Issue 43, 2008

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

Abstract

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 × 10−6 mol cm−3 < ρ < 1.9 × 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 ρ = 1.6 × 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 CH2O-forming channel, (CHO)2→ CH2O + 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 + H2, 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.

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

Supplementary files

Article information

Article type
Paper
Submitted
12 Jun 2008
Accepted
11 Aug 2008
First published
29 Sep 2008

Phys. Chem. Chem. Phys., 2008,10, 6520-6533

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

G. Friedrichs, M. Colberg, J. Dammeier, T. Bentz and M. Olzmann, Phys. Chem. Chem. Phys., 2008, 10, 6520 DOI: 10.1039/B809992H

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