Photochemistry and Spectroscopy of Hydrated Metal Ions

For many metal ions, chemical reactions depend sensitively on the exact number of water molecules in their immediate vicinity. These peculiar changes of chemical properties with the size of the hydration shell can be ideally studied in gas-phase clusters, where a single metal ion in an unusual oxidation state can be embedded in a water cluster with precisely defined size. For example, singly charged vanadium ions spontaneously react with water to form atomic or molecular hydrogen in the presence of eight to twenty-four water molecules. Also reactions with a third partner exhibit a strong size dependence. To understand these phenomena on a molecular level, spectroscopy will be performed over a wide wavelength region. Hydrated metal ions are generated in a laser vaporization source and stored in the cell of a Fourier transform ion cyclotron resonance mass spectrometer in a low temperature environment. Mass selection allows investigating individual cluster sizes. Optical parametric oscillators are used to irradiate the ions. Intense tunable light is available from 225 nm to 4450 nm. If photon absorption leads to water evaporation, the mass of the cluster is changed, and an absorption spectrum can be recorded with the mass spectrometer. Also photoinduced oxidation of the metal center and release of hydrogen is identified via the characteristic mass change. Experimental spectra will be compared with vibrational and electronic absorption spectra calculated with quantum chemical methods. Metals to be investigated include magnesium, aluminum, vanadium, manganese, and zinc. These studies provide unprecedented insight into electron transfer reactions at metal centers, like corrosion or photochemical water splitting. For the first time, electronic and vibrational spectra will be measured under identical conditions to correlate electronic with geometric structure and both with reactivity. Also for the first time, photochemical reactions of hydrated metal ions will be studied systematically, as a function of cluster size and excitation wavelength. The results will tell how individual water molecules fine tune the electronic properties of a metal center, relevant for metalloenzymes, removal of heavy metal ions from waste waters, or corrosion.

Hydrogen plays an important role for the energy transition towards a net-zero emission economy. The raw material for its production is water. A high school experiment illustrates how hydrogen is formed on metal surfaces: if a chunk of sodium is put into water, hydrogen is formed in an uncontrolled reaction. To analyze the fast and complex proceedings in detail, the high school experiment was simulated in a nanometer-sized water droplet containing less than 20 water molecules. With the help of infrared spectroscopy, we were able to show in the laboratory that before hydrogen evolution occurs, a metal hydride is formed. For hydrogen to actually evolve, a hydrogen bond between the hydride and another water molecule must be established. For the theoretical description, hundreds of possible structures were tested in the computer. Those structures, which matched the experimental spectrum, were also the energetically preferred ones, which leads to a concise explanation of the experimental observations. When sodium is immersed in water, hydrated electrons evolve for a fraction of a second, but the reaction in this case is so vigorous that there is no time for a detailed investigation of this important phenomenon. With a singly charged magnesium ion in a droplet of 20 to 70 water molecules, we succeeded to observed the hydrated electron in the mass spectrometer. The hydrated electron, donated from the metal, has a deep-blue color and is chemically highly reactive. Light can induce hydrogen evolution. The reaction pathways, however, are fundamentally different, because chemical bonds are modified by the excitation of an electron. This leads to a dazzling array of possibilities how atomic or molecular hydrogen evolves from the nano-droplet of water. We studied the details of the reaction mechanisms of photochemical hydrogen evolution with magnesium, aluminum and vanadium as representative examples. The understanding of individual reaction steps during hydrogen evolution provides researchers and engineers on the technology side with new ideas for improvement and increased efficiency of their processes. One of the fascinating properties of metal ions in nano-sized water droplets is that ions with a large radius, like the singly charged zinc ion, stay at the surface, i.e. they are not completely surrounded by water. This surprising results is explained by the fact that water molecules, which arrange on one side of the zinc ions, push the outermost electron to the opposite side of the metal ion. Additional water molecules then preferentially integrate to the hydrogen bonded network, instead of binding relatively weakly to the metal ion, which is shielded by the electron cloud. With the diatomic zinc molecular ion, only one zinc atom is in contact with water molecules, while the second stick out vertically from the surface of the water nanodroplet.