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 $$-$$ 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 $$ origin band spectrum has allowed an accurate determination of spectroscopic constants for both the $$ electronic ground and $$ 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 C₂H₂ polyenyl chain converging to a residual energy gap, suggesting the presence of residual nonuniform C-C bond lengths in a long polyenyl chain.