OPTICAL DETECTION OF TWO NEW ISOMERS OF C7H7 IN A TOLUENE DISCHARGE

In an effort to optically detect reactive intermediates implicated in mechanisms of C$_{7}$H$_{7}$ decomposition and formation, we have been interrogating radical products formed in supersonically cooled discharges of various C$_{7}$H$_{8}$ precursors. So far, two C$_{7}$H$_{7}$ isomers (neither of which is benzyl or tropyl) have been observed in the $470-455$\,nm region in resonant two-color two-photon ionization spectra. Both isomers were first observed in our laboratory in a toluene discharge, but they can be more efficiently produced from other precursors: one of them, for which we have measured an adiabatic ionization energy (AIE) of 6.92\,eV, is highly conspicuous in a discharge of 1,6-heptadiyne; the other, with an AIE of 7.16\,eV, is most efficiently generated from cycloheptatriene. Both species possess low frequency vibrational modes suggestive of acyclic structures, and because they absorb at similar wavelengths to the 1-vinylpropargyl radical, may also incorporate substituted propargyl chromophores. Quantum chemical calculations, additional chemical tests, and measurements of ground state vibrational frequencies by dispersed fluorescence are on-going to conclusively assign each spectrum to a particular carrier. The implications of our surprisingly facile discovery of two putatively unknown isomers will be discussed in the context of recent investigations of combustion and pyrolysis processes that begin or terminate with C$_{7}$H$_{7}$