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

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)) +/- 0.01) - (0.83 +/- 0.06 kJ mol(-1))/ RT In 10, for CHO + HBr, log(k/cm(3) molecule(-1) s(-1)) = (-12.59 +/- 0.06) - (0.13 +/- 0.44 kJ mol(-1))/ RT In 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 Delta H-f degrees(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 CH2O.