Saraswat Mayank, Portela-Gonzalez Adrian, Mendez-Vega Enrique, Sander Wolfram, Hemberger Patrick
Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, Bochum 44780, Germany.
Laboratory for Synchrotron Radiation and Femtochemistry, Paul Scherrer Institut (PSI), Villigen CH-5232, Switzerland.
J Phys Chem A. 2024 Nov 14;128(45):9747-9753. doi: 10.1021/acs.jpca.4c05855. Epub 2024 Oct 26.
Understanding the structure and properties of heterocyclic radicals and their cations is crucial for elucidating reaction mechanisms as they serve as versatile synthetic intermediates. In this work, the -carbazolyl radical was generated via pyrolysis and characterized using photoion mass-selected threshold photoelectron spectroscopy coupled with tunable vacuum-ultraviolet synchrotron radiation. The -centered radical is classified as a π-radical (B), with the unpaired electron found to be delocalized over the central five-membered ring of the carbazole. Adiabatic ionization energies corresponding to the transition from radical to its singlet (A) and triplet (B) cations were determined to be 7.70 ± 0.03 and 8.14 ± 0.03 eV, respectively. The antiaromatic nitrenium ion exhibits a singlet ground state with an experimental singlet-triplet energy gap (Δ) of -0.44 eV (10.1 kcal/mol), in very good agreement with theory. -centered radicals are found to have a higher ionization energy than their -centered analogues due to stabilization of the singly occupied molecular orbital.
了解杂环自由基及其阳离子的结构和性质对于阐明反应机制至关重要，因为它们是通用的合成中间体。在这项工作中，通过热解产生了咔唑基自由基，并使用光离子质量选择阈值光电子能谱结合可调谐真空紫外同步辐射对其进行了表征。以咔唑中心的自由基被归类为π自由基（B），发现未成对电子离域在咔唑的中心五元环上。从自由基到其单重态（A）和三重态（B）阳离子的绝热电离能分别确定为7.70±0.03和8.14±0.03 eV。反芳香性氮鎓离子表现出单重态基态，实验测得的单重态-三重态能隙（Δ）为-0.44 eV（10.1 kcal/mol），与理论非常吻合。由于单占据分子轨道的稳定化，发现以咔唑中心的自由基比其以氮中心的类似物具有更高的电离能。
