Vibrational Analyses of Di-mu-oxo-Bridged Manganese Dimers Based on Density Functional Theory Calculations.Theoretical Evaluation of Mn-O Vibrations of the Mn-Cluster Core for Photosynthetic Oxygen-Evolving Complex

摘要

Normal coordinate analyses of di-mu-oxo-bridged [Mn_2~(III,III)(mu-O)_2(ND_3)_8]~(2+),[Mn_2~(III,IV)(mu-O)_2(ND_3)_8]~(3+),and [Mn_2~(IV,IV)(MU-O)_2(ND_3)_8]~(4+) dimers were carried out systematically based on force-constants obtained by B3PW91 hybrid density functional theory (DFT) calculations on a broken-symmetry electronic state.Calculation results indicated that all Mn-O-Mn/Mn-O stretch vibrations are found at 697-387 cm~(-1) with the strongest infrared (IR) intensities occurring at 620,697,and 615cm~(-1) for Mn_2~(III,III),Mn_2~(III,IV),and Mn_2~(IV,IV) dimers,respectively,which are compatible with experimental data.These strong IR bands were assigned to the Mn-O-Mn asymmetric stretch 1 (B_(3u) vibration) for the equivalent Mn_2~(III,III) and Mn_2~(IV,IV) dimers with D_(2h) symmetry,and the Mn~(IV)-O symmetric stretch (A_1 vibration) for the mixed-valence Mn_2~(III,IV) dimer with C_(2v) symmetry.Based on the results of calculations,the reported ~(18)O-sensitive IR bands in the low-frequency S_2/S_1 spectrum in a photosynthetic oxygen-evolving complex (OEC) were assigned to the Mn~(IV)-O asymmetric stretch (B_2 vibration) in the di-mu-oxo-bridged Mn_2~(III,IV) dimer moiety for the S_1-state Mn-cluster core and the Mn-O-Mn asymmetric stretch 1 (B_(3u vibration) in the di-mu-oxo-bridged Mn_2~(IV,IV) dimer moiety in the S_2-state Mn-cluster core.