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

Abstract

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.