Hydrated singly charged magnesium ions [Mg(H 2 O) n ] + are thought to consist of an Mg 2+ ion and a hydrated electron for n > 15. This idea is based on mass spectra, which exhibit a transition from [MgOH(H 2 O) nÀ1 ] + to [Mg(H 2 O) n ] + around n ¼ 15-22, black-body infrared radiative dissociation, and quantum chemical calculations. Here, we present photodissociation spectra of size-selected [Mg(H 2 O) n ] + in the range of n ¼ 20-70 measured for photon energies of 1.0-5.0 eV. The spectra exhibit a broad absorption from 1.4 to 3.2 eV, with two local maxima around 1.7-1.8 eV and 2.1-2.5 eV, depending on cluster size. The spectra shift slowly from n ¼ 20 to n ¼ 50, but no significant change is observed for n ¼ 50-70. Quantum chemical modeling of the spectra yields several candidates for the observed absorptions, including fiveand six-fold coordinated Mg 2+ with a hydrated electron in its immediate vicinity, as well as a solvent-separated Mg 2+ /e À pair. The photochemical behavior resembles that of the hydrated electron, with barrierless interconversion into the ground state following the excitation.

The structures, along with solvation evolution, of size-selected Zn+(H2O)n (n = 2–35) complexes have been determined by combining infrared multiple photon photodissociation (IRMPD) spectroscopy and density functional theory.

Hydrated singly charged magnesium ions Mg(HO), n ≤ 5, in the gas phase are ideal model systems to study photochemical hydrogen evolution since atomic hydrogen is formed over a wide range of wavelengths, with a strong cluster size dependence. Mass selected clusters are stored in the cell of an Fourier transform ion cyclotron resonance mass spectrometer at a temperature of 130 K for several seconds, which allows thermal equilibration via blackbody radiation. Tunable laser light is used for photodissociation. Strong transitions to D states (correlating with the 3s-3p transitions of Mg) are observed for all cluster sizes, as well as a second absorption band at 4-5 eV for n = 3-5. Due to the lifted degeneracy of the 3p energy levels of Mg, the absorptions are broad and red shifted with increasing coordination number of the Mg center, from 4.5 eV for n = 1 to 1.8 eV for n = 5. In all cases, H atom formation is the dominant photochemical reaction channel. Quantum chemical calculations using the full range of methods for excited state calculations reproduce the experimental spectra and explain all observed features. In particular, they show that H atom formation occurs in excited states, where the potential energy surface becomes repulsive along the O⋯H coordinate at relatively small distances. The loss of HO, although thermochemically favorable, is a minor channel because, at least for the clusters n = 1-3, the conical intersection through which the system could relax to the electronic ground state is too high in energy. In some absorption bands, sequential absorption of multiple photons is required for photodissociation. For n = 1, these multiphoton spectra can be modeled on the basis of quantum chemical calculations.

Hydrated singly charged metal ions doped with carbon dioxide, Mg2+(CO2)−(H2O)n, in the gas phase are valuable model systems for the electrochemical activation of CO2. Here, we study these systems by Fourier transform ion cyclotron resonance (FT–ICR) mass spectrometry combined with ab initio calculations. We show that the exchange reaction of CO2 with O2 proceeds fast with bare Mg+(CO2), with a rate coefficient kabs = 1.2 × 10–10 cm3 s–1, while hydrated species exhibit a lower rate in the range of kabs = (1.2–2.4) × 10–11 cm3 s–1 for this strongly exothermic reaction. Water makes the exchange reaction more exothermic but, at the same time, considerably slower. The results are rationalized with a need for proper orientation of the reactants in the hydrated system, with formation of a Mg2+(CO4)−(H2O)n intermediate while the activation energy is negligible. According to our nanocalorimetric analysis, the exchange reaction of the hydrated ion is exothermic by −1.7 ± 0.5 eV, in agreement with quantum chemical calculations.

Investigating metal-ion solvation—in particular, the fundamental binding interactions—enhances the understanding of many processes, including hydrogen production via catalysis at metal centers and metal corrosion. Infrared spectra of the hydrated zinc dimer (Zn2+(H2O)n; n = 1–20) were measured in the O–H stretching region, using infrared multiple photon dissociation (IRMPD) spectroscopy. These spectra were then compared with those calculated by using density functional theory. For all cluster sizes, calculated structures adopting asymmetric solvation to one Zn atom in the dimer were found to lie lower in energy than structures adopting symmetric solvation to both Zn atoms. Combining experiment and theory, the spectra show that water molecules preferentially bind to one Zn atom, adopting water binding motifs similar to the Zn+(H2O)n complexes studied previously. A lower coordination number of 2 was observed for Zn2+(H2O)3, evident from the highly red-shifted band in the hydrogen bonding region. Photodissociation leading to loss of a neutral Zn atom was observed only for n = 3, attributed to a particularly low calculated Zn binding energy for this cluster size.