The essential role of symmetry in understanding 3He chemical shifts in endohedral helium fullerenes

0571079 - ÚOCHB 2024 RIV GB eng J - Journal Article
Vícha, Jan - Vaara, J. - Straka, Michal
The essential role of symmetry in understanding 3He chemical shifts in endohedral helium fullerenes.
Physical Chemistry Chemical Physics. Roč. 25, č. 15 (2023), s. 10620-10627. ISSN 1463-9076. E-ISSN 1463-9084
R&D Projects: GA ČR(CZ) GA21-17806S
Research Infrastructure: e-INFRA CZ - 90140
Institutional support: RVO:61388963
Keywords : nuclear magnetic resonance * ring currents * NMR
OECD category: Physical chemistry
Impact factor: 3.3, year: 2022
Method of publishing: Limited access
https://doi.org/10.1039/D3CP00256J

The 3He atom is an excellent NMR probe, particularly when enclosed in endohedral helium fullerenes. The 3He chemical shift, δ(3He), in fullerenes spans a range from ca. −50 to +10 ppm, and changes sensitively between different cages, isomers, and external substituents. Reduction of the fullerenes to anions changes the δ(3He) dramatically and unexpectedly, particularly for the most symmetric and also the most abundant C60 and C70 cages. While the 3He atom is shielded by ∼43 ppm upon charging the He@C60 to He@C606−, it is correspondingly deshielded by ∼37 ppm in the He@C70/He@C706− pair. Here, we show that such puzzling differences in δ(3He) relate to the high symmetry of the host fullerene cages. While similar shielding is induced at the 3He atom by the core orbitals of different cages, the symmetry of the cage allows or quenches large paramagnetic, i.e., deshielding orbital interactions of frontier orbitals upon charging of the cage, which is directly responsible for the large observed chemical shift range of endohedral 3He.
Permanent Link: https://hdl.handle.net/11104/0342382