Semi-Empirical ASMO SCF Method for the Valence Electron System. The Electronic Structures of Protonated Pyridinium and Diazinium Cations

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Abstract
Using a semi-empirical ASMO SCF method for valence electron systems previously proposed by the present authors, the valence shell electronic structures of various protonated azines are investigated. The compounds treated in the present paper are monoprotonated pyridinium, mono-, and diprotonated diazinium cations. The calculated results show that quite large charge redistributions on both σ and π electron systems in these cations take place in the protonation, while about a 0.7 e charge migrates to the attached protons. As to the transition energies, the lowest π–π* transitions exhibit slight red shifts, while the n–π* transitions would be expected to cause large blue shifts by means of protonation. Several models for protonated cations are compared, and the configuration where the proton is situated on the bisector of the CNC(N) angle and in the molecular plane is shown to be the most stable one. The changes in Fock’s operators upon protonation are also briefly discussed.