Electronic spectra of C₆H⁺ and C₆H₃⁺ in the gas phase

Abstract

Measurement of the 3 ⌸ Ϫ 3 ⌸ transition of C 6 H ϩ in the gas phase near 19486 cm Ϫ1 is reported. The experiment was carried out with a supersonic slit-jet expansion discharge using cavity ringdown absorption spectroscopy. Partly resolved P lines and observation of band heads permitted a rotational contour fit. Spectroscopic constants in the ground and excited-state were determined. The density of ions being sampled is merely 2 ϫ 10 8 cm Ϫ3 . Broadening of the spectral lines indicates the excited-state lifetime to be Ϸ100 ps. The electronic transition of HC 6 H 2 ϩ at 26402 cm Ϫ1 assumed to be 1 A 1 Ϫ X 1 A 1 in C 2v symmetry could not be rotationally resolved. (J Am Soc Mass Spectrom 2010, 21, 694 -697)

Spectroscopic Survey of Electronic Transitions of C6H, 13C6H, and C6D

“…12, indicated by #, was also observed in the aforementioned studies. 21,35 This again demonstrates that these features of decreased absorption are not just artifact features. Bands XIII and XIV: B 2 Π Ω ´X2 Π Ω vibronic band In the frequency range 19 587 ´19 600 cm ´1 another composite band feature has been observed which again is heavily overlaid by sharp C 2 (0,0) and (1,1) Swan band transitions.…”

“…Figure shows a recording of a band centered at 19 486 cm –1 . In a previous study this band feature was associated to the 2 A″–X 2 A″ electronic transition of the C 4 H 4 + cation, whereas in another study, it was identified as a 3 Σ – –X 3 Σ – transition of the C 6 H + cation . Because the spectrum is overlaid by a number of strong and sharp C 2 Swan transitions, particularly in the region where a Q branch of a possible 2 Π 3/2 – 2 Π 3/2 transition of C 6 H may be expected, an unambiguous assignment of this band is difficult.…”

“…In a previous study this band feature was associated to the 2 A″−X 2 A″ electronic transition of the C 4 H 4 + cation, 35 whereas in another study, it was identified as a 3 Σ − −X 3 Σ − transition of the C 6 H + cation. 21 Because the spectrum is overlaid by a number of strong and sharp C 2 Swan transitions, particularly in the region where a Q branch of possible 2 Π 3/2 − 2 Π 3/2 transition of C 6 H may be expected, an unambiguous assignment of this band is difficult. Several band contours were simulated, and by imposing a transition of 2 Σ− 2 Σ symmetry and keeping the ground state fixed at values for the μ 2 Σ, Band XII can be well reproduced with excited state constants (B′ = 0.04552 cm −1 and γ′ = 12.9 × 10 −3 cm −1 ) matching those of C 6 H. The fact that the observed interference between simulated P branches for both F 1 −F 1 and F 2 −F 2 subbands at ∼19 484.6 cm −1 is reproduced in the simulated spectrum, lends credit to the assignment of this band to a 2 Σ− 2 Σ feature and to assign the C 6 H molecule as its carrier.…”

“…9 Also of interest is the spectroscopic investigation of the C 6 H + cation, both in a neon matrix 20 and in the gas phase. 21 In the present work, cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma has been employed in a wider-ranging spectroscopic survey. All spectroscopic features observed are associated with the B 2 Π electronically excited state and its vibrational modes.…”

“…These states were predicted in ab initio electronic structure calculations. , Harmonic vibrational frequencies of the various modes in C 6 H were ab initio calculated, yielding the lowest mode, ν 11 , at some 100 cm –1 and the highest mode at about 3300 cm –1 . Also of interest is the spectroscopic investigation of the C 6 H + cation, both in a neon matrix and in the gas phase …”