Natural bond orbitals (NBO) analysis
The electronic wave functions are interpreted in terms a set of occupied Lewis and a set of unoccupied non-Lewis localized orbitals in the natural bond orbitals (NBO) analysis [51]. This method provides information about the interactions in both filled donor and virtual orbital spaces (empty) that facilitates the analysis of intra-and intermolecular interactions. A second-order perturbation theory analysis of the Fock matrix was carried out to consider all possible interactions between the donor-acceptor orbitals and their estimated energies. For each donor orbital and an acceptor orbital , the stabilization energy,E(2) , or the strengths of delocalization interactions is related to the electron delocalization between donor and acceptor , given by following equation: