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High-Resolution Electronic Spectrum of the 1,4,6-Heptatrienyl Radical in the Gas Phase.

We report the gas-phase identification of the 1,4,6-heptatrienyl (C7H9) radical via its A~2B1-X~2A2 electronic transition spectrum. The optical absorption spectrum in the 590-630 nm region is recorded using cavity ring-down spectroscopy in combination with a supersonic plasma jet. An analysis of the rotationally resolved 000 origin band spectrum has allowed an accurate determination of spectroscopic constants for both the X~2A2 electronic ground and A~2B1 excited states of this radical. Ab initio calculations at the CASPT2/cc-pVTZ level have been performed to predict the radical structure and molecular constants that are in good agreement with the experimental results. By combination with available experimental data for the allyl and 1,4-pentadienyl radicals, we extrapolate an excitation energy gap between the ground and first excited states of a long C2n-1H2n+1 polyenyl chain converging to a residual energy gap, suggesting the presence of residual nonuniform C-C bond lengths in a long polyenyl chain.