Gas-Phase Optical Detection of 3‑Ethynylcyclopentenyl: A Resonance-Stabilized C₇H₇ Radical with an Embedded 1‑Vinylpropargyl Chromophore

The 3-ethynylcyclopentenyl radical (3ecpr) has been identified as the carrier of an electronic spectrum with origin at 21792 cm^–1 using resonant ionization and laser-induced fluorescence spectroscopies. The radical was first detected in a toluene discharge and is most efficiently produced from 1,6-heptadiyne. Overwhelming spectroscopic and chemical evidence support our diagnosis: (1) the observed (6.93 eV) and calculated (CCSD­(T)/pVQZ) adiabatic ionization energies are the same; (2) the origin band rotational contour can be well simulated with calculated rotational constants; (3) convincing vibrational assignments can be made using computed frequencies; and (4) the same spectrum was observed in a discharge of 1-ethynylcyclopentanol, which contains the 3ecpr carbon framework. The π-chromophore is essentially that of trans-1-vinylpropargyl, a highly resonance-stabilized C₅H₅ radical that persists in conditions relevant to both combustion and circumstellar atmospheres. We suggest that 3ecpr may be a similarly important radical warranting inclusion in models of C₇H₇ chemistry. It is the second C₇H₇ isomer with a five-membered ring yet to be detected, the other being vinylcyclopentadienyl, a species crucially involved in a recently proposed mechanism of soot formation (Science, 2018, 361, 6406, 997–1000). We argue that 3ecpr should be a significant product of H addition to ethynylcyclopentadiene (C₇H₆), a known product of benzyl decomposition. Further, it is plausible that 3ecpr is the unidentified C₇H₇ product of sequential addition of acetylene to propargyl (J. Phys. Chem. Lett., 2015, 6, 20, 4153–4158) in which 1-vinylpropargyl is an intermediate. As such, the nC₂H₂ + C₃H₃ cascade could represent a facile synthesis of a substituted five-membered ring in flames and stellar outflows.