A computational NICS and 13C NMR characterization of the substitution patterns of C70−2x(BN)x fullerenes (x=1–25)

A density functional study has been performed to investigate the electronic and magnetic properties of BN substituted fullerenes C70−2x(BN)x (x=1, 2, 3, 6, 9, 12, 15, 17, 19, 21, 23 and 25) based on the NMR parameters and NICS index. The calculated 13C chemical shielding (CS) tensors are found to be perturbed at the first and second neighbors of the doped atoms while the other distant carbon atoms not to be influenced significantly. 13C Chemical shifts (δiso) of the second neighbors of nitrogen and boron are significantly shifted to upfield and downfield (the second neighboring effects), respectively. Besides, chemical shifts are also affected by the curvature of the corresponding sites; for example, the perturbed sites at the caps yield smaller absolute values of chemical shifts than those located at the equator. Nucleus independent chemical shifts (NICS) at the cage centers of heterofullerenes (from −25.29 to −8.80) demonstrate that all the substituted species are aromatic, but less than C70 (−27.29). The predicted NICS values may be useful for identification of the heterofullerenes through their endohedral 3He NMR chemical shifts.

Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► 13C chemical shielding tensors at the first and second neighbors of the doped atoms are perturbed. ► Chemical shifts of the second neighbor of nitrogen and boron nuclei are shifted to upfield and downfield. ► Chemical shifts are affected by the curvature of the corresponding sites. ► NICS values demonstrate that the heterofullerenes are aromatic but less than C70.