Speaker
Description
Carbocations, as reactive intermediates, play a significant role in many organic reactions. A detailed description of their structure and stability is relevant in the understanding of such reactions and thus crucial for modern synthetic chemistry. Carbocations are distinguished into two classes, classical (carbenium) and non-classical (carbonium) carbocations, where the non-classical carbocations are distinguished by a 2-center-3-electron bond at the central carbon atom. Historically, the debate around non-classical carbocations has been dominated by discussions about the structure of the 2-norbornyl cation. Its non-classical structure, proposed as early as 1949 by Saul Winstein, sparked a passionate debate, influencing and driving forward the advancement of several spectroscopic, experimental, and theoretical methods. In recent developments, XRD imaging of the crystal structure of the 2-norbornyl cation have shown its non-classical properties. Nevertheless, the gas phase structure remains unclear.
In this study, we investigated the non-classical structure and isomerization reactions of the norbornyl system in the gas phase, using infrared action spectroscopy in helium nanodroplets, employing coherent infrared photons from the Free-Electron Laser at the Fritz Haber Institute (FHI-FEL). The well resolved IR action spectra, obtained at the cryogenic temperature of 0.37 K, aided by quantum chemistry calculations, are extremely useful to determine the molecular structure of ionic species. For the norbornyl system, three possible isomerization products were investigated, and the product composition gave insight into energy contributions from experimental parameters via kinetic selectivity. By this, the isomerization of the norbornyl system was partially prevented, allowing the investigation of the non-classical structure of the 2-norbornyl cation in the gas phase for the first time.
