Static Indices of Reactivity

Some authors (Fukui et al. 1954 Klopman 1974 Eukui 1975) believe that the HOMO and LUMO orbitals have a particular importance for the reactivity of the molecules. 

In the case of a donor-acceptor type of interaction, an electron transfer takes place between molecule A and B. In the case of a change type of interaction, an electron transfer takes place between molecule A and B. As a rule, when two molecules A and B interact, the electron transfer that requires the smallest energy is the transfer from the HOMO orbital of a molecule to a LUMO orbital of the other molecule. Which molecule participates in the interaction with the HOMO orbital and which participates with the LUMO orbital depends on the relative value of the differences Al and A2 between the energies of those particular orbitals. 

For each heavy atom in the analyzed molecule, one can calculate (Fukui 1975) the reactivity indices 1 (nucleophilic), Ig (electrophilic), and 1r (one-electron). 

Reactivity Fukui Indices for Heavy Atoms in Guanine Isomers 

The Fukui Equations (4.16), (4.17), and (4.18) reactivity indices are considered a measure of the reactivity of the molecule (as nucleophil, electrophil, or radical) when the reaction center is the atom for which the indices have been calculated. The values of the Fukui indices, calculated for different atoms, allow the comparison of the atoms and the identification, as a rule, of the reaction center in different types of chemical reactions. The reactivity estimated in this way does not consider steric factors. Table 4.4 presents the values of the Fukui reactivity indices for the isomers of guanine that are analyzed here. The indices 1, Ie, and 1r do not reflect the deformation of the electron clouds of two molecules that are approaching in order to react 1 therefore they are called static. The reactivity indices that consider this factor also are called dynamic. Their calculation requires much more complicated formulas (Roothaan 1951) and involves the comparison of the energy of the molecular orbitals of the isolated molecules to the energy of the molecular orbitals of the supermolecule (i.e., the ensemble of the A and B molecules, situated at very short distances). 

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