Kinetic study of the OH radical reaction with phenylacetylene

Optical Identification of the Resonance-Stabilized para-Ethynylbenzyl Radical

We report the spectroscopic observation of the jet-cooled para-ethynylbenzyl (PEB) radical, a resonance-stabilized isomer of C₉H₇. The radical was produced in a discharge of p-ethynyltoluene diluted in argon and probed by resonant two-color two-photon ionization (R2C2PI) spectroscopy. The origin of the D₀(²B₁)-D₁(²B₁) transition of PEB appears at 19,506 cm^-1. A resonant two-color ion-yield scan reveals an adiabatic ionization energy (AIE) of 7.177(1) eV, which is almost symmetrically bracketed by CBS-QB3 and B3LYP/6-311G++(d,p) calculations. The electronic spectrum exhibits pervasive Fermi resonances, in that most a₁ fundamentals are accompanied by similarly intense overtones or combination bands of non-totally symmetric modes that would carry little intensity in the harmonic approximation. Under the same experimental conditions, the m/z = 115 R2C2PI spectrum of the p-ethynyltoluene discharge also exhibits contributions from the m-ethynylbenzyl and 1-phenylpropargyl radicals. The former, like PEB, is observed herein for the first time, and its identity is confirmed by measurement and calculation of its AIE and D₀-D₁ origin transition energy; the latter is identified by comparison with its known electronic spectrum (J. Am. Chem. Soc., 2008, 130, 3137-3142). Both species are found to co-exist with PEB at levels vastly greater than might be explained by any precursor sample impurity, implying that interconversion of ethynylbenzyl motifs is feasible in energetic environments such as plasmas and flames, wherein resonance-stabilized radicals are persistent.

Kinetics of the OH Radical Reaction with Fulvenallene from 298 to 450 K

Self-recombination and cross-reactions of large resonant stabilized hydrocarbon radicals such as fulvenallenyl (C7H5) are predicted to form polycyclic aromatic hydrocarbons in combustion and the interstellar medium. Although fulvenallenyl is likely to be present in these environments, large uncertainties remain about its formation mechanisms. We have investigated the formation of fulvenallenyl by reacting the OH radical with fulvenallene (C7H6) over the 298 to 450 K temperature range and at a pressure of 5 Torr (667 Pa). The reaction rate coefficient is found to be 8.8(±1.7) × 10(-12) cm(3) s(-1) at room temperature with a negative temperature dependence that can be fit from 298 to 450 K to k(T) = 8.8(±1.7) × 10(-12) (T/298 K)(-6.6(±1.1)) exp[-(8.72(±3.03) kJ mol(-1))/(R((1/T) - (1/298 K)))] cm(3) s(-1). The comparison of the experimental data with calculated abstraction rate coefficients suggests that over the experimental temperature range, association of the OH radical to fulvenallene plays a significant role likely leading to a low fulvenallenyl branching fraction.