Isomer Differentiation of Trapped C₁₆H₁₀⁺ Using Low-Energy Collisions and Visible/VUV Photons

Abstract

Polycyclic aromatic hydrocarbons are major species in astrophysical environments, and this motivates their study in samples of astrochemical interest such as meteorites and laboratory analogues of stardust. Molecular analyses of carbonaceous matter in these samples show a dominant peak at m/z = 202.078 corresponding to C 16 H 10 . Obtaining information on the associated isomeric structures is a challenge for the molecular analysis of samples available in very small quantities (mg or less). Here we show that coupling laser desorption ionization mass spectrometry with ion trapping opens up the possibility of unraveling isomers by activating ion fragmentation via collisions or photon absorption. We report the best criteria for differentiating isomers with comparable dissociation energies, namely pyrene, fluoranthene, and 9-ethynylphenanthrene, on the basis of the parent dissociation curve and the ratio of dehydrogenation channels. Photoabsorption schemes (multiple photon absorption in the visible range and single photon absorption at 10.5 eV) are more effective in differentiating these isomers than activation by low energy collisions. The impact of the activation scheme on the fragmentation kinetics and dehydrogenation pathways is discussed. By analyzing the 10.5 eV photodissociation measurements with a simple kinetic model, we were able to derive a branching ratio for the H and 2H/H 2 loss channels of the parent ions. The results suggest a role in the formation of H 2 for bay hydrogens that are present in both fluoranthene and 9-ethynylphenanthrene. In addition, we suggest for the latter the presence of a highly competitive 2H loss channel, possibly associated with the formation of a pentagonal ring.

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 .…”

“…Examples of other novel molecules with potential astrochemical significance include 2-aza-1,3-butadiene, 4 the C 7 H 5 radical, 5 functionalized buckyballs in the form of C 60 O + and C 60 OH + , 6 the 1quinolinium (C 9 H 7 NH + ) cation along with its corresponding neutral and hydrogenated radicals, 7 cyano-cyclopentadiene ions, 8 the simplest aminooxycarbene in aminohydroxymethylene (H 2 N−C ̈−OH), 9 1-cyanocyclobutene (C 5 H 5 N), 10 monocyclic carbon ring cations of the form C 2n + , 11 the phenalenyl radical, 12 and isomers of the PAH C 16 H 10 + . 13 Purely theoretical studies are providing characterization for other unique molecules including glycolic acid, 14 pyridyl radicals, 15 and diazirine along with its cyclic isomers. 16 Additionally, the molecular structure and spectra of noncovalent interactions are also characterized through a mixture of theory and experiment for glycerol−water clusters, 17 HCN and CH 3 Cl clusters, 18 and complexes of NH 3 •••CO.…”