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

Abstract

Reductions of O2, CO2, and CH3CN by the half-reaction of the Mg(II)/Mg(I) couple (Mg(2+) + 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 Mg(2+) as a solvent-separated ion pair (ssip) or directly coordinated to Mg(2+) as a contact-ion pair (cip) having the (1)η-[MgO2](•+) and (1)η-[MgOCO](•+) coordination modes. The (1)η-[MgO2](•+) core is more crowded than the (1)η-[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 Mg(2+)/Mg(•+) couple on the molecular level.

CO2/O2 Exchange in Magnesium–Water Clusters Mg+(H2O)n

“…54,59 Quantum chemical calculations corroborate that charge transfer occurs, resulting in a doubly charged metal center and a negative CO 2 • − . 53,54 Similar reactions are observed with O 2 for M = Mg, Cr, Co, Ni, and Zn.…”

“…Bond dissociation energies for Mg + complexes with a series of small molecules, including CO 2 , as well as the binding energies of the first four water molecules were determined by collision-induced dissociation (CID) experiments by the Armentrout group. , Williams and co-workers studied doubly charged hydrated magnesium by blackbody infrared radiative dissociation (BIRD) . Singly charged hydrated magnesium ions Mg + (H 2 O) n have been examined with respect to the influence of blackbody infrared radiation, photodissociation, and reactions with small molecules by FT-ICR mass spectrometry and methods of theoretical chemistry. − …”

Carbon Dioxide Activation at Metal Centers: Evolution of Charge Transfer from Mg .+ to CO2 in [MgCO2(H2O)n].+, n=0–8

“…Several reactivity and photochemical studies on Mg .+ (H 2 O) n confirmed the coexistence of Mg 2+ and a hydrated electron for n >15 . Quantum chemical calculations have corroborated the existence of Mg 2+ /O 2 .− and Mg 2+ /CO 2 .− ion pairs in clusters containing 3 and 16 water molecules . Infrared spectroscopy is an excellent tool for the structural investigation of metal–CO 2 interactions in clusters such as M + (CO 2 ) n (M=Mg, Al, Si, V, Fe, Co, Ni, Rh, Ir), or M − (CO 2 ) n (M=Ti, Mn, Fe, Co, Ni, Cu, Ag, Au, Sn, Bi) .…”

Aktivierung von Kohlenstoffdioxid an Metallzentren: Entwicklung des Ladungstransfers von Mg .+ auf CO2 in [MgCO2(H2O)n].+, n=0–8

“…Quantenchemische Rechnungen bestätigten die Existenz von Mg 2+ /O 2 .− ‐ und Mg 2+ /CO 2 .− ‐Ionenpaaren in Clustern, die 3 bzw. 16 Wassermoleküle enthalten . IR‐Spektroskopie eignet sich hervorragend zur Strukturuntersuchung von Metall‐CO 2 ‐Wechselwirkungen in Clustern, wie M + (CO 2 ) n (M=Mg, Al, Si, V, Fe, Co, Ni, Rh, Ir), oder M − (CO 2 ) n (M=Ti, Mn, Fe, Co, Ni, Cu, Ag, Au, Sn, Bi) .…”