Small carbon rings: dissociation, isomerization, and a simple model based on strain

Abstract

Injected ion drift tube techniques have been used to examine the properties of carbon rings with 10–36 atoms. Previous studies have shown that the monocyclic ring is the dominant isomer for annealed clusters with 10–36 atoms, while for unannealed clusters a bicyclic ring first appears at n ≈ 22 and becomes the dominant isomer for n > 30. A detailed study of the annealing of the bicyclic ring has been performed for clusters with 24–36 atoms. Activation energies for the bicyclic to monocyclic transition are around 2.4eV and show a slight systematic decrease with increasing cluster size. The fragmentation of carbon cluster ions containing 6–30 atoms has been studied and dissociation energies estimated. A simple model for the strain energy of the carbon rings accounts for many of their physical properties, including isomerization to monocyclic rings, their fragmentation patterns, and dissociation energies. From simple estimates of their stabilities, planar graphite fragments should be lower in energy than the monocyclic rings for medium sized clusters ( n > 30), but little, if any, of this isomer is observed even after annealing. The low abundance of the graphite fragments is attributed to the large activation energy (induced by strain energy) for their formation from monocyclic rings.