Friedrichs Gernot, Colberg Mark, Dammeier Johannes, Bentz Tobias, Olzmann Matthias
Institut für Physikalische Chemie, Olshausenstr. 40, Christian-Albrechts-Universität zu Kiel, 24098 Kiel, Germany.
Phys Chem Chem Phys. 2008 Nov 21;10(43):6520-33. doi: 10.1039/b809992h. Epub 2008 Sep 29.
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.
通过监测冲击波后HCO（调频光谱法，FMS）、(CHO)₂（紫外吸收）和H原子（原子共振吸收光谱法，H-ARAS）的浓度-时间曲线，在1106 K < T < 2320 K的温度范围以及1.7×10⁻⁶ mol cm⁻³ < ρ < 1.9×10⁻⁵ mol cm⁻³的总密度下，对乙二醛的多通道热单分子分解进行了实验研究。在T = 2300 K和ρ = 1.6×10⁻⁵ mol cm⁻³时，分支比为48%，之前被忽视的、能量上不利的生成HCO的分解通道(CHO)₂→2HCO，被发现起着关键作用，实际上它代表了高温和高总密度下的主要分解途径。根据Rice-Ramsperger-Kassel-Marcus理论（RRKM）、简化统计绝热通道模型（SACM）和能量粒度主方程（ME）对实验结果进行理论分析，其基于从头算（G3和MP2/6-311G(d,p)）的输入参数以及无碰撞光解实验中分支比的文献数据。实现了对温度和密度依赖性的一致描述，揭示出旋转和弱碰撞效应都反映在测量的分支比中。总体而言，发生了产物通道切换，生成CH₂O的通道(CHO)₂→CH₂O + CO在低温/低密度下占主导，而生成HCO的通道在高温/高密度下占主导。此外，所谓的三重击通道(CHO)₂→2CO + H₂在中等温度/密度下对总分解速率有显著贡献，而生成HCOH的途径(CHO)₂→HCOH + CO预计是最不重要的。不同分解通道的温度和压力依赖性用二维切比雪夫多项式进行参数化。
