Electronic Spectroscopy of Monocyclic Carbon Ring Cations for Astrochemical Consideration

Abstract

Gas phase electronic spectra of pure carbon cations generated by laser vaporization of graphite in a supersonic jet and cooled to below 10 K and tagged with helium atoms in a cryogenic trap are presented. The measured C 2 n + –He with n from 6 to 14, are believed to be monocyclic ring structures and possess an origin band wavelength that shifts linearly with the number of carbon atoms, as recently demonstrated through N 2 tagging by Buntine et al. ( 34879679 J. Chem. Phys. 2021 155 214302 ). The set of data presented here further constrains the spectral characteristics inferred for the bare C 2 n + ions to facilitate astronomical searches for them in diffuse clouds by absorption spectroscopy.

Photofragment Imaging of Carbon Cluster Cations: Explosive Ring Rupture

“…On the basis of extensive theory and experiments, small carbon clusters are known to exist as both linear-chain and cyclic isomers. − ,− ,,,− The details vary, but this is true for neutral clusters as well as cations and anions. The structures of cations have been investigated with ion mobility measurements conducted in different laboratories, − ,, with matrix isolation spectroscopy on certain cluster sizes, − and with gas phase electronic spectroscopy. − The ion mobility of C n + cations in several laboratories finds that the n = 6 cation is linear, that n = 7–9 ions are mostly linear with a small percent of cyclic structures, that the n = 10 ion is mostly cyclic with a minor percent of a linear structure, and that n = 11–20 ions have only monocyclic ring structures. − , Theory finds that the ions in the n = 6–9 size range are more stable in their cyclic structures, in contrast with the results of ion mobility. ,− This difference is presumably caused by the free energy at increased temperatures in the laser vaporization plasma environment where clusters grow; entropy favors linear chains over rings. Theory agrees that the n = 10–20 species are more stable in monocyclic ring structures. , Recent electronic spectroscopy of tagged cations in the C 2 n + ( n = 6–14) size range has been interpreted as arising from monocyclic ring structures. , …”

“…The structures of cations have been investigated with ion mobility measurements conducted in different laboratories, − ,, with matrix isolation spectroscopy on certain cluster sizes, − and with gas phase electronic spectroscopy. − The ion mobility of C n + cations in several laboratories finds that the n = 6 cation is linear, that n = 7–9 ions are mostly linear with a small percent of cyclic structures, that the n = 10 ion is mostly cyclic with a minor percent of a linear structure, and that n = 11–20 ions have only monocyclic ring structures. − , Theory finds that the ions in the n = 6–9 size range are more stable in their cyclic structures, in contrast with the results of ion mobility. ,− This difference is presumably caused by the free energy at increased temperatures in the laser vaporization plasma environment where clusters grow; entropy favors linear chains over rings. Theory agrees that the n = 10–20 species are more stable in monocyclic ring structures. , Recent electronic spectroscopy of tagged cations in the C 2 n + ( n = 6–14) size range has been interpreted as arising from monocyclic ring structures. , …”

“…The effects of other expansion gases (Ar and N 2 ) were also investigated. This experiment uses the same laser vaporization method and conditions employed in previous ion mobility measurements − , and electronic spectroscopy experiments. − Cluster structures are therefore assumed to be the same as in those experiments, i.e., linear for n < 10 and monocyclic rings for n = 10–20. Ions are analyzed and mass selected in a reflectron time-of-flight mass spectrometer.…”

“…26,56 Recent electronic spectroscopy of tagged cations in the C 2n + (n = 6−14) size range has been interpreted as arising from monocyclic ring structures. 49,50 The dissociation dynamics and energetics of carbon cluster ions have been investigated extensively. Collision-induced dissociation (CID) and photodissociation experiments identified the elimination of C 3 as a prominent dissociation product, although other fragments (C 2 or C 5 elimination) were found for larger clusters.…”

10 Years of the ACS PHYS Astrochemistry Subdivision

“…Most notably, one manuscript reports that silicon carbide grains in conditions like those found in late-stage circumstellar envelopes will produce buckyballs and nanotubes, confirming in large part Nobel Laureate Sir Harry Kroto’s original idea about the astrophysical formation of these carbon allotropes. Examples of other novel molecules with potential astrochemical significance include 2-aza-1,3-butadiene, the C 7 H 5 radical, functionalized buckyballs in the form of C 60 O + and C 60 OH + , the 1-quinolinium (C 9 H 7 NH + ) cation along with its corresponding neutral and hydrogenated radicals, cyano-cyclopentadiene ions, the simplest aminooxycarbene in aminohydroxymethylene (H 2 N–C̈–OH), 1-cyanocyclobutene (C 5 H 5 N), monocyclic carbon ring cations of the form C 2 n + , the phenalenyl radical, and isomers of the PAH C 16 H 10 + . Purely theoretical studies are providing characterization for other unique molecules including glycolic acid, pyridyl radicals, and diazirine along with its cyclic isomers .…”