The extended Hückel theory has been given an approximate deduction from first principles. The use of experimental ionization energies for the atoms in the molecule implies that the correlation energy is taken care of. Koopmans' theorem therefore cannot be used. Instead, a ΔESCF calculation is performed by using ionization energies, which imply the use of a transition state in the secular determinant. The eigenvalues therefore denote ionization energies, for both occupied and unoccupied orbitals. To find the electron affinities from the ionization energies two procedures are used. From experimental data the relation can be obtained directly for two molecules, but to find general rules theory is necessary. The result is that the electron affinities can be obtained from the eigenvalues for the unoccupied orbitals simply by adding about 7.0 eV (for hydrocarbons). The extended Hückel method is in these respects analogous to the HAM method. Electron affinities for σ* orbitals are calculated for a number of molecules and compared with experiment. It appears that the extended Hückel method is useful for such studies. The increased knowledge may be of importance for the understanding of certain chemical reactions.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry