HCO formation in the thermal unimolecular decomposition of glyoxal: rotational and weak collision effects.

The multi-channel thermal unimolecular decomposition of glyoxal was experimentally investigated in the temperature range 1106 K < T < 2320 K and at total densities of 1.7 x 10(-6) mol cm(-3) < rho < 1.9 x 10(-5) mol cm(-3) by monitoring HCO (frequency modulation spectroscopy, FMS), (CHO)(2) (UV absorption), and H atom (atom resonance absorption spectroscopy, H-ARAS) concentration-time profiles behind shock waves. With a branching fraction of 48% at T = 2300 K and rho = 1.6 x 10(-5) mol cm(-3), the so-far-neglected, energetically unfavourable HCO-forming decomposition channel, (CHO)(2)--> 2HCO, was found to play a crucial role and in fact represents the major decomposition pathway at high temperatures and high total densities. A theoretical analysis of the experimental results in terms of Rice-Ramsperger-Kassel-Marcus theory (RRKM), the simplified statistical adiabatic channel model (SACM), and an energy-grained master equation (ME) was based on input parameters from ab initio calculations (G3 and MP2/6-311G(d,p)) and literature data on branching ratios from collision-free photolysis experiments. A consistent description of the temperature and density dependences was achieved, revealing that both rotational and weak collision effects are reflected in the measured branching ratios. Overall, a product channel switching occurs with the CH(2)O-forming channel, (CHO)(2)--> CH(2)O + CO, dominating at low temperatures/densities and the HCO-forming channel dominating at high temperatures/densities. Additionally, the so-called triple-whammy channel, (CHO)(2)--> 2CO + H(2), significantly contributes to the total decomposition rate at intermediate temperatures/densities whereas the HCOH-forming pathway, (CHO)(2)--> HCOH + CO, is predicted to be the least important one. The temperature and pressure dependences of the different decomposition channels are parametrized in terms of two-dimensional Chebyshev polynomials.