Although the values of charge population of the studied molecules measured by Mulliken charge analysis and natural population analysis (NPA) (QM and Qn, respectively) are different, the charge distribution characteristics of the molecules calculated by them follow the same pattern. The C18 unit in H-C18-H exhibits a significant negative charge, indicating that C18 ring is inherently electron acceptor, as revealed in previous research ; this also makes it natural that the C18 attached to the electron-donating -NH2 group in NH2-C18-H still bears an obvious negative charge.