Gas phase chemistry of pyrene and related cations with molecules and atoms of interstellar interest Vale ´ry Le Page a , Yeghis Keheyan b , Theodore P. Snow a , Veronica M. Bierbaum a, * a Department of Chemistry and Biochemistry, and Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309, USA b Istituto di Chimica Nucleare del CNR, Monterotondo Stazione, 00016 Roma, Italy Received 9 September 1998; accepted 14 October 1998 Abstract The chemistry of the pyrene radical cation C 16 H 10 + and its derivatives C 16 H 9 + and C 16 H 11 + has been investigated in the gas phase using a flowing afterglow-selected ion flow tube. Rate coefficients have been determined for reactions between C 16 H n + ( n = 9, 10, 11) and H 2 , CO, H 2 O, and NH 3 molecules as well as H, O, and N atoms. These studies supplement previous investigations on the smaller benzene and naphthalene cations. It is found that C 16 H 10 + and C 16 H 11 + display consistent, predictable chemistry with reactivities very similar to those of benzene and naphthalene cations. On the other hand, the striking reactivity of C 16 H 9 + toward atoms, compared to the relative unreactivity of phenylium and naphthylium cations, is believed to result from the triplet nature of this ion in its ground state. Ab initio calculations have been carried out to validate this hypothesis in conjunction with experimental evidence. (Int J Mass Spectrom 185/186/187 (1999) 949 –959) © 1999 Elsevier Science B.V. Keywords: PAH; Ion–molecule reactions; Interstellar chemistry; Flowing afterglow 1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are thought to be ubiquitous in the interstellar medium (ISM) and they have been proposed, in cationic form, as possible carriers of the diffuse interstellar bands (DIBs) [1] which are visible absorption features in stellar spectra. Their simple composition (both C and H atoms are abundant in environments where DIBs are found) together with the ability of cyclic rings to withstand strong UV flux without undergoing fragmen- tation support the viability of this hypothesis [2– 4]. Although the chemical path leading to the forma- tion of PAHs in the ISM is still unknown and largely open to speculation [5–7], there are even fewer studies about their reactivity in the presence of the other constituents of the diffuse clouds where DIBs mostly appear. The existence of neutral PAHs in the ISM is largely accepted because of their ability to explain certain IR emission features; there are good matches between these emission lines and vibrational transitions in characteristic PAHs [8]. To account for the DIBs, however, PAHs are expected to be in cationic or radical form and the existence of neutrals does not guarantee the presence of cations for their * Corresponding author. Dedicated to Professor Michael Bowers on the occasion of his 60th birthday, and in recognition of his seminal contributions to gas phase ion chemistry. 1387-3806/99/$20.00 © 1999 Elsevier Science B.V. All rights reserved PII S1387-3806(98)14217-3 International Journal of Mass Spectrometry 185/186/187 (1999) 949 –959