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Laboratory Studies of Ion/Molecule Reactions of Fullerenes: Chemical Derivatization of Fullerenes within Dense Interstellar Clouds and Circumstellar Shells

We discuss recent experimental results for ion/molecule reactions of ionized and multiply-ionized fullerenes, and of derivatized fullerene ions, with molecules relevant to the chemistry of interstellar clouds and circumstellar envelopes These reactions were studied using a selected-ion flow tube (SIFT) at 294 ± 2 K in helium at a pressure of 0.35 ± 0.01 torr. The present study supplements an earlier discussion on aspects of interstellar fullerene ion chemistry explored by the same technique. Several implications are apparent for the chemical processing of fullerenes in various astrophysical environments. Triply charged fullerene ions, such as C603+, may be formed under conditions prevailing within dense IS clouds, but their abundance will be very low owing to the large number of loss processes identified for such species. Derivatization of fullerene ions under interstellar or circumstellar conditions is less probable for larger fullerenes than for fullerenes smaller than C60. Hydrogenation may severely impede the efficiency of fullerene ion association with polar molecules and small unsaturated molecules, but should not substantially affect the efficiency of addition of radicals or PAHs under these conditions. We discuss prospects for neutralization of ionized fullerene adducts. Four classes of adduct ions are described, differing in their structure and expected neutralization tendencies. Adducts of fullerene ions with interstellar isonitriles, with radicals, and with linear polycyclic aromatic hydrocarbons (PAHs; class 1) are most likely to form derivatized fullerenes on neutralization, while fullerene ion adducts of nitriles, most hydrocarbons (class 3), and nonlinear PAHs (class 4) are most likely to yield the bare fullerene cage upon neutralization. Adducts of ammonia (class 2) appear to have an intermediate probability of surviving neutralization with the functionalizing group(s) intact.