Absolute electronegativity and hardness: An analogy with classical electrostatics suggests an interpretation of the Parr 'electrophilicity index' as a 'global energy index' leading to the 'minimum electrophilicity principle'

It is shown that the electronegativity chi and the hardness eta of chemical entities play the same role as, respectively, the potential V and the reciprocal of the capacity (1/C) of conductors in classical electrostatics. This analogy accounts for the charge transfer and energy variation occurring by electronegativity equalization. Moreover, it allows to consider the Parr 'electrophilicity index' omega = 1/2 chi(2)/eta, as an index of global energy. This quantity is found to decrease during some basic exothermal processes, which supports a 'minimum electrophilicity principle'. Its variation fairly reproduces the ionization energy of atoms, and is linearly correlated to the bonding energy in bond formation reactions, within some families of diatomic and small molecules. (C) 2008 Elsevier B.V. All rights reserved.