Ab initio theoretical study of the small fullerenes carbon(20)-carbon(36)

Abstract

Ab initio SCF Hartree-Fock calculations have been carried out on all the fullerene isomers of C$\sb{20}$ to C$\sb{36}.\ C\sb{20},\ C\sb{24},$ and C$\sb{26}$ have only one fullerene isomer each, of $C\sb{2h},\ D\sb6$ and $D\sb{3h}$ symmetry respectively. C$\sb{28}$ has two distinct fullerene isomers, C$\sb{30}$ has three, C$\sb{32}$ and C$\sb{34}$ have six, and C$\sb{36}$ has fifteen. Their lowest energy structures were found to be of $T\sb{d}\ C\sb{2v},\ D\sb3,\ C\sb2,$ and $D\sb{2d}$ symmetry respectively. All ground-state isomers have closed-shell electronic configurations except C$\sb{26}$-$D\sb{3h}$ (open-shell $\rm\sp5A\sp\prime\sb1)$ and C$\sb{28}$-$T\sb {d}$ (open-shell $\sp5{\rm A}\sb2).$ A new mechanism, called "peeling", is proposed in order to explain the end of the C$\sb2$ loss fragmentation pattern at C$\sb{32}$ observed in photodissociation studies. It consists of opening the fullerene surface and excising long carbon chains. MNDO calculations show the "peeling" channel to be more competitive than the C$\sb2$ loss fragmentation process for C$\sb{32}.$