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

Content Provider	World Health Organization (WHO)-Global Index Medicus
Author	Friedrichs, Gernot Colberg, Mark Dammeier, Johannes Bentz, Tobias Olzmann, Matthias
Description	Country affiliation: Germany Author Affiliation: Friedrichs G ( Institut für Physikalische Chemie, Olshausenstr. 40, Christian-Albrechts-Universität zu Kiel, 24098 Kiel, Germany. friedrichs@phc.uni-kiel.de)
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 $CH_{2}O-forming$ 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.
File Format	HTM / HTML
ISSN	14639076
Issue Number	43
Journal	Physical Chemistry Chemical Physics
Volume Number	10
e-ISSN	14639084
Language	English
Publisher	Royal Society of Chemistry
Publisher Date	2008-11-21
Publisher Place	Great Britain (UK)
Access Restriction	Subscribed
Subject Keyword	Discipline Physics Discipline Biophysics Discipline Chemistry
Alternative Title	HCO formation in the thermal unimolecular decomposition of glyoxal: rotational and weak collision effects
Content Type	Text
Resource Type	Article
Subject	Physics and Astronomy Physical and Theoretical Chemistry

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