Atomic partial charges were estimated through fitting to quantum mechanical molecular restrained electrostatic potentials (RESP) computed at the RHF level with the 6-31 G(d,p) basis set using Gaussian 98 [29]. Electrostatic potential (ESP) charges fitting consists of using a least squares fitting algorithm in order to derive atom-centred point charges that could best reproduce the quantum mechanically calculated electrostatic potential of the molecule [30].