| Abstract
| - The thermal dissociation of formaldehyde proceeds on three channels, the molecular-elimination channelH2CO → H2 + CO (1), the radical-forming bond-fission channel H2CO → H + HCO (2), and the bond-fission-initiated, intramolecular-hydrogen-abstraction channel H2CO → H···HCO → H2 + CO (3) which alsoforms molecular products. The kinetics of this system in the low-pressure range of the unimolecular reactionis shown to be governed by a subtle superposition of collisional channel coupling to be treated by solving amaster equation, of rotational channel switching accessible through ab initio calculations of the potential aswell as spectroscopic and photophysical determinations of the threshold energies and channel branching abovethe threshold energy for radical formation which can be characterized through formaldehyde photolysis quantumyields as well as classical trajectory calculations. On the basis of the available information, the rate coefficientsfor the formation of molecular and radical fragments are analyzed and extrapolated over wide ranges ofconditions. The modeled rate coefficients in the low-pressure range of the reaction (neglecting tunneling)over the range 1400−3200 K in the bath-gas Ar in this way are represented by k0,Mol/[Ar] ≈ 9.4 × 10-9exp(−33 140 K/T) cm3 molecule-1 s-1 and k0,Rad/[Ar] ≈ 6.2 × 10-9 exp(−36 980 K/T) cm3 molecule-1 s-1.The corresponding values for the bath-gas Kr, on which the analysis relies in particular, are k0,Mol/[Kr] ≈ 7.7× 10-9 exp(−33 110 K/T) and k0,Rad/[Kr] ≈ 4.1 × 10-9 exp(−36 910 K/T) cm3 molecule-1 s-1. While thethreshold energy E0,2 for channels 2 and 3 is taken from spectroscopic measurements, the threshold energyE0,1 for channel 1 is fitted on the basis of experimental ratios k0,Rad/k0,Mol in combination with photolysisquantum yields. The derived value of E0,1(1) = 81.2 (±0.9) kcal mol-1 is in good agreement with resultsfrom recent ab initio calculations, 81.9 (±0.3) kcal mol-1, but is higher than earlier results derived fromphotophysical experiments, 79.2 (±0.8) kcal mol-1. Rate coefficients for the high-pressure limit of the reactionare also modeled. The results of the present work markedly depend on the branching ratio between channels2 and 3. Expressions of this branching ratio from classical trajectory calculations and from photolysis quantumyield measurements were tested. At the same time, a modeling of the photolysis quantum yields was performed.The formaldehyde system so far presents the best characterized multichannel dissociation reaction. It mayserve as a prototype for other multichannel dissociation reactions.
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