<i>N</i>‑Carbazolyl π‑Radical\nand Its Antiaromatic Nitrenium Ion: A Threshold Photoelectron Spectroscopic\nStudy

Understanding the\nstructure and properties of heterocyclic\nradicals\nand their cations is crucial for elucidating reaction mechanisms as\nthey serve as versatile synthetic intermediates. In this work, the <i>N</i>-carbazolyl radical <b>1</b> was generated via pyrolysis\nand characterized using photoion mass-selected threshold photoelectron\nspectroscopy coupled with tunable vacuum-ultraviolet synchrotron radiation.\nThe <i>N</i>-centered radical <b>1</b> is classified\nas a π-radical (<sup>2</sup>B<sub>1</sub>), with the unpaired\nelectron found to be delocalized over the central five-membered ring\nof the carbazole. Adiabatic ionization energies corresponding to the\ntransition from radical <b>1</b> to its singlet <b>1</b><sup>+</sup>(<sup>1</sup>A<sub>1</sub>) and triplet <b>1</b><sup>+</sup>(<sup>3</sup>B<sub>2</sub>) cations were determined to\nbe 7.70 ± 0.03 and 8.14 ± 0.03 eV, respectively. The antiaromatic\nnitrenium ion <b>1</b><sup><b>+</b></sup> exhibits a singlet\nground state with an experimental singlet–triplet energy gap\n(Δ<i>E</i><sub>S–T</sub>) of −0.44 eV\n(10.1 kcal/mol), in very good agreement with theory. <i>N</i>-centered radicals are found to have a higher ionization energy than\ntheir <i>C</i>-centered analogues due to stabilization of\nthe singly occupied molecular orbital.