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 (1)A1 ? X (1)A1 electronic transition of 1234 pentatetraenylium H2CCCCCH(+) (isomer B(+)) and 1 (1)B2 ? X (1)A1 of penta 14 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 (2)B1 ? X (2)B1 and 1 (2)A2 ? X (2)B1 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.