Simple electrostatic model for the ionization energies of alkenes, alkynes, bromoalkanes, and iodoalkanes

It is postulated that the trends in the ionization energies within several families of organic compounds are caused primarily by the attractions between the center of charge in the ion and the induced dipole moments in the alkyl groups of the ion. Simple expressions are developed for the stabilization energy, which depends on the inverse fourth power of the distance between the charge center and the carbon atoms in the alkyl groups. Dipoles induced in the bonds directly attached to the charged center affect the magnitude of the atomic charge. The mutual repulsion between induced dipoles in different branches of an isopropyl or tertbutyl group is also considered. The method allows the calculation of ionization energies by simple arithmetic. There are strong linear relationships between the predictions of the theory and the experimental data for 108 alkenes, alkynes, bromoalkanes, and iodoalkanes with linear alkyl chains. The square roots of the mean square deviations (SRMSD) between the data and the linear relation are between 1.3 and 2.0 kJ mol(-1) (0.013 and 0.021 eV) for each class of compound. These relationships have also been used to predict ionization energies for 18 branched and cyclic compounds, with the resulting SRMSD being 3.4 kJ mol(-1).