Substituent effect in n-hexanes and n-hexatrienes based on core-electron binding energies calculated with density-functional theory

It was shown that core-electron binding energy (CEBE) is a very convenient quantity to monitor substituent effect at each carbon atom in a substituted n-hexane (1-X-hexane), a chain σ-system, and a substituted n-hexatriene (1-X-hexatriene), a chain π-system. The core-electron binding energy was calculated using the density-functional theory with a scheme:ΔEKS(PW86-PW91)/TZP+Crel//HF/6-31G∗The calculated CEBE(i) of ith orbital is equal to the sum of the ionization energy (−εi) due to the Koopmans' theorem and relaxation energy (R). The variation of the ionization energy (−εi) parallels closely to that of CEBE(i). The relaxation energy curve does not follow the CEBE curve. The behavior of CEBE in a molecule M depends almost exclusively upon the electronic structure of its neutral parent molecule M, and not upon its core-ionized cation M+. The substituent effect in the σ-system is considered as inductive effect. The substituent effect in the π-system consists of inductive and resonant/π-electron effects. Assuming that the inductive effect of the π-system, 1-X-hexatriene, can be approximated by that of the σ-systems, 1-X-hexane, resonant effect of the π-system was estimated.