Electronic spectra of H(2)C(2n)H+ radicals (n=2-6): an ab initio study

Electronic Spectra of the Triacetylene Cation (HC6H+) and Protonated Triacetylene (HC6H2+) Tagged with Ar

Polyacetylene cations (HC2nH+) play important roles in combustion processes and in the chemistry of planetary atmospheres and interstellar clouds. Here we report the electronic spectrum for the triacetylene cation (HC6H+) recorded over the 300–610nm range by photodissociating mass-selected ions tagged with argon atoms in a tandem mass spectrometer. The spectrum shows three band systems that are assigned to (origin transition 16665cm−1), (origin transition 23916cm−1), and (origin transition 29920cm−1). Although the band system is well known, the and band systems are observed for the first time in the gas phase. In addition, the electronic spectrum of the protonated triacteylene cation tagged with an argon atom (HC6-Ar) is reported, providing the first gas-phase spectrum for this species.

Theoretical studies on structures and electronic spectra of linear free radicals CnH (n=5-12)

The B3LYP, CAM-B3LYP, and coupled cluster CCSD(T) calculations have been utilized to determine the equilibrium structures of linear carbon radicals CnH (n=5-12) in their ground states, as well as the CASSCF method used to optimize the ground and selected low-lying excited states. DFT-calculations show that even-n radicals C2nH have polyacetylene-like structures with significant single-triple bond length alternation, whereas the odd-numbered analogues C2n+1H exhibit a trend from polyacetylene-like characters into cumulenic-like arrangement towards C ends along the carbon chains. The stabilities of the system under study have been evaluated by analyses of the vibrational frequencies and incremental binding energies. For the whole CnH (n=5-12) series, the vertical excitation energies and oscillator strengths have been calculated at the CASPT2/cc-pVTZ level of theory. At the B3LYP optimized geometries, the lowest 1(2)Δ←X2Π transitions for C5H and C7H occur at 2.36 and 2.14 eV, respectively, comparing well with the observed values of 2.33 and 2.09 eV. Moreover, the strongest 2(2)Π←X2Π transitions for C2nH (n=3-6) are predicted to be at 2.39, 2.00, 1.80, and 1.64 eV, respectively, which are in agreement with the experimental observations. Additionally, the possible dissociation channels and the fragmentation energies of CnH (n=5-12) series are discussed in the paper.