Predictions of special signatures of fullerenes. Second-order Jahn–Teller effects on the structures of C44, C56, C68 and C92

Abstract

It is argued that pseudo closed-shell fullerenes (those with two electrons in every occupied bonding orbital but with a bonding LUMO in Hückel theory) often fulfil the conditions for second-order Jahn–Teller distortion. Semiempirical (MNDO, AM1 and PM3) calculations on isomers of C₄₄, C₆₈and C₉₂ yield optimal structures of C₂ symmetry (C₄₄) and D₂ symmetry (C₆₈ and C₉₂), rather than the maximal possible T symmetry while C₅₆ and a second C₉₂ isomer give structures of S₄ rather than the maximal T_d symmetry. The distortions to lower symmetry are accompanied by a smoothing out of inequalities in atomic charge. Tetrahedral (T_d) C₈₄ has a properly closed shell in Hückel theory and shows no loss of symmetry in a semiempirical calculation. Consequences of the distortions for spectroscopic signatures are discussed.