Linear correlation models for the redox potential of organic molecules in aqueous solutions
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ORIGINAL PAPER
Linear correlation models for the redox potential of organic molecules in aqueous solutions Jessica C. Ortiz-Rodríguez 1 & Juan A. Santana 1
&
Dalvin D. Méndez-Hernández 1
Received: 15 June 2019 / Accepted: 23 February 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract In this study, we use the molecular orbital energy approximation (MOEA) and the energy difference approximation (EDA) to build linear correlation models for the redox potentials of 53 organic compounds in aqueous solutions. The molecules evaluated include nitroxides, phenols, and amines. Both the MOEA and EDA methods yield similar correlation models, however, the MOEA method is less computationally expensive. Correlation coefficients (R2) below 0.3 and mean absolute errors above 0.25 V were found for correlation models built without solvent effects. When explicit water molecules and a continuum solvent model are added to the calculations, correlation coefficients close to 0.8 are reached, and mean absolute errors below 0.18 V are obtained. The incorporation of solvent effects is necessary for good correlation models, particularly for redox processes of charged molecules in aqueous solutions. A comparison of the correlation models from different methodologies is provided. Keywords Redox potentials . Aqueous solutions . Linear correlations . Explicit solvation
Introduction Energy technologies such as organic solar cells [1], lightemitting diodes [2], dye-sensitized solar cells [3–10], and artificial photosynthetic systems [11–22] rely on the reduction and oxidation (redox) of different organic molecules. The calculation of the redox potential of organic molecules in solutions is crucial for the rational design of such technologies. Various methodologies are currently employed to calculate redox potential in solutions: (i) the Born-Haber thermodynamic cycle (BHTC) [23–26], (ii) the S0–D0 energy difference approximation (EDA) [27–29], and (iii) the molecular orbital energy approximation (MOEA) [27, 30, 31]. In the BHTC method, the redox potential is calculated from the standard free energy of product and reactants in a redox half-reaction. In the EDA and MOEA methods, a different approach is used where calculated total (or frontier molecular orbital) energies are used to build a correlation model with available experimental redox potentials. The built correlation model (typically a linear model) is then used to calculate the unknown redox * Dalvin D. Méndez-Hernández [email protected] 1
Department of Chemistry, University of Puerto Rico at Cayey, Cayey, PR 00736, USA
potential of a given molecule from calculated total (or frontier molecular orbital) energies. In the EDA method, the total energy of the initial and final ground states in the redox process is used to build the correlation models. For the MOEA method, the energies of the frontier molecular orbitals of the S0 state are employed to build the correlation models; the energy of the lowest unoccupied molecular orbital (LUMO) and the highest occupi
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