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 <i>cis-</i> (<i>cis</i>-HCC–CHCH–CCH)⁺ and <i>trans</i>-3-hexene-1,5-diyne (<i>trans</i>-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 <b>1</b> ²A″ and <b>2</b> ²A″ ← X̃ ²A″, <b>1</b>²A₂ and <b>2</b> ²A₂ ← X̃ ²B₁, and <b>1</b> ²B_g and <b>2</b> ²B_g ← X̃ ²A_u electronic transitions of these cations. Two overlapping systems are detected at around 420 nm and tentatively assigned to the <b>1</b> ²A″ ← X̃ ²A″ electronic transitions of propargyl cyclopropene and <b>2</b> ²B₁ ← X̃ ²A₂ of <i>o</i>-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<sup>+