Electronic absorption spectra of H2C6O⁺ isomers: produced by ion-molecule reactions.

Three absorption systems with origins at 354, 497, and 528 nm were detected after mass-selected deposition of H2C6O(+) in a 6 K neon matrix. The ions were formed by the reaction of C2O with HC4H(+) in a mixture of C3O2 and diacetylene in a hot cathode source, or by dissociative ionization of tetrabromocyclohexadienone. The 497 and 354 nm systems are assigned to the 1(2)A″ ← X(2)A″ and 2(2)A″ ← X(2)A″ electronic transitions of B(+), (2-ethynylcycloallyl)methanone cation, and the 528 nm absorption to the 1(2)A2 ← X(2)B1 transition of F(+), 2-ethynylbut-3-yn-1-enone-1-ylide, on the basis of calculated excitation energies with CASPT2.