Production of H and O(3P) Atoms in the Reaction of CH2 with O2

CH2 + O2: reaction mechanism, biradical and zwitterionic character, and formation of CH2OO, the simplest Criegee intermediate

The singlet and triplet potential surfaces for the title reaction were investigated using the CBS-QB3 level of theory. The wave functions for some species exhibited multireference character and required the CASPT2/6-31+G(d,p) and CASPT2/aug-cc-pVTZ levels of theory to obtain accurate relative energies. A Natural Bond Orbital Analysis showed that triplet ³CH₂OO (the simplest Criegee intermediate) and ³CH₂O₂ (dioxirane) have mostly polar biradical character, while singlet ¹CH₂OO has some zwitterionic character and a planar structure. Canonical variational transition state theory (CVTST) and master equation simulations were used to analyze the reaction system. CVTST predicts that the rate constant for reaction of ¹CH₂ + ³O₂ is more than ten times as fast as the reaction of ³CH₂ (X³B₁) + ³O₂ and the ratio remains almost independent of temperature from 900 K to 3000 K. The master equation simulations predict that at low pressures the ¹CH₂O + ³O product set is dominant at all temperatures and the primary yield of OH radicals is negligible below 600 K, due to competition with other primary reactions in this complex system.

Direct Dynamics Simulations of the 3CH2 + 3O2 Reaction at High Temperature

Direct dynamics simulations with the M06/6-311++G(d,p) level of theory were performed to study the ³CH₂ + ³O₂ reaction at 1000 K temperature on the ground state singlet surface. The reaction is complex with formation of many different product channels in highly exothermic reactions. CO, CO₂, H₂O, OH, H₂, O, H, and HCO are the products formed from the reaction. The total simulation rate constant for the reaction at 1000 K is (1.2 ± 0.3) × 10^-12 cm³ molecule^-1 s^-1, while the simulation rate constant at 300 K is (0.96 ± 0.28) × 10^-12 cm³ molecule^-1 s^-1. The simulated product yields show that CO is the dominant product and the CO:CO₂ ratio is 5.3:1, in good comparison with the experimental ratio of 4.3:1 at 1000 K. On comparing the product yields for the 300 and 1000 K simulations, we observed that, except for CO and H₂O, the yields of the other products at 1000 K are lower at 300 K, showing a negative temperature dependence.

Direct Dynamics Simulation of the Thermal 3CH2 + 3O2 Reaction. Rate Constant and Product Branching Ratios

The reaction of ³CH₂ with ³O₂ is of fundamental importance in combustion, and the reaction is complex as a result of multiple extremely exothermic product channels. In the present study, direct dynamics simulations were performed to study the reaction on both the singlet and triplet potential energy surfaces (PESs). The simulations were performed at the UM06/6-311++G(d,p) level of theory. Trajectories were calculated at a temperature of 300 K, and all reactive trajectories proceeded through the carbonyl oxide Criegee intermediate, CH₂OO, on both the singlet and triplet PESs. The triplet surface leads to only one product channel, H₂CO + O(³P), while the singlet surface leads to eight product channels with their relative importance as CO + H₂O > CO + OH + H ∼ H₂CO + O(¹D) > HCO + OH ∼ CO₂ + H₂ ∼ CO + H₂ + O(¹D) > CO₂ + H + H > HCO + O(¹D) + H. The reaction on the singlet PES is barrierless, consistent with experiment, and the total rate constant on the singlet surface is (0.93 ± 0.22) × 10^-12 cm³ molecule^-1 s^-1 in comparison to the recommended experimental rate constant of 3.3 × 10^-12 cm³ molecule^-1 s^-1. The simulation product yields for the singlet PES are compared with experiment, and the most significant differences are for H, CO₂, and H₂O. The reaction on the triplet surface is also barrierless, inconsistent with experiment. A discussion is given of the need for future calculations to address (1) the barrier on the triplet PES for ³CH₂ + ³O₂ → ³CH₂OO, (2) the temperature dependence of the ³CH₂ + ³O₂ reaction rate constant and product branching ratios, and (3) the possible non-RRKM dynamics of the ¹CH₂OO Criegee intermediate.

Direct measurement of site-specific rates of reactions of H with C3H8, i-C4H10, and n-C4H10

We measured the rates of abstraction of a hydrogen atom from specific sites in propane C₃H₈, 2-methyl propane (i-C₄H₁₀), and butane (n-C₄H₁₀); the sites are a primary hydrogen of C₃H₈ and i-C₄H₁₀ and a secondary hydrogen of n-C₄H₁₀. The excellent reproducibility of conditions of a diaphragm-less shock tube enabled us to conduct comparative measurements of the evolution of H atoms in three mixtures-(i) 0.5 ppm C₂H₅I + Ar, (ii) 0.5 ppm C₂H₅I + 50-100 ppm alkane as C₃H₈ or i-C₄H₁₀ or n-C₄H₁₀ + Ar, and (iii) the same concentrations of alkane + Ar without C₂H₅I-in the temperature range 1000-1200 K and at a pressure of 2.0 bars. The net profile of rise and decay of H atoms in the C₂H₅I + alkane mixture was derived on subtracting the absorbance of (iii) from that of (ii). Measurements of the mixture (iii) are important because the absorption of alkanes at 121.6 nm is not negligible. In the temperature range 1000-1100 K, the rate of decomposition of C₂H₅I was evaluated directly on analyzing the exponential growth of H atoms in the mixture (i). The rate of decomposition of C₂H₅I is summarized as ln(k/s^-1) = (33.12 ± 1.4) - (25.23 ± 1.5) 10³/T (T = 1000-1100 K, P = 2.0 bars); the broadening factor F(T) in the Lindemann-Hinshelwood formula was evaluated in the fall-off region. The site-specific rates of H + (C₃-C₄) alkanes are summarized as follows: H + C₃H₈ → H₂ + 1-C₃H₇, ln(k_1a) = -(21.34 ± 0.86) - (5.39 ± 0.93)10³/T, H + i-C₄H₁₀ → H₂ + i-C₄H₉, ln(k_2a) = -(20.50 ± 1.36) - (6.14 ± 0.13)10³/T, H + n-C₄H₁₀ → H₂ + 2-C₄H₉, ln(k_3b) = -(21.37 ± 1.15) - (4.83 ± 1.26)10³/T. The present experimental results are compared with published results from quantum-chemical calculations of potential-energy surfaces and transition-state theory. The present experiments are consistent with those calculations for the reaction rates for the attack at the primary site for H + C₃H₈ and H + i-C₄H₁₀, but for the attack at the secondary site of n-C₄H₁₀, our results are substantially smaller than the computational prediction, which might indicate a hindrance by the C-H bonds of the primary sites that serves to decrease the rate of abstraction from the secondary site of n-C₄H₁₀. The influence on the total rates of reactions H + alkane and the group additivity rule are discussed.