a".Rev. 1992, 92, 1487-1508 1487 PAH and Fullerene Ions and Ion/Molecule Reactions in Interstellar and Circumstellar Chemistry Diethard K. Bohme Deparrment of chermshy and Cenire lor Research in Earth and Space Science, York Unlversky, North Yo&, Ontarb, Canada M3J 1P3 Received Februaty 24, 1992 (Revised Manuscript Received August 11, 1992 ) Contents I. Introductlon 1487 11. Thermochemical Propertles of PAH and 1488 Fullerene Ions A. Recombination Energies 1488 6. Proton Affinities 1489 C. Hydrogen-Atom Affinities 1490 111. Reactions of Ions with PAH and Fullerene 1490 Molecules A. Reactions wlth Atomic Ions 1490 B. Reactions with Molecular Ions 1493 IV. Reactions of PAH and Fullerene Ions 1495 A. Recombination with Electrons and Negative 1495 Ions Diethard Kurt Bohme received his early education In Germany and 6. Reactions wkh Atoms 1498 Canada. He first specialized in chemistryat McGill University where C. Reactions with Molecules 1497 he received his Ph.D. in Physical Chemistry in 1965. After V. Synthesis of PAH and Fullerene Ions and 1500 postdoctoral fellowships in the Physics Department at University Molecules by IonlMolecule Reactions College, London, andthe Aeronomy Laboratoryat the Environmental Science Services Administration in Boulder. CO, he joined York A. Condensation of Carbon Chains 1500 University where he is now Professor of Chemistry, HIS early 6. Growth of Polyhedral Carbon Ions 1501 research interests included ionospheric chemistry. the physlcal- C. Growth of Closed-Shell Polyhedral Carbon 1501 organic chemistry of ions including fundamental aspects of proton- transfer and nucleophilic displacement reactions. and flame-ion Ions chemistry. Currently his research is directed to general aspects VI. Gas-Phase Surface Chemistry with PAH and 1502 of the growth of ions and molecules by ionlmolecuie reactions in Fullerene Ions and Molecules the gas phase (particularly as it relates to interstellar and A. Thermochemistry of Adduct Ions 1502 circumstellar ion chemistry), ion-induced polymerization. and the ion chemistry of fuilerene molecules. B. Structures and Bonding 1503 C. Reactivities of Adduct Ions 1504 crystallized with the discovery3 that some astronomical VII. Acknowledgments 1508 objects emit a broad infrared emission band which peaks VIII. References 1506 at 3050 cm-' and other emission features4 which peak in theregionbetween 1610and890cm-'. Astronomical objects which emit these features include regions I. Introduction associated with individual stars such as H-I1 regions and reflection nebulae, as well as interstellar clouds Although it is well known that polycyclic aromatic such as the IR Cirrus, both in our own and other hydrocarbons (PAHs, or polynuclear aromatic hydro- galaxie~.~Soon after, these infrared emission features carbons) are widespread in the earth's environment, were attributed to infrared fluorescence from molecular- the likely presence of PAHmolecules and ions beyond sized emitters excited by the absorption of ultraviolet the earth's atmosphere in space is still somewhat of a and visible photons.6 The idea that the fluorescence novelty for many chemists. Even though they have originated from vibrations of chemical group3 attached not yet been unequivocally identified, aromatic and to aromatic constituents of amorphous carbon particles7 polycyclic aromatic hydrocarbon molecules and ions led to the proposal that individual PAH molecules are are now generally accepted by the astrophysics com- responsible for the infrared emission based on their munity to be present in interstellar and circumstellar stability against photodissociation and the resemblance environments. Some have even argued for the presence of laboratory infrared fluorescence data of such species of nonplanar PAHs and of hollow cages of carbon atoms with the astrophysical spectra.8 PAH molecules have known as fullerene molecules. also been proposed to be carriers of visible diffuse The hypothesis that PAHs are present in interstellar interstellar bands.g Nowadays the PAHs responsible and circumstellar environments already has a substan- for the infrared features are thought to be more tial historical record' which began with the visionary abundant (- 17 % of the cosmic carbon) than all of the suggestion*in 1956 that related carbonaceous species other known gaseous interstellar organic molecules are responsible for visible diffuse absorption bands and combined.'" 