EXPERIMENTAL RESULTS FOR ION-MOLECULE REACTIONS OF FULLERENES - IMPLICATIONS FOR INTERSTELLAR AND CIRCUMSTELLAR CHEMISTRY

Reactions of the ions D+, He+., and Si+. with C60, and of the ions C-6 0(+.), C-60(2+), C-70(+). and C-70(2+) with several neutrals of releva nce to the chemistry of interstellar clouds and circumstellar envelope s recently have been studied with a selected-ion flow tube (SIFT) appa ratus at 294 + 3 K in helium at pressures of 0.35-0.40 Torr. The impli cations of the results of these studies are discussed for the chemical evolution of interstellar clouds and circumstellar shells surrounding carbon-rich stars. Notable features of the ion-molecule reactivity of fullerenes include: the production of fullerene dications in the reac tion of He'' with C60; the high selectivity of C-60(+.) and C-70(+.) t owards radicals and amines; the predominance of association as a react ion channel of C-60(+.) with various neutral molecules; the tendency o f fullerene dications to undergo association with unsaturated hydrocar bons; and the existence of activation energy barriers towards many cha rge-transfer and hydride abstraction reactions of fullerene dications, and proton transfer from functionalised fullerene dications. Other as pects of fullerene reactivity, not directly related to ion-molecule in teractions, are also discussed. These include difficulties inherent in the dissociative recombination of fullerene ions and dications with e lectrons; the possible role of neutral and ionized fullerenes as a mod el for interstellar dust grain surface recombination of atomic hydroge n and other reactive species; production of fullerene dications by seq uential cosmic-ray and UV photoionization and by cosmic-ray double ion ization; and the possibility of fullerene cages acting as ''traps'' fo r cosmic rays within the interstellar medium. Possibilities for the in terstellar detection of fulleranes, and fullerenes functionalised by a mine and hydrocarbon moieties, are also discussed. It is argued that t he fullerene cage structures, especially C60, should be very durable w ithin the interstellar environment because of the lack of destruction mechanisms available for these species. In contrast, substituents atta ched to fullerene cages as a consequence of ion-molecule chemistry occ urring within the interstellar medium are likely to be readily removed by UV irradiation within diffuse regions. For this reason, functional ised fullerenes (except for the slightly hydrogenated fulleranes) are not expected to be abundant in diffuse regions although they may be pr esent in reasonable quantities within dense clouds.