Substitution Reactions in the Pyrolysis of Acetone Revealed through a Modeling, Experiment, Theory Paradigm

The development of high-fidelity
mechanisms for chemically reactive
systems is a challenging process that requires the compilation of
rate descriptions for a large and somewhat ill-defined set of reactions.
The present unified combination of modeling, experiment, and theory
provides a paradigm for improving such mechanism development efforts.
Here we combine broadband rotational spectroscopy with detailed chemical
modeling based on rate constants obtained from automated ab initio
transition state theory-based master equation calculations and high-level
thermochemical parametrizations. Broadband rotational spectroscopy
offers quantitative and isomer-specific detection by which branching
ratios of polar reaction products may be obtained. Using this technique,
we observe and characterize products arising from H atom substitution
reactions in the flash pyrolysis of acetone (CH3C­(O)­CH3) at a nominal temperature of 1800 K. The major product observed
is ketene (CH2CO). Minor products identified include acetaldehyde
(CH3CHO), propyne (CH3CCH), propene (CH2CHCH3), and water (HDO). Literature mechanisms
for the pyrolysis of acetone do not adequately describe the minor
products. The inclusion of a variety of substitution reactions, with
rate constants and thermochemistry obtained from automated ab initio
kinetics predictions and Active Thermochemical Tables analyses, demonstrates
an important role for such processes. The pathway to acetaldehyde
is shown to be a direct result of substitution of acetone’s
methyl group by a free H atom, while propene formation arises from
OH substitution in the enol form of acetone by a free H atom.

高保真化学活性系统机制的构建是一项极具挑战性的工作，它需要汇编大量定义模糊的反应速率描述。目前，通过将建模、实验与理论相结合，我们形成了一种改进此类机制开发工作的范式。在本研究中，我们结合了宽带旋转光谱与基于从自动化初态理论计算中获得速率常数及高级热化学参数化所进行的详细化学建模。宽带旋转光谱能够提供定量和异构体特异性的检测，从而获得极性反应产物的分支比。利用该技术，我们在1800 K的名义温度下观察并表征了丙酮（CH3C(O)CH3）闪速裂解中由氢原子取代反应产生的产物。观察到的主要产物为丙酮二烯（CH2CO）。鉴定出的次要产物包括乙醛（CH3CHO）、丙炔（CH3CCH）、丙烯（CH2CHCH3）和水（HDO）。有关丙酮裂解的文献机制未能充分描述这些次要产物。将多种取代反应纳入其中，并从自动化初态动力学预测和活性热化学表分析中获得速率常数和热化学数据，展示了此类过程的重要作用。乙醛的形成路径被证明是丙酮甲基被自由氢原子取代的直接结果，而丙烯的形成则源于丙酮烯醇形式中的羟基被自由氢原子取代。