Abstract
Hydrated singly charged magnesium ions [Mg(H2O)n]+ are thought to consist of an Mg2+ ion and a hydrated electron for n > 15. This idea is based on mass spectra, which exhibit a transition from [MgOH(H2O)n-1]+ to [Mg(H2O)n]+ around n = 15-22, black-body infrared radiative dissociation, and quantum chemical calculations. Here, we present photodissociation spectra of size-selected [Mg(H2O)n]+ in the range of n = 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 = 20 to n = 50, but no significant change is observed for n = 50-70. Quantum chemical modeling of the spectra yields several candidates for the observed absorptions, including five- and six-fold coordinated Mg2+ with a hydrated electron in its immediate vicinity, as well as a solvent-separated Mg2+/e- pair. The photochemical behavior resembles that of the hydrated electron, with barrierless interconversion into the ground state following the excitation.
Highlights
IntroductionHydrated metal ions are a useful and well-de ned model to examine fundamental chemical reactions including hydrogen production or corrosion mechanisms and to investigate the link from small metal–water complexes to bulk aqueous solutions.[1,2,3,4,5,6,7,8,9,10,11] Among various hydrated metal ions, [Mg(H2O)n]+ has received continuous attention.[12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40] Hydrated singly charged magnesium ions can be found in two different species, depending on hydration number
Hydrated singly charged magnesium ions [Mg(H2O)n]+ are thought to consist of an Mg2+ ion and a hydrated electron for n > 15
This idea is based on mass spectra, which exhibit a transition from [MgOH(H2O)nÀ1]+ to [Mg(H2O)n]+ around n 1⁄4 15–22, black-body infrared radiative dissociation, and quantum chemical calculations
Summary
Hydrated metal ions are a useful and well-de ned model to examine fundamental chemical reactions including hydrogen production or corrosion mechanisms and to investigate the link from small metal–water complexes to bulk aqueous solutions.[1,2,3,4,5,6,7,8,9,10,11] Among various hydrated metal ions, [Mg(H2O)n]+ has received continuous attention.[12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40] Hydrated singly charged magnesium ions can be found in two different species, depending on hydration number. In our measured photodissociation spectra, we could assign speci c isomers, providing insight into the electronic structure and dissociation mechanisms, which involve water evaporation and hydrogen dissociation
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