Natural bond orbital analysis (NBO) [119] provides good information regarding the change in charge densities of donor and acceptor ions. While the interaction energy is defined as the difference between the energy of the ion pairs and the sum of the energies of the purely cationic and anionic species (Equation ) however, for each donor NBO(i) and acceptor NBO(j), the stabilization energy E(2) (Equation ) associated with delocalization of electron pair from donor orbital (i) to acceptor orbital (j) and is estimated as: