Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
J Phys Chem A. 2010 Jan 21;114(2):765-77. doi: 10.1021/jp906919w.
A novel theoretical framework for predicting the branching between roaming and bond fission channels in molecular dissociations is described and applied to the decomposition of acetaldehyde. This reduced dimensional trajectory (RDT) approach, which is motivated by the long-range nature of the roaming, bond fission, and abstraction dynamical bottlenecks, involves the propagation of rigid-body trajectories on an analytic potential energy surface. The analytic potential is obtained from fits to large-scale multireference ab initio electronic structure calculations. The final potential includes one-dimensional corrections from higher-level electronic structure calculations and for the effect of conserved mode variations along both the addition and abstraction paths. The corrections along the abstraction path play a significant role in the predicted branching. Master equation simulations are used to transform the microcanonical branching ratios obtained from the RDT simulations to the temperature- and pressure-dependent branching ratios observed in thermal decomposition experiments. For completeness, a transition-state theory treatment of the contributions of the tight transition states for the molecular channels is included in the theoretical analyses. The theoretically predicted branching between molecules and radicals in the thermal decomposition of acetaldehyde is in reasonable agreement with the corresponding shock tube measurement described in the companion paper. The prediction for the ratio of the tight to roaming contributions to the molecular channel also agrees well with results extracted from recent experimental and experimental/theoretical photodissociation studies.
一种用于预测分子离解过程中 roaming 和 bond fission 通道分支的新理论框架被描述并应用于乙醛的分解。这种简化维度轨迹（RDT）方法的动机是 roaming、bond fission 和 abstraction 动力学瓶颈的远程性质，涉及在解析势能表面上传播刚体轨迹。解析势能是通过拟合大规模多参考从头算电子结构计算获得的。最终的势能包括来自更高阶电子结构计算的一维修正以及沿加成和抽象路径的守恒模式变化的影响。沿抽象路径的修正在预测的分支中起着重要作用。主方程模拟用于将从 RDT 模拟获得的微正则分支比转换为热分解实验中观察到的温度和压力相关的分支比。为了完整性，在理论分析中还包括对分子通道紧过渡态贡献的过渡态理论处理。乙醛热分解中分子和自由基之间的理论预测分支与伴生论文中描述的相应激波管测量结果相符。对分子通道中紧过渡态和 roaming 贡献的比值的预测也与最近的实验和实验/理论光解研究中提取的结果很好地吻合。
