Reductions of oxygen, carbon dioxide, and acetonitrile by the magnesium(II)/Magnesium(I) couple in aqueous media: Theoretical insights from a nano-sized water droplet

Reductions of O₂, CO₂, and CH₃CN by the half-reaction of the Mg(II)/Mg(I) couple (Mg²⁺ + e^- → Mg^+•) confined in a nanosized water droplet ([Mg(H₂O)₁₆]^•+) have been examined theoretically by means of density functional theory based molecular dynamics methods. The present works have revealed many intriguing aspects of the reaction dynamics of the water clusters within several picoseconds or even in subpicoseconds. The reduction of O₂ requires an overall doublet spin state of the system. The reductions of CO₂ and CH₃CN are facilitated by their bending vibrations and the electron-transfer processes complete within 0.5 ps. For all reactions studied, the radical anions, i.e., O₂ ^•-, CO₂ ^•-, and CH₃CN^•-, are initially formed on the cluster surface. O₂ ^•- and CO₂ ^•- can integrate into the clusters due to their high hydrophilicity. They are either solvated in the second solvation shell of Mg²⁺ as a solvent-separated ion pair (ssip) or directly coordinated to Mg²⁺ as a contact-ion pair (cip) having the ¹n-[MgO₂]^•+ and ¹n-[MgOCO]^•+ coordination modes. The ¹n-[MgO₂]^•+ core is more crowded than the ¹n-[MgOCO]^•+ core. The reaction enthalpies of the formation of ssip and cip of [Mg(CO₂)(H₂O)₁₆]^•+ are -36 ± 4 kJ mol^-1 and -30 ± 9 kJ mol^-1, respectively, which were estimated based on the average temperature changes during the ion-molecule reaction between CO₂ and [Mg(H₂O)₁₆]^•+. The values for the formation of ssip and cip of [Mg(O₂)(H₂O)₁₆]^•+ are estimated to be -112 ± 18 kJ mol^-1 and -128 ± 28 kJ mol^-1, respectively. CH₃CN^•- undergoes protonation spontaneously to form the hydrophobic [CH₃CN, H]^•. Both CH₃CN and [CH₃CN, H]^• cannot efficiently penetrate into the clusters with activation barriers of 22 kJ mol^-1 and ∼40 kJ mol^-1, respectively. These results provide fundamental insights into the solvation dynamics of the Mg²⁺/Mg^•+ couple on the molecular level.