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 (C 6 H 5 CC˙) and its cation (C 6 H 5 C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 ( 3 B 1 ) ground state, while the closed-shell singlet ( 1 A 1 ) state lies just 2.8 kcal mol -1 (0.12 eV) higher in energy. The reactive phenylethynyl radical abstracts hydrogen to form ethynylbenzene (C 6 H 5 CCH) but also isomerizes via H-shift to the o -, m -, and p -ethynylphenyl isomers (C 6 H 4 CCH). These radicals are very reactive and undergo ring-opening followed by H-loss to form a mixture of C 8 H 4 triynes, along with low yields of cyclic 3- and 4-ethynylbenzynes (C 6 H 3 CCH). At higher temperatures, dehydrogenation from the unbranched C 8 H 4 triynes forms the linear tetraacetylene (C 8 H 2 ), an astrochemically relevant polyyne.