ROLE OF SP(3) CARBON AND 7-MEMBERED RINGS IN FULLERENE ANNEALING AND FRAGMENTATION

WHEN fullerenes1,2 are fragmented by laser irradiation, they lose C2 f ragments and retain a closed carbon cage3. The detailed mechanism of t his process remains unknown, although survival of the cage implies tha t annealing (rearrangement of the bonding) must play an important role 3,4. Here we use ab initio quantum-chemical calculations to show that fullerene annealing happens more readily than fragmentation, and that both are intimately related. Our findings imply that the assumptions c ommonly made about fullerenes5-that they are composed of five- and six -membered rings of sp2 carbons- are not valid under high-energy condit ions. In particular, the appearance of sp3 carbon and seven-membered r ings is central in both the annealing and fragmentation processes. Our theoretical predictions imply that the high-energy processes of fulle rene growth6-11 and coalescence12 are much richer than previously thou ght, and that their mechanisms may also involve structures containing sp3 carbon and seven-membered rings. Our results may aid in the design of experimental methods for controlling the nature of fullerene cages (for example, doping, opening and re-closing them).