Induced Magnetic Field of Fullerenes: Role of σ- and π- Contributions to Spherical Aromatic, Nonaromatic, and Antiaromatic Character in C₆₀^q (q = +10, 0, −6, −12), and Related Alkali-Metal Decorated Building Blocks, Li₁₂C₆₀ and Na₆C₆₀

Abstract

The induced magnetic field of fullerenes is strongly dependent on the charge state, where C 60 is depicted as a nonaromatic species, in contrast to C 60 10+ which exhibits a strong spherical aromatic character. Here, we account for the response of relevant charged stable building blocks for novel extended networks with variable applications, as observed in A 12 C 60 and A 6 C 60 phases (A = alkali metal), given by, Li 12 C 60 and Na 6 C 60 , as well as four different charge states of C 60 q (q = +10, 0, −6, −12), to an external magnetic field is studied in detail, focusing on the contributions from the π and σ systems to the induced magnetic field. C 60 , C 60 6− , and C 60 12− accounts for the variation of their isolated species upon addition of charge, whereas C 60 10+ is a hypothetical highly aromatic counterpart. Our results show that each spherical shell and each canonical molecular orbital exhibit characteristic patterns, revealing the direct dependence of the magnetic response, and therefore of spherical aromatic character, with regard to electron configuration. In particular, low-lying S, P, D, and F π-type shells exhibit identical strong and long-range shielding character among the four charge states. The G shell exhibits a weak shielding response, precluding the strong deshielding contribution from high-lying H and I shells. A similar analysis is given for σ-type orbitals. Thus, the aromatic, nonaromatic, and antiaromatic character of C 60 among the different charge states is ruled by the population of the high-lying π-shells, which is explained in terms of π → π* excitations of high-lying canonical molecular orbitals. Hence, in spherical aromatic fullerenes, the formation of a shielding cone is given mainly by the π-type shells, extending characteristic features from planar aromatics to three-dimensional structures, which is useful for further rationalization and characterization of spherical/nonaromatic and antiaromatic spherical structures.

Noble gas endohedral fullerenes

“…In further studies, the shielding of the encapsulated 3 He core in He@C 60 6– was rationalized by the presence of a long-range shielding cone along the entire cage due to its spherical aromatic character, whereas He@C 70 6– exhibits a strong deshielding response inside the cage to the induced magnetic field. 31 , 32 Also, the 3 He shielding was assessed in terms of the number of addends in arylated and cloro[60]fullerenes. 33 Based on the 3 He upfield shift, Birkett et al 33 noted an enhanced aromatic character for all the aryl and chlorine C 60 derivatives (C 60 Cl 6 – δ He = –12.30 ppm, C 60 Ph 5 Cl– δ He = –15.14 ppm, C 60 (4-FC 6 H 4 ) 5 Cl– δ He = –15.04 ppm, C 60 (4-FC 6 H 4 ) 5 + – δ He = –12.61 ppm and 1,4-C 60 Ph 2 – δ He = –10.50 ppm, C 60 Ph 4 – δ He = –14.40 ppm) relative to the parent compound due to the introduction of an sp 3 -hybridised carbon in adjacent pentagonal rings and an improved electronic delocalization in adjacent hexagons.…”

“…In further studies, the shielding of the encapsulated 3 He core in He@C 60 6À was rationalized by the presence of a long-range shielding cone along the entire cage due to its spherical aromatic character, whereas He@C 70 6À exhibits a strong deshielding response inside the cage to the induced magnetic eld. 31,32 Also, the 3 He shielding was assessed in terms of the number of addends in arylated and cloro [60]fullerenes. 33 Based on the 3 He upeld shi, Birkett et al 33 relative to the parent compound due to the introduction of an sp 3hybridised carbon in adjacent pentagonal rings and an improved electronic delocalization in adjacent hexagons.…”

The pseudo-π model of the induced magnetic field: fast and accurate visualization of shielding and deshielding cones in planar conjugated hydrocarbons and spherical fullerenes

“…[16][17][18] However visualizations of the magnetic field induced only from the p subset of orbitals (B ind pz ) precisely corresponds to the ring current (RC) analysis of monocyclic 12,19,20 and polycyclic 21 anti/aromatic hydrocarbons and spherical carbon fullerenes. 22 Moreover, the dissection of B ind pz into contributions from canonical molecular orbitals (CMOs) allows the interpretation of the magnetic response based on relevant chemistry concepts such as symmetry selection rules, orbital overlap and energy gaps. 21,22 Yet, the calculation and visualization of B ind pz , apart from the large computational time needed, produces a bulky amount of complex data that requires a custom code to be further processed.…”

“…22 Moreover, the dissection of B ind pz into contributions from canonical molecular orbitals (CMOs) allows the interpretation of the magnetic response based on relevant chemistry concepts such as symmetry selection rules, orbital overlap and energy gaps. 21,22 Yet, the calculation and visualization of B ind pz , apart from the large computational time needed, produces a bulky amount of complex data that requires a custom code to be further processed. Thus, the computational cost of B ind pz calculations rapidly increases with the number of electrons and practically restricts the scope of the analysis to small and medium sized molecules.…”

Aromatic character of Oh‐C24N24. A cavernous nitride fullerene bearing N4‐macrocycle motifs

“…In the NMR calculations, we used a three‐dimensional grid of 41 × 41 × 41 with a separation of 0.635 Å. The dissection of chemical shielding to π , σ , and core contributions was derived from the summation of the corresponding π , σ, and core canonical orbitals, utilizing the EPR module of ADF, as described previously . The 24 π orbitals where chosen by having the cage surface as a nodal plane.…”