Conformational Analysis of [60]PCBM from DFT Simulations of Electronic Energies, Bond Strain and the 13C NMR Spectrum:Input Geometry Determination and Ester Bond Rotation Dynamics

With the aim of determining the best input geometry for DFT calculations of [60]PCBM,the geometry of 24 chemically possible [60]PCBM conformers were optimised and their electronicenergies and average bond strains were determined. A DFT analysis of the relevant dihedral anglesprovided insights into the dynamical behaviour of the ester group through sterically restricted bondrotations. In addition, the 13C NMR spectra of the six better performing conformers were simulatedand compared with an experiment. There is a close correlation between average bond strain, totalelectronic energy and mean absolute error of the simulated 13C NMR spectra of the ester carbons.The best overall candidate conformer for the input geometry had the C61-C4, C4-C3 and C3-C2 singlebonds of the alkyl chain in syn, anti and anti arrangements, respectively, and had the C2-C1 and C1-Osingle bonds of the ester in syn and anti arrangements, respectively. This contrasts strikingly withmost representations of PCBM in the literature, which depict all relevant bonds in anti arrangements.