Nakamura Masaaki, Tsai Po-Yu, Kasai Toshio, Lin King-Chuen, Palazzetti Federico, Lombardi Andrea, Aquilanti Vincenzo
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.
Faraday Discuss. 2015;177:77-98. doi: 10.1039/c4fd00174e.
Recent experimental and theoretical advances in the study of the dissociation of excited molecules are revealing unexpected mechanisms, when their outcomes are tackled by combining (i) space-time ion imaging of translational features, with (ii) spectroscopic probing of rotational and vibrational distributions; crucial is the assistance of (iii) the quantum chemistry of structural investigations of rearrangements of chemical bonds, and of (iv) the simulations of molecular dynamics to follow the evolution of selective bond stretching and breaking. Here we present results of such an integrated approach to methyl formate, HCOOCH3, the simplest of esters; the main focus is on the rotovibrationally excited CO (v=1) product and in general on the energy distribution in the fragments. Previous laser studies of dissociation into CO and CH3OH at a sequence of various wavelengths discovered signatures of a roaming mechanism by the late arrival of CO (v=0) products in time-of-flight ion imaging. Subsequent detailed investigations as a function of excitation energy provided the assessment of the threshold, which opens for triple breakdown into CO and further fragments H and CH3O, as spectroscopically characterized by ion imaging and FTIR respectively. Accompanying quantum mechanical electronic structure calculations and classical molecular dynamics simulations clarify the origin of these fragments through "roaming" pathways involving incipient radical intermediates at energies below the triple fragmentation threshold: a specific role is played by nonadiabatic transitions at a conical intersection between ground and excited states; alternative pathways focalize our attention to regions of the potential energy surfaces other than those in the neighbourhoods of saddle points along minimum energy paths: eventually this leads us to look for avenues in reaction kinetics beyond those of venerable transition state theories.
在激发态分子解离研究方面，近期的实验和理论进展揭示了一些意想不到的机制，这些机制是通过将以下方法结合起来得出的：（i）对平动特征进行时空离子成像，以及（ii）对转动和振动分布进行光谱探测；关键在于（iii）对化学键重排结构研究的量子化学，以及（iv）对分子动力学进行模拟以追踪选择性键拉伸和断裂的演化过程。在此，我们展示了对甲酸甲酯（HCOOCH₃，最简单的酯类）采用这种综合方法所取得的结果；主要关注点是转动 - 振动激发的CO（v = 1）产物，总体上是碎片中的能量分布。先前在一系列不同波长下对甲酸甲酯解离为CO和CH₃OH的激光研究，通过飞行时间离子成像中CO（v = 0）产物的延迟到达发现了漫游机制的特征。随后作为激发能量函数的详细研究给出了阈值评估，该阈值开启了三重分解为CO以及进一步的碎片H和CH₃O的过程，分别通过离子成像和傅里叶变换红外光谱进行光谱表征。伴随的量子力学电子结构计算和经典分子动力学模拟通过涉及低于三重碎片化阈值能量下的初始自由基中间体的“漫游”途径阐明了这些碎片的起源：基态和激发态之间锥形交叉处的非绝热跃迁起到了特定作用；替代途径将我们的注意力集中到势能面中除沿着最小能量路径的鞍点附近区域之外的其他区域：最终这引导我们在反应动力学中寻找超越传统过渡态理论的途径。
