Mechanistic DFT Insights into Biomimetic Mn-Porphyrin Catalysts for Peroxymonosulfate-Driven Degradation of Carbamazepine in Water

One of the most recalcitrant and environmentally persistent micropollutants detected worldwide in surface waters, groundwater, and treated effluents at concentrations ranging from ng L−1 to µg L−1 is carbamazepine (CBZ), a commonly prescribed anticonvulsant and mood-stabilizing drug. Conventional wastewater treatment technologies are not effective in removing CBZ; therefore, advanced remediation strategies are required for its complete mineralization. Herein, we present a detailed mechanistic study of the oxidation of CBZ by Mn(III) meso-tetraphenylporphyrin (Mn-TPP) in the presence of peroxymonosulfate (PMS) using the spin-unrestricted density functional theory (DFT) at the Becke, 3-parameter, Lee–Yang–Parr (B3LYP-D3BJ) level of theory with dispersion corrections. The full free energy profiles of catalysis were constructed by optimizing the geometries using the LACVP basis set on Mn and the 6-31G(d,p) basis set on all other non-metal atoms, followed by single-point calculations at the 6-311 + G(d,p) level with the SMD implicit solvation model to mimic the aqueous environment. The data suggest that the oxidation of Mn-TPP by PMS to form the key high-valent intermediate [Mn(V)=O(TPP)] + is thermodynamically and kinetically feasible. The activation barrier for PMS-mediated formation of [Mn(V)=O(TPP)]+ is ΔG‡ = 17.8 kcal mol−1 (SMD water, 298 K), indicating kinetic accessibility under ambient conditions. CBZ subsequently oxidizes the dibenzazepine ring system via concerted radical and non-radical mechanistic pathways. The computed ET pathway has an activation barrier of ΔG‡ = 16.4 kcal mol−1 and a strongly exergonic reaction free energy (ΔGr = −47.3 kcal mol−1). Analysis of the transition state shows that the spin density is localized on the Mn-Oxo unit and adjacent porphyrin π-system, characteristic of Mn-oxo species and consistent with the computed preference for the ET pathway over the OAT pathway on the examined spin surface. These mechanistic insights provide quantitative, molecular-level design parameters to rationally optimize next-generation porphyrin-based quantum catalysts for persistent pharmaceutical contaminant remediation.