Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139.
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439.
Proc Natl Acad Sci U S A. 2020 Jan 7;117(1):146-151. doi: 10.1073/pnas.1911326116. Epub 2019 Dec 18.
The 193-nm photolysis of CHCHCN illustrates the capability of chirped-pulse Fourier transform millimeter-wave spectroscopy to characterize transition states. We investigate the HCN, HNC photofragments in highly excited vibrational states using both frequency and intensity information. Measured relative intensities of = 1-0 rotational transition lines yield vibrational-level population distributions (VPD). These VPDs encode the properties of the parent molecule transition state at which the fragment molecule was born. A Poisson distribution formalism, based on the generalized Franck-Condon principle, is proposed as a framework for extracting information about the transition-state structure from the observed VPD. We employ the isotopologue CHCDCN to disentangle the unimolecular 3-center DCN elimination mechanism from other pathways to HCN. Our experimental results reveal a previously unknown transition state that we tentatively associate with the HCN eliminated via a secondary, bimolecular reaction.
193nm 光解 CHCHCN 说明了啁啾脉冲傅里叶变换毫米波光谱学在表征过渡态方面的能力。我们利用频率和强度信息研究了高激发振动态下的 HCN、HNC 光碎片。通过测量 = 1-0 转动跃迁线的相对强度，得到了振动能级布居（VPD）。这些 VPD 编码了母体分子过渡态的特性，分子碎片就是在这个过渡态中产生的。基于广义 Franck-Condon 原理的泊松分布形式被提出来作为一种从观测到的 VPD 中提取关于过渡态结构信息的框架。我们采用同位素 CHCDCN 将 DCN 消除的三中心单分子反应机制与其他途径（到 HCN）区分开来。我们的实验结果揭示了一个以前未知的过渡态，我们初步将其与通过二次双分子反应消除的 HCN 相关联。
