DENSITY-FUNCTIONAL STUDY ON THE ELECTRONIC-STRUCTURES OF MODEL PEROXIDASE COMPOUND-I AND COMPOUND-II

The electronic structures of [Fe(Por)(Im)O](1+) and [Fe(Por)(Im)O] (mo del compounds I and LI, respectively) have been studied on the basis o f density functional theory or DFT (Por = porphine, Im = imidazole). T he a(2u) pi-cation radical state ((4)A(2u)) was determined to be the g round state of compound I with total spin equal to 3/2, while the a(1u ) pi-cation state ((4)A(1u)) was found to be 0.15 eV higher in energy than the (4)A(2u) state. Since, in both states, the spins were localiz ed to the porphyrin ring (S = 1/2) and the Fe-O center (S = 1), the ma gnetic coupling interaction between the two spin sites was examined by using a broken symmetry method. The calculated J value revealed very weak magnetic coupling for the A(2u) state, which corresponded to the experimental data, The calculated J value revealed strong antiferromag netic coupling for the A(1u) state. The calculated Mossbauer spectrum parameters (quadrupole splitting and asymmetry) were similar for both the A(1u) and A(2u) states, and both agreed well with experimental val ues. On the other hand, the calculated hyperfine coupling constants fo r the nitrogen and the proton of the porphyrin ring were different in the two states. Although the experimental coupling constant values of the pyrrole nitrogen atoms were intermediate between the calculated va lues for the A(2u) and A(1u) states, the experimental values for the m eso protons were closer to the values calculated for the Ar, state, Th ese results suggest that the electronic structure of compound I is clo ser to the A(2u) state than to the A(1u) state. However, these results also suggest that there is the possibility that the electronic struct ure of compound I is an admixture of the A(1u) state and the A(2u) sta te. The electronic structure of compound II was calculated and compare d with the electronic structure of compound I. The energetics of the r edox reaction between the two compounds is discussed.