If regarding monosubstitued methane (MeZ) and monosubstitued paraffin (RZ) as the unperturbed and perturbed molecules respectively, the first ionization potential, Ip~1~(~R~Z~), of the latter can be obtained by the expression: Ip~1~(~R~Z~)＝Ip~1~(~M~e~Z~)＋7.1702△qz－ 1.3949△PEI, where the Ip~1~(~M~e~Z~) is the first ionization potential of monosubstitued methane, the △qz is the difference of partial charge on Z in RZ and MeZ molecules, and the △PEI is the difference of polarizability effect index (PEI) between group R and Me. The results obtained show that the mean relative error is only 0. 20% between the calculated and experimental Ip~1~(~R~Z~) values for 61 monosubstitued paraffins.

Key words: SUBSTITUTION, ALKANE P, POLARIZATION EFFECT, IONIZATION ENERGY, PERTURBATION MOLECULAR ORBITAL, QUANTUM MECHANICS, PERTURBATION THEORY