Abstract: The electronic structures of the porphin and its dimer have been calculated by EHMO.From the differential charge distribution in the porphin dimer it has been shown that the porphin molecule belongs to the amphoteric electron donor-aceeptor in its dimer, having steady donor centers and acceptor centers. The system has minimum energy at the orientation angle ψ=45°. 4-fold barriers should be overcome when two monomers rotate each other. The barrier is ～0.09 eV. The degenerate perturbation theory has been expanded to deal with the dimer system, and according to this result the energy level corespondence between the monomer and the dimer has been explained quantitatively. It has been shown that the cause of the interaction is the charge transfer between the donor center and the acceptor center in the dimer.
The model for calculation is shown in Fig.1. The frontier orbitals and those nearby them are all π ones (ref. Table 1), and the difference between the π-π interaction energy and the total energy is less than 10% of the total one. Thereby, we only calculate the π-π interaction for simplicity.
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