Refined Analysis of the Thermal Dissociation of Formaldehyde

Abstract

New experimental results for the thermal dissociation of formaldehyde to radical and molecular products (Proc. Combust. Inst. 2007, 31, in press) form the basis of the present analysis of the respective low-pressure rate coefficients k(Rad,0) and k(Mol,0) of the reaction. The article supersedes an earlier analysis (J. Phys. Chem. A 2005, 109, 8320) which used less accurate and more preliminary input information. In addition, refined rotational factors F(rot) are determined and specific energy and angular momentum dependent branching ratios from a more detailed analysis of photolysis quantum yields (J. Phys. Chem. A 2007, 111, 3868) are employed as well. It is emphasized again that pyrolysis and photolysis are intimately related and should be analyzed in an internally consistent manner. The combination of the new with earlier experimental results for pyrolysis rates allows one to fit the height of the energy barrier for the molecular elimination channel with improved precision. A value of E0,1 = 81.7(+/-0.5) kcal mol(-1) is obtained. In addition, employing anharmonicity factors F(anh) from the earlier work, a total average energy transferred per collision of -DeltaE/hc = 100(+/-20) cm(-1) is fitted from the experiments in the bath gas Ar. This value is consistent with the value -DeltaE/hc = 80(+/-40) cm(-1) for the bath gas N(2) such as fitted from photolysis quenching experiments (using the same molecular parameters as for the pyrolysis). Rate coefficients for the temperature range 1200-3500 K are represented in the form k(Mol,0)/[Ar] = 7.3 x 10(14) T -6.1 exp(-47300 K/T) cm(3) molecule(-1) s(-1) and k(Rad,0)/[Ar] = 2.1 x 10(12) T -5.5 exp(-47300 K/T) cm(3) molecule(-1) s(-1) (accuracy +/-25%) and recommended for use in combustion chemistry.