Douglas Kevin M, West Niclas A, Lucas Daniel I, Van de Sande Marie, Blitz Mark A, Heard Dwayne E
School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K.
Leiden Observatory, Leiden University, P.O. Box 9513, Leiden 2300 RA, The Netherlands.
ACS Earth Space Chem. 2024 Nov 20;8(12):2428-2441. doi: 10.1021/acsearthspacechem.4c00188. eCollection 2024 Dec 19.
Rate coefficients for the reaction of CH with CHO were measured for the first time over the temperature range of 37-603 K, with the CH radicals produced by pulsed laser photolysis and detected by CH radical chemiluminescence following their reaction with O. The low temperature measurements (≤93 K) relevant to the interstellar medium were made within a Laval nozzle gas expansion, while higher temperature measurements (≥308 K) were made within a temperature controlled reaction cell. The rate coefficients display a negative temperature dependence below 300 K, reaching (1.3 ± 0.2) × 10 cm molecule s at 37 K, while only a slight positive temperature dependence is observed at higher temperatures above 300 K. Ab initio calculations of the potential energy surface (PES) were combined with rate theory calculations using the MESMER master-equation program in order to predict rate coefficients and branching ratios. The three lowest energy entrance channels on the PES all proceed via the initial formation of a weakly bound prereaction complex, bound by ∼5 kJ mol, followed by either a submerged barrier on the route to the H-abstraction products (CH + CHO), or emerged barriers on the routes to the C- or O-addition species. MESMER calculations indicated that over the temperature range investigated (10-600 K) the two addition channels were uncompetitive, accounting for less 0.3% of the total product yield even at 600 K. The PES containing only the H-abstraction product channel was fit to the experimentally determined rate coefficients, with only a minor adjustment to the height of the submerged barrier (from -2.6 to -5.9 kJ mol) required. Using this new submerged barrier height, and including the subsequent dissociation of the CHO product into CO + H in the PES, rate coefficients and branching ratios were calculated over a wide range of temperatures and pressures and these used to recommend best-fit modified Arrhenius expressions for use in astrochemical modeling. Inclusion of the new rate coefficients and branching ratios in a UMIST chemical model of an outflow from an asymptotic giant branch (AGB) star yielded no significant changes in the abundances of the reactants or the products of the reaction, however, removal of the C-addition channel currently in the UMIST Rate22 database did result in a significant reduction in the abundance of propynal (HCCCHO).
首次在37 - 603 K的温度范围内测量了CH与CHO反应的速率系数。CH自由基由脉冲激光光解产生，并在其与O反应后通过CH自由基化学发光进行检测。与星际介质相关的低温测量（≤93 K）在拉瓦尔喷管气体膨胀装置内进行，而较高温度测量（≥308 K）在温度控制的反应池中进行。速率系数在300 K以下呈现负温度依赖性，在37 K时达到(1.3 ± 0.2) × 10 cm³分子⁻¹ s⁻¹，而在300 K以上的较高温度下仅观察到轻微的正温度依赖性。对势能面（PES）进行了从头算，并结合使用MESMER主方程程序进行速率理论计算，以预测速率系数和分支比。PES上三个最低能量的入口通道均通过初始形成一个弱束缚的预反应复合物进行，该复合物的束缚能约为5 kJ/mol，随后在通往氢提取产物（CH₂ + CHO）的路径上有一个隐式势垒，或者在通往碳或氧加成物种的路径上有显式势垒。MESMER计算表明，在所研究的温度范围（10 - 600 K）内，两个加成通道不具有竞争力，即使在600 K时，其占总产物产率的比例也不到0.3%。仅包含氢提取产物通道的PES与实验测定的速率系数相拟合，仅需对隐式势垒的高度进行微小调整（从 -2.6到 -5.9 kJ/mol）。使用这个新的隐式势垒高度，并在PES中考虑CHO产物随后分解为CO + H的过程，在广泛的温度和压力范围内计算了速率系数和分支比，并用于推荐用于天体化学建模的最佳拟合修正阿伦尼乌斯表达式。将新的速率系数和分支比纳入渐近巨星分支（AGB）恒星外流的UMIST化学模型中，反应物或反应产物的丰度没有显著变化，然而，去除UMIST Rate22数据库中目前的碳加成通道确实导致丙炔醛（HCCCHO）的丰度显著降低。
