Density Functional Study of Manganese Complexes: Protonation Effects on Geometry and Magnetism

Abstract

Protonation processes are ubiquitous in various biochemical reactions such as the water-oxidizing reaction in photosystem II and detoxications of active oxygen species in Mn catalase and Mn superoxide dismutase. In order to investigate them, experiments to probe protons often need supplementary computational results to support the experimental spectra, for which reliable DFT methods are required for description of protonation processes. In this study, we investigated manganese complexes, [Mn(IV)2O2Hn(salpn)2]n+ (n = 0,1,2), of which geometries and magnetism show systematic changes due to protonations to bridged oxygen anions. We examined the performance of B3LYP, B3LYP-D, BP86, BP86-D, and LC-ωPBE on these changes. With all methods, the observed changes during protonation processes can be reproduced, and the quantitatively best procedure is found to be LC-ωPBE/LACVP* for geometry optimization calculations and LC-ωPBE/chem for calculations of magnetic interactions. This conclusion is expected to be a numerical foundation for theoretical investigation of reaction centers in manganese-containing proteins.