Reaction Rate Constant of CH₂O + H = HCO + H₂Revisited: A Combined Study of Direct Shock Tube Measurement and Transition State Theory Calculation

Abstract

The rate constant of the H-abstraction reaction of formaldehyde (CH2O) by hydrogen atoms (H), CH2O + H = H2 + HCO, has been studied behind reflected shock waves with use of a sensitive mid-IR laser absorption diagnostic for CO, over temperatures of 1304-2006 K and at pressures near 1 atm. C2H5I was used as an H atom precursor and 1,3,5-trioxane as the CH2O precursor, to generate a well-controlled CH2O/H reacting system. By designing the experiments to maintain relatively constant H atom concentrations, the current study significantly boosted the measurement sensitivity of the target reaction and suppressed the influence of interfering reactions. The measured CH2O + H rate constant can be expressed in modified Arrhenius from as kCH2O+H(1304-2006 K, 1 atm) = 1.97 × 10(11)(T/K)(1.06) exp(-3818 K/T) cm(3) mol(-1)s(-1), with uncertainty limits estimated to be +18%/-26%. A transition-state-theory (TST) calculation, using the CCSD(T)-F12/VTZ-F12 level of theory, is in good agreement with the shock tube measurement and extended the temperature range of the current study to 200-3000 K, over which a modified Arrhenius fit of the rate constant can be expressed as kCH2O+H(200-3000 K) = 5.86 × 10(3)(T/K)(3.13) exp(-762 K/T) cm(3) mol(-1)s(-1).

Ab Initio Chemical Kinetics for the CH3 + O(3P) Reaction and Related Isomerization–Decomposition of CH3O and CH2OH Radicals

“…The barriers of the forward and reverse hydrogen atom abstraction reactions are predicted to be 6.1 and 22.5 kcal/mol, respectively, which are almost the same as those calculated at the G2M//B3LYP/6-311+G(3df,2p) level. In 2000, Oehlers et al 35 gave the forward reaction activation energy of 4.4 ± 0.4 kcal/mol by analyzing their kinetic results in the temperature range of 296−780 K. Two years later, another The Journal of Physical Chemistry A Article direct measurement of the reaction by Friedrichs et al 36 determined the activation energy to be 9.7 kcal/mol for the temperatures from 1510 to 1960 K. Recently, Wang et al 37 have located a transition state for the H-abstraction process at the CCSD(T)-F12/VTZ-F12 level to be 6.4 kcal/mol, which is close to our values of 6.1 kcal/mol. The energy of CHO + H 2 relative to that of CH 2 O + H was predicted to be −16.4 and −16.1 kcal/mol at the CCSD(T) and G2M levels, respectively, which agree quantitatively with the heat of reaction, −16.5 ± 0.2 kcal/mol, calcualted by the heats of formation of the related species (Δ f H°0(CHO) = 10.0 kcal/mol 69 and others in Table 3).…”

Atom Tunneling in Chemistry

“…Hydrogen atoms are frequently found to tunnel, even at room temperature,inclassical reactions of organic chemistry. In the case of reactions of closed-shell molecules with radicals, hydrogen is abstracted if the emerging radical is more stable than the previous one.V arious reactions of small organic molecules with hydrogen atoms, [153][154][155][156] hydroperoxyl radicals, [64] chlorine atoms, [157][158][159][160] or the activation of H 2 [161] have been studied experimentally as well as computationally. These reactions are influenced by tunneling,e ven at room temperature,t hereby raising the reaction rate constants.F or Claisen rearrangements it was necessary to computationally include am odel of tunneling through ap arabolic barrier to explain the experimental 13 CK IEs, [162][163][164] and in aS wern oxidation, multidimensional tunneling had to be included in computations to reproduce the experimental results.…”

Uncertainty quantification of a newly optimized methanol and formaldehyde combustion mechanism

“…The optimized H-atom abstraction reactions from CH2O were studied extensively. For the reaction CH2O + Ḣ = HĊO + H2 (R38), experimental data [145][146][147][148][149][150][151][152] over a wide temperature range (143/8) and theoretical calculations of Wang et al [151] and Irdam et al [152] were used. Only direct measurements [140,153] were available for the H-atom abstraction reaction via O2 (R41; 59/3), while the reaction CH2O + ȮH = HĊO + H2O (R40) was studied both experimentally [114,129,[154][155][156][157] 63/6and theoretically [154,158,159].…”