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

The induced magnetic field of fullerenes is strongly dependent on the charge state, where C₆₀ is depicted as a nonaromatic species, in contrast to C₆₀¹⁰⁺ 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 A₁₂C₆₀ and A₆C₆₀ phases (A = alkali metal), given by, Li₁₂C₆₀ and Na₆C₆₀, as well as four different charge states of C₆₀^<i>q</i> (<i>q</i> = +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. C₆₀, C₆₀^6–, and C₆₀^12– accounts for the variation of their isolated species upon addition of charge, whereas C₆₀¹⁰⁺ 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 C₆₀ 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

Interpretation of Electron Delocalization in Benzene, Cyclobutadiene, and Borazine Based on Visualization of Individual Molecular Orbital Contributions to the Induced Magnetic Field

The magnetic response of the valence molecular orbitals (MOs) of benzene, cyclobutadiene, and borazine to an external magnetic field has been visualized by calculating the chemical shielding in two-dimensional grids of points on the molecular plane and on a plane perpendicular to it, using gauge-including atomic orbitals (GIAOs). The visualizations of canonical MO contributions to the induced magnetic field (CMO-IMF) provide a clear view of the spatial extension, the shape, and the magnitude of shielding and deshielding areas within the vicinity of the molecule, originating from the induced currents of each valence orbital. The results are used to investigate the delocalization of each valence MO and to evaluate its contribution to the aromatic character of systems under study. The differentiation of the total magnetic response among the three molecules originates exclusively from π-HOMO orbitals because the magnetic response of the subsets of the remaining MOs is found to be almost identical. Borazine is classified as nonaromatic as the four electrons that occupy the π-HOMO are found to be strongly localized on nitrogen centers. CMO-IMF can clarify the interpretation of various NICS indexes and can be applied for the investigation of various types of electron delocalization