Double aromaticity of the B fullerene: induced magnetic field analysis of π and σ delocalization in the boron cavernous structure.

The induced magnetic field of B40 was dissected into contributions from π, σ and core electrons revealing the origins for the formation of the strong long range shielding response characterizing the spherical aromatic nature of the cavernous D2d structure. Our analysis showed the complementary role of π and σ orbitals for the formation of the global shielding cone, with weak π contributions at a long range and strong σ contributions inside the cage, supporting the molecule as double aromatic with weak π and strong σ delocalization. Similar local variations of both π and σ magnetic responses were identified portraying peripheral diatropic and local paratropic currents. The weak π aromaticity is explained on the basis of symmetry rules pertaining to its electronic structure which forbid small gap paratropic rotational excitations.