Laboratory Infrared Spectroscopy of Gaseous Negatively Charged Polyaromatic Hydrocarbons

Abstract

Based largely on infrared spectroscopic evidence, polycyclic aromatic hydrocarbon (PAH) molecules are now widely accepted to occur abundantly in the interstellar medium. Laboratory infrared spectra have been obtained for a large variety of neutral and cationic PAHs, but data for anionic PAHs are scarce. Nonetheless, in regions with relatively high electron densities and low UV photon fluxes, PAHs have been suggested to occur predominantly as negatively charged ions (anions), having substantial influence on cloud chemistry. While some matrix spectra have been reported for radical anion PAHs, no data is available for even-electron anions, which are more stable against electron detachment. Here we present the first laboratory infrared spectra of deprotonated PAHs ( [PAH -H] − ) in the wavelength ranges between 6 and 16 μm and around 3 μm. Wavelength-dependent infrared multiple-photon electron detachment is employed to obtain spectra for deprotonated naphthalene, anthracene, and pyrene in the gas phase. Spectra are compared with theoretical spectra computed at the density functional theory level. We show that the relative band intensities in different ranges of the IR spectrum deviate significantly from those of neutral and positively charged PAHs, and moreover from those of radical anion PAHs. These relative band intensities are, however, well reproduced by theory. An analysis of the frontier molecular orbitals of the even-and odd-electron anions reveals a high degree of charge localization in the deprotonated systems, qualitatively explaining the observed differences and suggesting unusually high electric dipole moments for this class of PAH molecules.

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging

“…The three C−H bond dissociation energies of anthracene are predicted to be very similar, leading to three anthracenyl radical isomers nearly degenerate in energy (1,28,29,31). The anionic isomers are calculated to be more separated in energy, with the 9-anthracenyl anion being the most stable and the 1-and 2-anthracenyl anions lying 0.14 eV and 0.18-0.21 eV higher in energy, respectively (6,29).…”

“…The reactivity of the 9-anthracenyl radical with naphthalene and toluene has also been measured experimentally (26). A recent multiple-photon electron detachment study yielded the infrared action spectrum of the 9-anthracenyl anion (6). Anionic anthracene derivatives in various protonation states are also used in electron transfer dissociation mass spectrometry as an electron source to induce fragmentation of peptide backbones (27).…”

“…Anionic anthracene derivatives in various protonation states are also used in electron transfer dissociation mass spectrometry as an electron source to induce fragmentation of peptide backbones (27). A fair amount of additional theoretical work has been reported detailing the energetics, electronic structure, geometries, vibrational frequencies, and reactivities of the anthracenyl radicals and anions (1,6,(28)(29)(30)(31)(32). The three C−H bond dissociation energies of anthracene are predicted to be very similar, leading to three anthracenyl radical isomers nearly degenerate in energy (1,28,29,31).…”

“…PAHs are therefore common environmental pollutants and have well-documented mutagenic and carcinogenic biological activity (4,5). PAHs are also believed to be abundant in the interstellar medium (6) and may be carriers of the anomalous IR emission bands (7)(8)(9). Recent molecular beam studies indicate that PAH growth can proceed through cold collisions of smaller hydrocarbons under interstellar conditions (10,11).…”

“…Models of dense interstellar clouds find that anionic PAHs are the major carriers of negative charge, rather than free electrons (16). Closed-shell, singly deprotonated PAH carbanions have large electron affinities compared with radical anionic parent species and may therefore be reasonably robust in the interstellar medium (6,13). In this article, we investigate the three dehydrogenated isomers of anthracene, the 9-, 1-, and 2-anthracenyl radicals, via slow photoelectron velocity-map imaging (cryo-SEVI) of the corresponding cryogenically cooled, deprotonated anions.…”