Defining the pH* Scale in Methanol: Determination of Accurate Values for the Solvation Free Energies of CH₃OH₂⁺ and CH₃O^- in Methanol.

Authors

Antonio R Cunha, José M Riveros, Sylvio Canuto, Kaline Coutinho

Published in

The journal of physical chemistry. A. Oct 25, 2025. Epub Oct 25, 2025.

Abstract

A correction factor for the autoprotolysis constant of methanol is proposed in the present work to obtain thermodynamic data for the standard solvation free energies of CH₃OH₂⁺ and CH₃O^- ions in methanol and pK_a^*. Using this corrected constant, K_MOH^*, along with known values for the standard solvation free energy of proton ΔG_sol^*(H⁺) and of the methanol molecule ΔG_sol^*(CH₃OH), in its own liquid, in three different thermodynamic cycles, we obtain ΔG_sol^*(CH₃OH₂⁺) = -91.41 ± 2.76 kcal mol^-1, ΔG_sol^*(CH₃O^-) = -88.36 ± 2.10 kcal mol^-1, and pK_a^*(methanol) = 22.67 ± 2.97. To validate our approach, we applied the same thermodynamic cycles for water in its own liquid, resulting in experimental values of ΔG_sol^*(H₃O⁺) = -110.20 ± 1.91 kcal mol^-1, ΔG_sol^*(OH^-) = -104.60 ± 0.25 kcal mol^-1, and pK_a(water) = 15.73 ± 1.42. Employing quantum mechanics calculations combined with Monte Carlo simulation, we calculated the standard deprotonation free energy and the pK_a values of water and methanol in their respective liquids. These calculations were performed using an explicit model of the solvent, with Free Energy Perturbation theory in Monte Carlo simulation (FEP-MC), three different pure implicit solvation models (HF-PCM, the Conductor-like Polarizable Continuum Model (C-PCM), the solvation model based on density (SMD)), and a hybrid model (cluster-SMD). Excellent agreement with experimental data was achieved using FEP-MC, HF-PCM, and cluster-SMD. Methanol is the simplest alcohol, and its pK_a^* value is a critical parameter in chemical and biological systems. Hence, its understanding, along with its pH* scale, enables better control and utilization of methanol in diverse applications, ranging from pharmaceuticals to industrial processes.

PMID:
41137771
Bibliographic data and abstract were imported from PubMed on 25 Oct 2025.

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