Mechanism of photochemical reaction of permanganate ion

The electronic mechanism of the photodecomposition reaction of MnO₄ ⁻ was investigated using an ab initio theory, taking into account electron correlation. The SAC (symmetry adapted cluster)/SAC-CI method was used to calculate the ground and excited states of the permanganate ion. It was found that to obtain reasonable descriptions of the ground and excited states of MnO₄ ⁻, the inclusion of electron correlation is quite important. The experimental absorption spectrum of MnO_{4 −} was well reproduced by the present calculations. All the observed peaks were assigned to the electronic allowed transitions to the ¹T₂ excited states. We then calculated the potential-energy curves of the ground and excited states of MnO₄ ⁻ along the reaction pathway. The resulting potential-energy curves for the ground and excited states show that the excited 1¹A₂ state plays a key role in the photochemical reaction. The energy barrier in the 1¹A₂ state determines the strong wavelength dependence of the quantum yield and the slight temperature dependence at longer wavelengths. The electronic mechanism clarified by the present ab initio study is consistent with previously reported experimental results for the photochemical decomposition of MnO₄ ⁻.