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.

Identification of Isoprene Oxidation Reaction Products via Anion Photoelectron Spectroscopy

“…Pulsed discharge in combination with pulsed molecular beam valves are useful tools in the study of anions, cations, and unstable radical species, though species formed in discharge sources can be surprising in terms of connecting the structures produced by the precursor molecules. − The results presented above show that the source used in this study produces a combination of fragments, oxidized fragments, and unfragmented species such as the O 2 – ·isoprene IMC, methylglyoxal, and dimethylglyoxal. ,, While in many cases our results do not allow for an unambiguous structural assignment of species that have clearly undergone some fragmentation, we can unambiguously distinguish oxidized and unoxidized species, and there is evidence that the most prevalent fragment ions are formed by breaking the weakest carbon–carbon or carbon–hydrogen bonds and that single O-atom or atomic ion addition takes place. Indeed, the fact that the mass spectra show distinct product distributions under different conditions, rather than a common, statistical distribution of ions ranging from carbon clusters to sequentially reduced and oxidized clusters, is evidence of a hierarchy of processes, with low-energy events such as electron attachment and ion–molecule association being the most important.…”

Optical Identification of the Resonance-Stabilized para-Ethynylbenzyl Radical

“…The 2 2 B 1 state of propargyl has a double minimum in the ∠CCH coordinate, , and attempts have been made to assign the propargyl 1 2 B 1 –2 2 B 1 transition in terms of C 2 v and C s selection rules . Similarly, the first A ″ excited state of the 1-vinylpropargyl chromophore appears to harbor a ∠CCH double minimum, , presumably owing to the interaction with a nearby A ′ state via the a ″ CCH bend. A relatively sophisticated vibronic coupling calculation may therefore be necessary for accurate vibrational intervals in D 1 , but the B3LYP harmonic D 0 frequency may be sufficiently reliable that upper-state levels involving mode 24 can be identified by SVLE.…”

“…The vacuum chamber used for R2C2PI experiments has been described previously. , The precursor, p -ethynyltoluene, was stored at room temperature in a stainless-steel reservoir and entrained in Ar at 3 atm, admitting a seed ratio smaller than 0.1%. para -Ethynylbenzyl (PEB) was generated using a pulsed discharge nozzle, consisting of annular copper electrodes and Vespel insulators mounted to the faceplate of a pulsed solenoid valve with 0.5 mm orifice.…”

“…Because experimental AIEs of RSRs are generally ca. 0.1 eV higher (respectively, lower) than those predicted by B3LYP/6-311G++(d,p) (respectively, CBS-QB3) calculations, the latter method was also applied to neutral and cationic ground states for the purposes of bracketing AIEs. Franck–Condon (FC) factors for the D 0 – D 1 transition of PEB were calculated using the FCLab II program of Pugliesi and co-workers, , which accounts for the effects of normal coordinate (i.e., Duschinsky) rotation; it does not include treatment of Herzberg–Teller effects.…”

“…27 Within this context, we have embarked on optical surveys for new transient species using resonant two-color two-photon ionization (R2C2PI) and laser-induced fluorescence (LIF). We recently reported electronic spectra of previously unobserved resonance-stabilized isomers of C 7 H 7 (3-ethynylcyclopentenyl), 28 and much more latterly, C 9 H 9 (cis-and trans-metavinylbenzyl). 29 The present study concerns isomers with the molecular formula C 9 H 7 , which appears regularly in the mass spectra of hydrocarbon flames, 30−32 and is highly conspicuous in the soot formation study by Johansson et al 13 The C 9 H 7 potential energy hypersurface has not been well explored spectroscopically, with most gas-phase studies having taken place only in the last 15 years.…”