Conformational stability and rotational energy barrier of RC60–C60R dimers: hyperconjugation versus steric effect

Abstract

Open image in new window The conformational stability of 4, 4′ disubstituted HC60–C60R and RC60–C60R dimers were calculated at ONIOM approach (AM1:B3LYP/6-31+G**) and density functional theory (B3LYP/6-31G**). The new evidences for stability and rotational energy barriers of these dimers were obtained by natural bond orbital, natural steric and molecular orbital analyses. Based on B3LYP/6-31G** calculations, except for RC60–C60R (R = hydrogen, tert-butyl and trimethylsilyl) where gauche is the most stable conformer, trans is a global energy minimum. The greater stability of the gauche conformer of HC60–C60H over trans is the result of hyperconjugation, which dominates the instability caused by the steric effect. By increasing the size of the substituent of HC60–C60R dimers, the trans becomes sterically unstable but the hyperconjugation of bulky substituents dominates (the trans is global energy minimum). The hyperconjugation stability of RC60–C60R dimers dominates until R = iso-propyl (higher stability of trans). In the case of bulky tert-butyl and trimethylsilyl substituents, the steric energy of trans is large and overweighs the hyperconjugation effect. This favors gauche as the most stable conformer. The calculated rotational energy barrier for HC60–C60R and RC60–C60R dimers is less than 7.3 and more than 10 kcal/mol, respectively (depending on substitution).