Electronic Transitions of C6H4+ Isomers: Neon Matrix and Theoretical Studies

Three open-chain isomers of C6H4+ and two cyclic ones were detected following mass-selective trapping in 6 K neon matrixes. The open-chain cations 5-hexene-1,3-diyne (CH2═CH–CC–CC–H)+ and cis- (cis-HCC–CH═CH–CCH)+ and trans-3-hexene-1,5-diyne (trans-HCC–CH═CH–CCH)+, possess two absorption systems commencing at 609 and 373, 622 and 385, and 585 and 373 nm, respectively. They are assigned to the 1 2A″ and 2 2A″ ← X̃ 2A″, 12A2 and 2 2A2 ← X̃ 2B1, and 1 2Bg and 2 2Bg ← X̃ 2Au electronic transitions of these cations. Two overlapping systems are detected at around 420 nm and tentatively assigned to the 1 2A″ ← X̃ 2A″ electronic transitions of propargyl cyclopropene and 2 2B1 ← X̃ 2A2 of o-benzyne cation structures. The assignment of the electronic transitions is based on theoretical vertical excitation energies calculated with CASPT2 and EOMEE-CCSDT methods for 12 isomers of C6H4+. These have been carried out at the geometries optimized using several ab initio methods. Adiabatic excitation energies were calculated for the five identified isomers of C6H4+.