DFT MD Simulations Decode the Role of Explicit Solvation in Spin-Dependent O–O Bond Formation for a High-Valent Mn ₄ Catalyst

Density functional theory molecular dynamics simulations were performed to investigate the critical O-O bond formation step mediated by the high-valent Mn₄^{IV, IV, IV, IV}-O^• intermediate in a tetranuclear manganese complex catalyzed water oxidation reaction. The direct coupling mechanism between the oxyl radical and the OH group to form the O-O bond was explored in the octet, 12-tet, and sextet states using an explicit solvation model. The calculated results indicate that the O-O bond formation is kinetically feasible in the octet and sextet states, exhibiting quantitatively similar barriers. In contrast, the O-O bond generation in the 12-tet state is unfavorable. The present study indicates that the O-H bond breaks before the formation of the O-O bond, with a water molecule accepting the released proton. In addition, the DFT-MD simulations incorporating explicit solvation water molecules yielded relatively lower barriers compared to static DFT calculations employing an implicit solvation model. Upon switching from DFT-MD simulations to the static DFT calculations, the barriers increase by 8.0, 15.4, and 15.9 kcal/mol for the octet, 12-tet, and sextet, respectively, highlighting the crucial role of explicit water solvation.