Ring currents and magnetic properties of the cyclopropenyl cation and isoelectronic triangular 2π electron systems

A coupled Hartree-Fock method is used to compute the magnetizability tensors and carbon and hydrogen shielding tensors of the isoelectronic series of cyclic molecules consisting of the cations C3 H3 + (cyclopropenyl), C2 H2 N+ (azirinyl), CHN2 + (diazirinyl) and N3 + (triazirinyl) and the neutral molecules C2 H3 B (borirene), CH2 NB (borazirene), N2 BH (boradiazirine), and NB2 H3 (azadiboridine). A distributed-origin method is also used to map the electron current densities induced in these by an external magnetic field. Comparisons are made with the benzene molecule. The computations show that whereas the anisotropy of the magnetizability of benzene is dominated by the contribution of the π current, that in the cyclopropenyl cation is due almost entirely to the valence σ current. The current density maps of the two molecules also differ sharply. The maps for benzene show a localized circulation of σ charge in each of the bonds, and a simple circulation of π charge around the principal axis with maximum density on the carbons of the ring: a π ring current. The maps for cyclopropenyl show a weak π ring current and a circulation of σ charge around the perimeter of the molecules that can be interpreted as a global σ ring current. The corresponding maps for the heterocyclic systems show that the π and σ ring currents are maintained with little change when CH in C3 H3 + is replaced by N, but that the introduction of B results in partial localization of these currents.