Reactivity of Hydrated Monovalent First Row Transition Metal Ions [M(H2O)n](+), M = Cr, Mn, Fe, Co, Ni, Cu, and Zn, n < 50, Toward Acetonitrile.

Reactions of [M(H2O)n](+), M = Cr, Mn, Fe, Co, Ni, Cu, and Zn, n < 50, with CH3CN are studied in the gas phase by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Sequential uptake of 4-6 acetonitrile molecules is observed for all metals. Rate constants show a weak dependence on both the metal and the number of acetonitrile molecules already in the cluster. Nanocalorimetry yields the enthalpy of the first reaction step. For most metals, this is consistent with a ligand exchange of water against acetonitrile. For M = Cr, however, the strong exothermicity of ΔE(nc) = -195 ± 26 kJ mol(-1) suggests an electron transfer from Cr(+) to CH3CN. Exclusively for M = Zn, a relatively slow oxidation of the metal center to Zn(2+), with formation of ZnOH(+) and release of CH3CNH(•) or CH3CHN(•) is observed. Density functional theory molecular dynamics simulations and geometry optimizations show that charge transfer from Zn(+) to CH3CN as well as the subsequent proton transfer are associated with a barrier.

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 (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.

Reduction of Acetonitrile by Hydrated Magnesium Cations Mg+ (H2 O)n (n≈20-60) in the Gas Phase.

Ion-molecule reactions of Mg+ (H2 O)n (n≈20-60) with CH3 CN are studied by Fourier-transform ion-cyclotron resonance mass spectrometry. Collision with CH3 CN initiates the formation of MgOH+ (H2 O)n-1 together with CH3 CHN. or CH3 CNH. , which is similar to the reaction of hydrated electrons (H2 O)n - with CH3 CN. In subsequent reaction steps, three more CH3 CN molecules are taken up by the clusters, to form MgOH+ (CH3 CN)3 after a reaction delay of 60 seconds. Density functional theory (DFT) calculations at the M06/6-31++G(d,p) level of theory suggest that the bending motion of CH3 CN allows the unpaired electron that is solvated out from the Mg center to localize in a π*(CN)-like orbital of the bent CH3 CN.- , which undergoes spontaneous proton transfer to form CH3 CNH. or CH3 CHN. , with the former being kinetically more favorable. The reaction energy for a cluster with the hexacoordinated Mg center is more exothermic than that with the pentacoordinated Mg. The CH3 CNH. or CH3 CHN. is preferentially solvated on the cluster surface rather than at the first solvation shell of the Mg center. By contrast, the three additional CH3 CN molecules taken up by the resulting MgOH+ (H2 O)n clusters coordinate directly to the first solvation shell of the MgOH+ core, as revealed by DFT calculations.