Ab initio MO calculations on stabilities and binding energies of bidentate ethylene diamine with ions (Na +, Li +, and H +)

Abstract

Geometrical relaxations and binding energies of the complexes of ions (Na +, Li +, and H +) and bidentate ethylene diamine were studied by using an ab initio MO method, and are compared with those of the monodentate form of ethylene diamine and methylamine. Computed 6-31G * binding energies of Na + and Li + ions with bidentate ethylene diamines are 42.3 and 66.3 kcal/mol, which are larger than those with monodentate form of ethylene diamine by 9.9 and 19.7 kcal/mol, respectively. These energies (9.9 and 19.7 kcal/mol) correspond to the cyclization stabilization, i.e. binding energies due to the second NH 2 addition. Morokuma's energy decomposition analyses were performed to analyze the interaction energy components. Larger binding energies of Li + and amines are due to the larger contribution of electrostatic stabilization; smaller contribution of polarization stabilization cancels the increase of electron-exchange destabilization. Proton affinity of bidentate ethylene diamine, computed at the level of Hartree—Fock MO theory with the 6-31G * basis set, was smaller than that of monodentate ethylene diamine, in contrast to the experimental findings. Third-order Møller—Plesset perturbation calculations with the 6-31G ** basis set (in the model system) suggest the importance of the electron correlation contribution in the proton affinity computations of bidentate ethylene diamine.