Orbital contributions to the magnetic shielding of cyclo[2n]carbons (n = 3–12)

Abstract

Natural chemical shielding (NCS) analysis yields erroneous magnetic shielding values (or NICS, which is the negative of magnetic shielding) for the core, σ and in-plane π orbitals of cyclo[2n]carbons (n = 3–12), assuming that the external magnetic field acts perpendicular to the ring plane. The most diagnostic are the unphysically large contributions of core electrons to the shielding inside the rings. An alternative way to estimate the contribution of the in-plane π-system to shielding values is proposed and is based on the addition of a hydrogen atom outside each carbon atom in the plane of the cyclo[2n]carbon ring, while keeping its geometry intact. The π_in contributions are then estimated as the difference between the total shielding values of cyclo[2n]carbon and the total shielding values of a ring with added Hs. This, however, includes the C−H bond effects near the carbon framework.