The aromaticity of verdazyl radicals and their closed-shell charged species

Abstract

The aromaticity of fourteen 3-oxo-verdazyl (1–8, 15) and Kuhn verdazyl (9–14) radicals with different substituents has been investigated computationally using the gauge-including magnetically induced current-density (GIMIC) method. The strength of the local and global diatropic and paratropic ring current has been obtained by performing numerical integration of the current density flow. Detailed studies of the current densities show that the aromatic character of the verdazyl ring obeys Hückel's aromaticity rule. The cations with formally 6π electrons in the verdazyl ring are weakly aromatic and the anions with 8π electrons are antiaromatic. The verdazyl ring of the neutral radicals is found to be weakly antiaromatic. The studied molecules sustain a significant global diatropic ring current that flows around the verdazyl and the phenyl rings suggesting that there is a strong coupling between the rings. For the 3-oxo-verdazyl cations, the global ring current is weaker. The spin-current density of the radicals is paratropic forming a ring current in the studied 3-oxo-verdazyls, whereas in the Kuhn verdazyls we found that the spin current density is mainly localized on the nitrogens. The calculated ionization potentials are also compared to the values deduced from cyclic voltammetry data.