The elusive phenylethynyl radical and its cation: synthesis, electronic structure, and reactivity

Alkynyl radicals and cations are crucial reactive intermediates in chemistry, but often evade direct detection. Herein, we report the direct observation of the phenylethynyl radical (C₆H₅CC˙) and its cation (C₆H₅CC⁺), which are two of the most reactive intermediates in organic chemistry. The radical is generated via pyrolysis of (bromoethynyl)benzene at temperatures above 1500 K and is characterized by photoion mass-selected threshold photoelectron spectroscopy (ms-TPES). Photoionization of the phenylethynyl radical yields the phenylethynyl cation, which has never been synthesized due to its extreme electrophilicity. Vibrationally-resolved ms-TPES assisted by ab initio calculations unveiled the complex electronic structure of the phenylethynyl cation, which appears at an adiabatic ionization energy (AIE) of 8.90 ± 0.05 eV and exhibits an uncommon triplet (³B₁) ground state, while the closed-shell singlet (¹A₁) state lies just 2.8 kcal mol^-1 (0.12 eV) higher in energy. The reactive phenylethynyl radical abstracts hydrogen to form ethynylbenzene (C₆H₅CCH) but also isomerizes via H-shift to the o-, m-, and p-ethynylphenyl isomers (C₆H₄CCH). These radicals are very reactive and undergo ring-opening followed by H-loss to form a mixture of C₈H₄ triynes, along with low yields of cyclic 3- and 4-ethynylbenzynes (C₆H₃CCH). At higher temperatures, dehydrogenation from the unbranched C₈H₄ triynes forms the linear tetraacetylene (C₈H₂), an astrochemically relevant polyyne.