Spin-Spin Coupling in Nitrogen Atom Encapsulated C60, C59N, and Their Respective Dimers

Density functional theoretical calculations were performed to study the stability and magnetic properties of nitrogen-encapsulated C60, C59N, and their respective dimers at B3LYP/6-311G* and B3LYP-GD2/6-311G* levels of theory. For the most stable spin state of each of the above complexes, spin density transfer and spin-spin coupling between different components are investigated. The nature of bonding between the guest and the host is analyzed based on the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap and the respective molecular orbital diagrams. The analysis of spin density showed that the encapsulated nitrogen retained its atomic state in N@C60 and N@C59N. Depending on the multiplicity of N@C59N, the unpaired electrons of the encapsulated nitrogen are coupled with those of the cage anti-ferromagnetically or ferromagnetically. The present study also showed that the complex (N@C60)2 can exist in two isoenergetic spin states, namely, (7)[(N@C60)2] and (1)[(N@C60)2]. In the former, the encapsulated nitrogens are ferromagnetically coupled, whereas they are coupled anti-ferromagnetically in the latter. A similar coupling between the guest species occurs in the nitrogen analogues (7)[(N@C59N)2] and (1)[(N@C59N)2]. The stabilization energy of the endohedral nitrogen complexes indicated that the interaction between the guest and the host cage is purely noncovalent.