Ring currents and magnetic properties of pyracylene

A distributed-origin coupled Hartree-Fock method is used to compute the current density induced in the pyracylene (cyclopent[fg]acenaphthylene) molecule by an external magnetic field, and hence to plot the pi ring currents and obtain ab initio values of the total magnetisability, C-13 and H-1 nuclear magnetic shieldings. The calculated map shows paramagnetic ring currents over the pentagons and diamagnetic circulation around the central naphthalenoid unit of pyracylene, and accounts semi-quantitatively for the measured difference in chemical shift between protons attached to the pentagons and hexagons of this molecule.