Derivatives of molecular valence as a measure of aromaticity

Derivatives of the molecular valence have been calculated ab initio within the new non-finite-difference approach elaborated on earlier for the global hardness and the Fukui function indices. The group of 10 five-membered-ring molecules C4H4X has been chosen for a test, using the exaltation of the total magnetic susceptibility (Lambda) as a reference measure of their aromaticity. An excellent correlation has been found between the molecular valence derivatives in the nucleophilic regime and the exaltation II, for both the aromatic and antiaromatic molecules. Calculation of the valence derivatives provides an absolute measure of aromatic character that is not directly dependent on the size of the molecule and does not require adopting any standard reference molecule.