RING CURRENTS AND MAGNETIC-PROPERTIES OF THE CYCLOPROPENYL CATION ANDISOELECTRONIC TRIANGULAR 2-PI ELECTRON-SYSTEMS

A coupled Hartree-Fock method is used to compute the magnetizability t ensors and carbon and hydrogen shielding tensors of the isoelectronic series of cyclic molecules consisting of the cations C3H3+ (cycloprope nyl), C2H2N+ (azirinyl) CHN2+ (diazirinyl) and N-3(+) (triazirinyl) an d the neutral molecules C2H3B (borirene), CH2NB (borazirene), N2BH (bo radiazirine), and NB2H3 (azadiboridine). A distributed-origin method i s also used to map the electron current densities induced in these by an external magnetic held. Comparisons are made with the benzene molec ule. The computations show that whereas the anisotropy of the magnetiz ability of benzene is dominated by the contribution of the pi current, that in the cyclopropenyl cation is due almost entirely to the valenc e sigma current. The current density maps of the two molecules also di ffer sharply. The maps for benzene show a localized circulation of sig ma charge in each of the bonds, and a simple circulation of pi charge around the principal axis with maximum density on the carbons of the r ing: a pi ring current. The maps for cyclopropenyl show a weak pi ring current and a circulation of sigma charge around the perimeter of the molecules that can be interpreted as a global sigma ring current. The corresponding maps for the heterocyclic systems show that the pi and sigma ring currents are maintained with little change when CH in C3H3: is replaced by N, but that the introduction of B results in partial localization of these currents.