Beyond NICS: estimation of the magnetotropicity of inorganic unsaturated planar rings

Abstract

A simple classical model of magnetic-field induced electron flow is used to evaluate the ring current strength for a few inorganic monocyclic compounds: B₃H₃N₃, B₃H₃O₃, P₆, N₆, Si₆H₆, N²⁻₄, Al²⁻₄ and H₆. It is shown that, for these neutral and charged systems, sustaining delocalized electron currents in the presence of a magnetic field B_ext orthogonal to the σ_h plane, the out-of-plane component of the nuclear magnetic shielding along the central axis is connected to the out-of-plane magnetizability by a simple equation, involving the radius of an average loop of current. A novel estimate of this effective radius is provided. Reliable ring current susceptibilities (that is, current strengths) can be evaluated by a simple relationship, using the out-of-plane components of nuclear shielding and magnetizability tensors. The accuracy of the current susceptibilities calculated by the classical model is established by comparison with corresponding ab initio estimates obtained by integrating the quantum mechanical current-density vector field. The out-of-plane components of nuclear shielding and magnetizability are both strongly biased by the molecular geometry. Their combined use to estimate the ring current susceptibility offers a quantifier of magnetotropicity more reliable than (i) the ξ_∥ out-of-plane component of magnetizability, (ii) the σ^CM_∥ out-of-plane component of the magnetic shielding at the center of mass, widely reported as NICS_∥(0) = −σ^CM_∥. The inadequacy of these commonly adopted magnetotropicity measures is demonstrated by comparing a set of related molecules, C₆H₆ and Si₆H₆, N₆ and P₆.