Relation Between Ring Currents and Hydrogenation Enthalpies for Assessing the Degree of Aromaticity.

Magnetically induced ring-current strength susceptibilities and nucleus independent chemical shifts (NICS) have been studied for 15 single-ring aromatic, antiaromatic, and nonaromatic molecules. The current densities have been calculated at the density functional theory (DFT), Hartree-Fock (HF) theory, and second-order Møller-Plesset perturbation theory (MP2) levels using the gauge-including magnetically induced current method (GIMIC). The ring-current strength susceptibilities have been obtained by numerical integration of the current density flowing around the molecular ring. The calculated ring-current strength susceptibilities are almost independent of the level of theory. The relative degree of aromaticity deduced from the magnetic properties has been compared with the ones deduced from hydrogenation enthalpies that are considered to be proportional to aromatic stabilization energies (ASE). For the studied single-ring molecules, GIMIC, NICS, and ASE calculations yield similar trends. The study shows that there is a linear correlation between the magnetic and energetic criteria of aromaticity. The largest uncertainty originates from the accuracy of the energy data, because they are much more dependent on the employed computational level than the calculated magnetic properties. Thus, ring-current strength susceptibilities can be used for assessing the degree of aromaticity.