Spectroscopic characterization of C7H3+ and C7H3˙: electronic absorption and fluorescence in 6 K neon matrices

Abstract

Mass selective deposition of C₇H₃⁺ (m/z = 87) into solid neon reveals the 1¹A₁ ← X¹A₁ electronic absorption system of hepta-1,2,3,4,5,6-heptahexaenylium cation B⁺ [H₂CCCCCCCH]⁺ with an origin band at 441.3 nm, 1¹A′ ← X¹A′ transition of 2,4-pentadiynylium,1-ethynyl cation C⁺ [HCCCHCCCCH]⁺ starting at 414.6 nm and the 1¹A₁ ← X¹A₁ one of cyclopropenylium,1,3-butadiynyl cation A⁺ [HCCCCC<(CHCH)]⁺ with an onset at 322.2 nm. Vibrationally resolved fluorescence was observed for isomer B⁺ upon laser excitation of the absorption bands in the 1¹A₁ ← X¹A₁ transition. After neutralization of the cations in the matrix five absorption systems of the C₇H₃ neutral radicals starting at 530.3, 479.4, 482.3, 325.0 and 302.5 nm were detected. These were identified as the 1²A′ ← X²A′ and 2²A′ ← X²A′ electronic transitions of 2-(buta-1,3-diynyl)cycloprop-2yl-1-1ylidene E˙ [HCCCCC<(CCH₂)]˙, 1²B₁ ← X²B₁ of 1,2,3,4,5,6-heptahexaenyl B˙ [H₂CCCCCCCH]˙, 3²B₁ ← X²B₁ of 3-buta-1,3-diynyl-cyclopropenyl A˙ [HCCCCC<(CHCH)]˙ and 2²B₁ ← X²A₂ transition of 1,2-divinylidene-cyclopropanyl radical F˙ [HCC-cyc-(CCHC)-CCH]˙, respectively. The assignment is based on calculated vertical excitation energies using the CASPT2 method. Comparison of the calculated harmonic vibrational frequencies with those inferred from the spectra supports the assignment.