Hydrated Metal Ions Photochemistry: New Insight from Theory

Hydration of ions, i.e. their interaction with water, is scientific problem of ever-growing interest. In particular, the photochemistry of hydrated metal ions, i.e. their interaction with light, poses many relevant but unanswered questions: What happens when such ions absorb UV radiation? Are highly reactive radicals created? How are these processes connected to photocorrosion the corrosion of metal surfaces exposed to light? What can we learn for the photocatalytic water splitting reaction? To answer such questions, chemists have resorted to model systems that are fully under their control, namely to hydrated metal ions in the gas phase, i.e. metal ions with several (up to about 500) water molecules. Structure and reactions of such hydrated metal ions are traditionally explored experimentally using mass spectrometry and infrared spectroscopy. Their photochemical behavior, however, is hardly understood. Within the present project, we would like to reveal fundamental aspects of hydrated metal ions photochemistry using methods of theoretical chemistry. We will investigate Mg+, V+, and Fe+ ions as species that have already been explored experimentally. It was found that after photoexcitation, the hydrated ions release water molecules or react to dissociate H and H2. The mechanistic details of these photochemical processes remain unknown. We will use various computational approaches to analyze photochemical pathways and model photodynamics, i.e. follow the fate of molecules after photoexcitation, retrieving information about reaction mechanisms, relative importance of various photochemical channels, and the timescale of the processes. In the field of theoretical chemistry, many advanced techniques have been developed and considerable computational power is nowadays available owing to the rapid advancement in computer hardware. Still, the present project represents a considerable challenge due to the complex electronic structure of the studied ions as well as the non-black-box character of the methods employed. The research will be conducted in the group of mass spectrometry of Prof. Martin K. Beyer at the University of Innsbruck; such close cooperation with experiment is the key ingredient of the project. Our theoretical modelling will rationalize experimental observations and suggest new experiments; at the same time, the computational approaches will be improved based on the feedback from the experiment. Experience shows that direct cooperation between experimentalists and theoreticians is the most fruitful way to reach a complete description of such complicated chemical processes as those occurring in the photochemistry of hydrated metal ions.

Hydrated metal ions are fascinating systems to follow fundamental processes in nature: emerging solvation from small clusters to bulk, fundamental steps in chemical reactions or the influence of hydration on chemistry that is initiated by photons, i.e. photochemistry. Apart from the significance in fundamental research, the study of hydrated metal ions can be of practical relevance in, e.g., studies of reaction intermediates and (photo)corrosion. We investigated the photochemistry of several hydrated metal ions (Mg +, V+, Fe+) by a combination of theoretical chemistry and mass spectrometric experiments. Our photochemical studies have also indirectly contributed to the study of the chemistry in the ground electronic state through interpretation of photodissociation spectra and mapping photochemical pathways that are encountered after excitation. We showed that, already for a magnesium ion hydrated by 20 water molecules, a hydrated electron emerges. The hydrated electron is an important species in radiation chemistry as it might e.g. attack our DNA following exposure of our bodies to X-ray radiation. In all investigated systems, we reached perfect agreement between experiment and theory. Within the project, computational protocols for modeling absorption spectra, photochemical channels and radiationless transitions were developed, with special attention to larger systems with up to 20 water molecules and challenging electronic structure. Although this size might look small for practical applications, it represents the state-of-the-art of current photochemical calculations with fully quantum mechanical treatment. The project led to development of a new methodology to calculate so-called action spectra, i.e. spectra that describe the absorption of a photon followed by a chemical event. In the mass spectrometry experiment, photodissociation spectra are measured, where absorption is followed by dissociation of the excited cluster ion. However, in theoretical chemistry, only absorption spectra are usually calculated, making comparison with the experimental results difficult. Here, we developed a method that accounts for several stages of the photodissociation process: absorption, fluorescence, absorption of a second photon, and eventually photodissociation. The approach enabled us to interpret the experiments at an unprecedented level of detail. For example, we found that two-photon processes form a significant part of the photochemistry of Mg + hydrated with a low number of water molecules. The method will be further refined in the future to allow for modeling of more complex phenomena.