0009-2665/92/0792-1487$10.00/0 0 1992 American Chemical Society 1488 Chemical Revlews, 1992, Vol. 92, No. 7 Bohme Current models of interstellar and circumstellar “cages” in promoting chemical reactions in the gas chemistry have emphasized planar PAHs with ar- phase? Many such intriguing questions are now being rangements of hexagonal rings which are more or less addressed by laboratory ion chemists and chemical compact (“catacondensed”)with the general formula theoreticians worldwide. They have also captured the C6&,2&p such as coronene, C24H12, or elongated poly- interest of the author and provided the motivation to acenes with the general formula C4n+2H~n+4such as write this review. naphthalene, anthracene, and tetracene.l* PAHs with The remainder of this article is organized as follows: loose arrangements of hexagonal rings, such as PAHs First, in section 11, available knowledge is presented on bound by single C-C bonds, have been neglected, as the energetics of several fundamental processes in- have aromatic molecules beyond hydrocarbons con- volving PAH and fullerene ions and molecules: the taining elements in addition to H and C. Neutral and capture of protons by the neutral molecules, the capture positively-charged fused-ringmolecules such as pyrene, of electrons and H atoms by their cations, and the coronene, and ovaline, either completely or partially capture of electrons by their dications. Section I11 hydrogenated, have been invoked to account for both documents known reactions of atomic and molecular the observed broad IR emission features in nebulae8 ions with PAH and fullerene molecules with a view to and the observed diffuse interstellar absorption bandse9 the role of such molecules as sinks for ions, their possible The very recent detection of additional sharp emission destruction by ion/molecule reactions, their ability to features” has led to the proposal that much simpler bond chemically with ions, and, in the case of the linear fused-ring molecules such as naphthalene, an- fullerene molecules, their ability to form endohedral thracene, and tetracene are responsible for the infrared (intracage) molecules. Section IV describes reactions emission and that benzene may also be present in these of ionized PAH and fullerene molecules with atoms environments.12 For example, anthracene has been and molecules. Special attention is given to recombi- suggested as the most abundant of these linear poly- nation reactions with electrons and negative ions, the acenes in the Orion Ridge.12 hydrogenation of fullerene ions, ion-catalyzed recom- The possibility that the fullerene molecule Cw might bination of H atoms, and reactions involving charge be widely distributed in the universe, particularly in transfer, condensation,and adduct formation (including the outflows of carbon stars, was suggested at the time the formation of endohedral adducts). Reactions of of the original discovery of its ~tabi1ity.l~Also, there benzene dimer ions and fullerene dications are also highlighted in this section. Section V draws together have been indications that c60, or a derivative, might be responsible for such features as the diffuse inter- results described in sections I11 and IV in a discussion stellar bands.13 So far spectroscopicsearches for CMin of the synthesisof PAH and fullerene ions and molecules space have led to negative results, and it seems clear by ion/moleculereactions. The condensation of carbon- now that neutral Cw is not responsible for the diffuse chain molecules to form ring molecules is considered interstellar bands.14 As a consequence, attention has first, as a basis for an ensuing discuasion of the growth been directed to the possible astrophysical importance of polyhedral carbon ions. Section V culminates in a of ionized CMand other ionized fullerene molecules, review of current proposals for the growth of closed- as shell polyhedral carbon ions such as fullerene ions. well as their derivatives such as,for example,endohedral (intracage) and exohedral (extracage) complexes in- Section VI enters the realm of surface chemistry in volving alkali, alkali earth, or other elements such which PAH and fullerene ions and molecules are viewed as in terms of their potential role as surfaces in the He and 0.15 Very recently a theory has been proposed occurrence of heterogeneous chemical change. Fun- which makes a strong case for the contribution of damental thermochemical aspects of the formation of fulleranes (hydrides of fullerenes) to the diffuse in- adducts between ions and PAH or fullerene molecules terstellar bands and the far-ultraviolet extinction.16 are treated first, together with the structure of these For astrophysicists and astrochemists, the proposition adducts and the nature of the bonding. This section that polycyclic and other aromatic hydrocarbon (PAH) concludes with a survey of the kinds of neutralization molecules and ions, and possibly larger fullerene and chemical reactions (together with some specific molecules and ions, are present in interstellar and examples) which have been proposed for such adduct circumstellar environments has