Time-Resolved Studies of the Kinetics of the Reactions of CHO with HI and HBr:  Thermochemistry of the CHO Radical and the C−H Bond Strengths in CH₂O and CHO

Abstract

Time-resolved studies of formyl radical, CHO, generated by laser flash photolysis of acetaldehyde, have been carried out to obtain rate constants for its bimolecular reactions with HI and HBr. Each reaction was studied in the gas-phase at five different temperatures in the range 293-540 K. The pressure-independent rate constants gave the following Arrhenius equations: for CHO + HI, log(k 2 /cm 3 molecule -1 s -1 ) ) (-11.51 ( 0.01) -(0.83 ( 0.06 kJ mol -1 )/ RT ln 10, for CHO + HBr, log(k 4 /cm 3 molecule -1 s -1 ) ) (-12.59 ( 0.06) -(0.13 ( 0.44 kJ mol -1 )/ RT ln 10. The Arrhenius parameters are consistent with those for reactions of other free radicals with HI and HBr and support the idea of intermediate complexes in these processes. Combination with the known kinetic data on the reverse reactions leads to the thermodynamic estimate for ∆H f °(CHO) ) 44.29 ( 0.43 kJ mol -1 (third law method, 298 K value). Second law values are in reasonable agreement. This value corresponds to the bond dissociation energies D(H-CHO) ) 370.86 ( 0.28 kJ mol -1 , and D(H-CO) ) 63.18 ( 0.46 kJ mol -1 . These values are in close agreement with several previous determinations although more precise. A small unresolved discrepancy exists with the most recent value for the photodissociation threshold for CH 2 O.

Validation of a thermal decomposition mechanism of formaldehyde by detection of CH2O and HCO behind shock waves

“…Rate constants for reverse reactions were calculated automatically on the basis of the thermodynamic data set included in the Chemkin‐II package 17. For CH 2 OH and HCO the enthalpies of formation were scaled to recent literature values of Δ f H 298 0 (CH 2 OH) = −16.6 kJ mol −1 18 and Δ f H 298 0 (HCO) = 44.3 kJ mol −1 19. For sensitivity analysis the sensitivity coefficient σ (i,j,t) of the i ‐th reaction of species j at the reaction time t was normalized with respect to the maximum concentration of the species j over the time history.…”

“…The unimolecular decomposition of CH 2 O is a well known example for a system with two reaction channels having similar threshold energies and in thermal systems both channels can proceed with comparable rates. With threshold energies of E 1a = 364.6 kJ mol −1 (based on enthalpy of formation Δ f H 298 0 (HCO) = 44.3 kJ mol −1 19) and E 1b = 332.6 kJ mol −1 22 the molecular channel (1b) is favored by 32 kJ/mol and should dominate the overall decomposition at low temperatures and pressures. However, in their theoretical analysis of two‐channel thermal unimolecular reactions Just and Troe 23 demonstrated the distinct pressure and temperature dependences of the branching fraction obtained in the case of similar threshold energies and Just 24 reviewed some practical examples.…”

Quantitative detection of HCO behind shock waves: The thermal decomposition of HCO

“…Because of the weakly bound hydrogen atom in HCO (58.3 kJ mol À1 bond energy, based on ref. 3) the unimolecular decomposition of HCO is one of the rare examples where the direct decay of a radical can compete with its bimolecular reactions even at practical combustion conditions. Whereas the unimolecular decomposition with H and CO as products is a chain-propagating process the bimolecular reactions like HCO + (H,OH) !…”

“…At the temperatures and pressures used in this work (298 < T/K < 1230, 0.28 < p/bar < 1.90) the radical production via thermal decomposition of formaldehyde (1a) is negligibly small. Instead, the 308 nm photolysis generates an H atom and a HCO radical, which in turn decomposes rapidly by reaction (3) forming another H atom and stable carbon monoxide. The subsequent reaction of a hydrogen atom with excessive CH 2 O by reaction (2) yields a formyl radical and in turn the H atom is regained by reaction (3).…”

Mechanism of HCS + O2 reaction: Hydrogen- or oxygen-transfer?

“…In contrast, the depletion mechanism of the isovalent analogue radical, HCO, has been the subject of extensive studies. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] The HCO reaction with molecular oxygen is of particular interest due to its importance in hydro-carbon oxidation in both low-temperature atmospheric chemistry and high-temperature combustion chemistry. The recent theoretical investigation at the QCISD(T)/6-311G(2df,2p)//QCISD/6-311G(d,p) level has shown that the HCO þ O 2 reaction can competitively proceed through either a direct or indirect hydrogen-transfer process with the respective energy barriers 2.98 and 2.26 kcal mol À1 .…”