Electronic Transitions of C5H3+ and C5H3: Neon Matrix and CASPT2 Studies

Two absorption systems of C5H3+ starting at 350 and 345 nm were detected following mass-selective deposition of m/e = 63 ions in a 6 K neon matrix. These are assigned to the 1 1A1 ← X 1A1 electronic transition of 1,2,3,4-pentatetraenylium H2CCCCCH+ (isomer B+) and 1 1B2 ← X 1A1 of penta-1,4-diyne-3-ylium HCCCHCCH+ (C+). The absorptions of neutral C5H3 isomers with onsets at 434.5, 398.3, 369.0, and 267.3 nm are also detected. The first two systems are assigned to the 1 2B1 ← X 2B1 and 1 2A2 ← X 2B1 transitions of isomer B and C, respectively, and the latter two to ethynylcyclopropenyl (A) and 3-vinylidenecycloprop-1-enyl (D) radicals. The structural assignments are based on the adiabatic excitation energies calculated with the MS-CASPT2 method. A vibrational analysis of the electronic spectra, based on the calculated harmonic frequencies, supports this.