Electronic Transitions of C₆H₄⁺ Isomers: Neon Matrix and Theoretical Studies

Abstract

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 cis- (cis-HCC-CH═CH-CCH)(+) and trans-3-hexene-1,5-diyne (trans-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 1 (2)A" and 2 (2)A" ← X (2)A", 1(2)A2 and 2 (2)A2 ← X (2)B1, and 1 (2)Bg and 2 (2)B(g) ← X (2)A(u) electronic transitions of these cations. Two overlapping systems are detected at around 420 nm and tentatively assigned to the 1 (2)A" ← X (2)A" electronic transitions of propargyl cyclopropene and 2 (2)B1 ← X (2)A2 of o-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(+).

Hydrocarbon growth via ion-molecule reactions: computational studies of the isomers of C4H2+, C6H2+ and C6H4+ and their formation paths from acetylene and its fragments

“…Fulara et al did not locate some relatively low-energy isomers including C3, C10, C12, C15 and C17. For isomers in common, the relative energy ordering presented in Table7matches the ordering calculated by Fulara et al45…”

Structural investigation of doubly-dehydrogenated pyrene cations

“…This is the strongest transition for this isomer. A similar bicyclic structure was determined as the most stable isomer for the C 6 H 4 + radical (m-benzyne) cation with CASPT2, and DFT, CCSD(T) and MP2 calculations, 43,44 supporting that the cyclopropenyl deformation is real and not simply an artefact of the computational methods used. 1,3-ddPy + was also investigated with the BP86 and wB97XD functionals using the same 6-311++G(2d,p) basis set to ensure that the 790 cm À1 band was real.…”

Direct observation ofo-benzyne formation in photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions

“…There are only few investigations of electronic absorption spectra in the ultraviolet/ visible (UV/VIS) range, in the gas phase or in low-temperature matrices. [76][77][78][79][80][81][82] To the best of our knowledge, the electronic absorption spectrum, i.e., the UV/VIS spectrum, of o-benzyne in solution, has not been reported.…”