Abstract
The dynamics of central protons in porphins is investigated within the vibronic approach. The ground A1g and the excited Eg electronic terms of the initial D4h symmetry configuration are mixed by the Eg nuclear displacements. Due to this multimode pseudo-Jahn-Teller effect four minima occur on the lowest potential-energy surface, the system at the minimum corresponding to the localization of the protons near pairs of opposite nitrogen atoms, accompanied by out-of-plane Eg static distortions of the macrocycle. The rate of transitions between the minima induced by the interaction with the dissipative subsystem (the redetermined Eg vibrations of the porphin skeleton and its environment) increases with temperature. These transitions are appropriate to the proton migration and to a wave of out-of-plane deformation of the skeleton. The results of theoretical estimations within this model are in good agreement with the data of NMR experiments on proton migration in tetraphenylporphin.
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