Paul Mathias, Peckelsen Katrin, Thomulka Thomas, Neudörfl Jörg, Martens Jonathan, Berden Giel, Oomens Jos, Berkessel Albrecht, Meijer Anthony J H M, Schäfer Mathias
University of Cologne, Department of Chemistry, Greinstrasse 4, 50939 Köln, Germany.
Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525 ED Nijmegen, The Netherlands.
Phys Chem Chem Phys. 2019 Aug 14;21(30):16591-16600. doi: 10.1039/c9cp02316j. Epub 2019 Jul 18.
A charge-tagged phenyl pyruvic acid derivative was investigated by tandem-MS, infrared (IR) ion spectroscopy and theory. The tailor-made precursor ions efficiently lose CO in collision induced dissociation (CID) experiments, offering access to study the secondary decay reactions of the product ions. IR ion spectroscopy provides evidence for the formation of an enol acid precursor ion structure in the gas phase and indicates the presence of enol products formed after CO loss. Extensive DFT computations however, suggest intermediate generation of hydroxycarbene products, which in turn rearrange in a secondary process to the enol ions detected by IR ion spectroscopy. Quantum mechanical tunneling of the hydroxycarbene can be excluded since no evidence for aldehyde product ion formation could be found. This finding is in contrast to the behavior of methylhydroxycarbene, which cleanly penetrates the energy barrier to form exclusively acetaldehyde at cryogenic temperatures in an argon matrix via quantum mechanical hydrogen tunneling. The results presented here are attributed to the highly excited energy levels of the product ions formed by CID in combination with different barrier heights of the competing reaction channels, which allow exclusive access over one energy barrier leading to the formation of the enol tautomer ions observed.
通过串联质谱、红外（IR）离子光谱和理论研究了一种带有电荷标签的苯丙酮酸衍生物。在碰撞诱导解离（CID）实验中，特制的前体离子有效地失去了CO，从而能够研究产物离子的二级衰变反应。红外离子光谱为气相中烯醇酸前体离子结构的形成提供了证据，并表明了CO损失后形成的烯醇产物的存在。然而，广泛的密度泛函理论（DFT）计算表明会生成羟基卡宾产物中间体，这些中间体又会在二级过程中重排为通过红外离子光谱检测到的烯醇离子。由于未发现醛产物离子形成的证据，因此可以排除羟基卡宾的量子力学隧穿。这一发现与甲基羟基卡宾的行为形成对比，甲基羟基卡宾在低温下于氩气基质中通过量子力学氢隧穿干净利落地穿透能垒，仅形成乙醛。此处呈现的结果归因于CID形成的产物离子的高激发能级，以及竞争反应通道的不同势垒高度，这使得能够唯一地越过一个能垒，从而形成所观察到的烯醇互变异构体离子。
