Trapping N₅ rings and N₃ chains on the outer surface of fullerene C₆₀: a theoretical study

The capture of N₃-chains and N₅-rings on the outer surface of C₆₀ was studied using density functional calculations. For the neutral N₅-ring, it was found that a N₅-ring trapped by a C₆₀ cage becomes more stable than an isolated N₅-ring radical, and a C₆₀–N₅ compound with a C–N bond at an exohedral position of C₆₀ is more stable than an isomer with the N₅-ring encapsulated in C₆₀. Such stability arises from the reduction in molecular strain energy, and charge transfer from C₆₀ to N₅. Dynamics calculations indicate that capture of the N₅-ring on the outer surface of C₆₀ is a barrierless process. Furthermore, the trapping sites of more N₅-rings on the C₆₀ were determined using condensed Fukui functions, where the N₅-rings prefer to be trapped on the surface to form addition products across 6,6-junctions. Based on the optimized geometries of C₆₀–(N₅)_n (n = 2, 6, 10), their chemical stabilities were found to be comparable with that of C₆₀ in terms of the gap between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals. Similar phenomena were found for an N₃-chain wrapped on the surface of C₆₀. However, the results of the average adsorption energies show that C₆₀ can capture N₅-rings more effectively than N₃-chains.