The classical Bohr model of the hydrogen atom consists of a “planetary” electron that orbits a central, positively charged nucleus. The supramolecular equivalent of the Bohr atomic model can be constructed in which H+ or H– are centered within the inner cavity of Buckminsterfullerene, C60. In the case of endohedral H+, the proton serves as the positively charged nucleus while the surface of the fullerene sphere provides the “valence” shell. Because of its icosahedral geometry, the 13C NMR spectrum of C60 consists of a single line. Density functional quantum chemical analysis at the EDF2 level of theory, with a Pople 6-31G(d) basis set, yields a chemical shift of 144.4 ppm for C60. Our calculations show that an endohedral proton, H+, causes an upfield shift in the resonance absorption of the fullerene carbon atoms to 139.5 ppm. Conversely, an endohedral hydride anion, H-, causes a strong downfield shift to 163.3 ppm. The magnetic shielding and deshielding effects are due to the electrical asymmetry of the fullerene sphere. Electrostatic potential maps show that C60 is made up of an inner electron-deficient locus surrounded by an electron-rich surface of π-electrons. Endohedral H+ causes the π-electrons to radially migrate inwards. Therefore, the electron density at carbon increases, inducing a magnetic shielding effect that decreases the NMR chemical shift. Conversely, endohedral H– causes the π-electrons to radially migrate outwards. Therefore, the electron density at carbon decreases, inducing a magnetic deshielding effect that increases the NMR chemical shift.

Towards carbon based nanotechnology: Electrostatic shielding and deshielding effects on the 13C NMR spectra of endohedral H+ and H– guests within a buckyball host

ACS Spring 2022

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