Applications of calculated local surface ionization energies to chemical reactivity

Abstract

A recently introduced property, the average local ionization energy I ( r), is discussed in relation to chemical reactivity. I ( r), is defined within the framework of self-consistent-field molecular-orbital (SCF-MO) theory and is interpreted as the average energy required to remove an electron from any point r in the space of a molecule. A survey of applications of ab initio SCF I ( r), computed on molecular surfaces defined by the 0.002 e bohr −3 contour of the electronic density is presented. The positions at which the surface I ( r), has its lowest values ( I S,min) are indicative of sites most reactive toward electrophiles. An excellent correlation was found between the ring carbon ( I S,min) of monosubstituted benzenes and the Hammett constants. For a large variety of carbon, oxygen and nitrogen acids, we have shown that good correlations exist between p K a and the conjugate base ( I S, min) Finally, a good relationship is demonstrated to exist between the conjugate base ( I S,min) of substituted acetic acids and the inductive substituent constants σ i (which were derived from the p K a values). ( I S, min) found near the C-C bond midpoints of the three-membered ring portions of saturated strained hydrocarbons are interpreted as reflecting the σ aromaticity of these rings.