate calculations at the MP2 level of theory demonstrate that the reaction is predominantly direct, i.e., proceeds without formation of intermediates, which is consistent with a recent molecular beam experiment. The reaction barrier as determined from MP2 calculations is found to be 2.24 kcal/mol and by DMC it is computed to be 2.39(49) kcal/mol.     

Vibrational mode analysis for the multichannel reaction of CH3Cl + OH

The recently presented ab initio calculations for the reaction system of CH₃Cl + OH (Dehestani and Shojaie, Int J Quantum Chem, in press) are applied to the vibrational mode analysis. Extending previous work, we use the vibrational mode analysis to elucidate the relationships of the reactants, the transition state, the intermediates (IM), and the products. The extensive investigation shows that the reaction mechanism is reliable. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

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.     

Crossed beams study of the reaction 1CH2+C2H2-->C3H3+H

The reaction of electronically excited singlet methylene (1CH2) with acetylene (C2H2) was studied using the method of crossed molecular beams at a mean collision energy of 3.0 kcal/mol. The angular and velocity distributions of the propargyl radical (C3H3) products were measured using single photon ionization (9.6 eV) at the advanced light source. The measured distributions indicate that the mechanism involves formation of a long-lived C3H4 complex followed by simple C-H bond fission producing C3H3+H. This work, which is the first crossed beams study of a reaction involving an electronically excited polyatomic molecule, demonstrates the feasibility of crossed molecular beam studies of reactions involving 1CH2.     

Theoretical study of the reactions CF3CH2OCHF2 + OH/Cl and its product radicals and parent ether(CH3CH2OCH3) with OH

A dual-level direct dynamic method is employed to study the reaction mechanisms of CF3CH2OCHF2 (HFE-245fa2; HFE-245mf) with the OH radicals and Cl atoms. Two hydrogen abstraction channels and two displacement processes are found for each reaction. For further study, the reaction mechanisms of its products (CF3CH2OCF2 and CF3CHOCHF2) and parent ether CH3CH2OCH3 with OH radical are investigated theoretically. The geometries and frequencies of all the stationary points and the minimum energy paths (MEPs) are calculated at the B3LYP/6-311G(d,p) level. The energetic information along the MEPs is further refined at the G3(MP2) level of theory. For reactions CF3CH2OCHF2 + OH/Cl, the calculation indicates that the hydrogen abstraction from --CH2-- group is the dominant reaction channel, and the displacement processes may be negligible because of the high barriers. The standard enthalpies of formation for the reactant CF3CH2OCHF2, and two products CF3CH2OCHF2 and CF3CHOCHF2 are evaluated via group-balanced isodesmic reactions. The rate constants of reactions CF3CH2OCHF2 + OH/Cl and CH3CH2OCH3 + OH are estimated by using the variational transition state theory over a wide range of temperature (200-2000 K). The agreement between the theoretical and experimental rate constants is good in the measured temperature range. From the comparison between the rate constants of the reactions CF3CH2OCHF2 and CH3CH2OCH3 with OH, it is shown that the fluorine substitution decreases the reactivity of the C--H bond.