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Information on the development of the research project

The project aimed at a molecular level understanding of carbon dioxide activation with a focus on bond-forming reactions, in particular carbon-carbon bonds. Since the submission of the project, the Innsbruck Laser Core Facility was funded, and additional laser systems became available. As a consequence, the project focus shifted towards spectroscopy and photochemistry of cluster ions with relevance for carbon dioxide activation. As outlined in the proposal, the experiments started with CO2-(H2O)n, Mg+(CO2)(H2O)n, and Co+/-(CO2)(H2O)n. Since hydrated electrons are the precursors of CO2-(H2O)n species, we also performed electronic spectroscopy of these species. Preliminary experiments on the spectroscopy and photochemistry of copper formate clusters in the framework of a Master’s Thesis showed that this system provides access to a number of highly relevant stationary point on the reaction potential energy surface from CO2 to HCOOH. This area became the focus of a third PhD thesis completed in the project. All in all, the actual project work spanned a much wider area than initially envisioned, yielding a series of interesting results and publications in high-ranking journals.

Most important results and a brief description of their significance

The project started with reactivity studies on CO2-(H2O)n and Mg+(CO2)(H2O)n. In reactions with CO2-(H2O)n, we were able to demonstrate C-C bond formation with 3-butyn-1-ol. Nanocalorimetry in combination with quantum chemical calculations indicated that the reaction is associated with a significant barrier [Int. J. Mass Spectrom. 435 (2019) 101–106]. The exchange reaction of Mg+(CO2)(H2O)n with O2 to form Mg+(O2)(H2O)n is 1.7±0.5 eV exothermic and involves a CO4- intermediate, which is unstable [J. Phys. Chem. A 123 (2019) 73−81]. Both studies underline that the carbon dioxide radical anion is reactive, but the reactions proceed through tight transition states requiring specific alignments.
The first highlight of the project is the electronic spectroscopy of hydrated electrons (H2O)n-, with n ≤ 200. Here mass selected clusters were irradiated with tunable laser light in the near infrared and visible range. A significant blue shift of the absorption was observed with increasing cluster size up to 100 water molecules, but then the spectra did not change much in the range n = 100 – 200. We calculated the gyration radii from the measured spectra as a function of cluster size. Comparison with previous
modeling and experimental literature values led to the conclusion that the hydrated electron in the gas phase occupies a partially embedded hydration site for n > 50, while for smaller clusters, the partially embedded isomer coexists with a surface isomer [J. Am. Chem. Soc. 141 (2019) 18000−18003]. This study, which was conducted in cooperation with Daniel M. Neumark, settled the debate on the nature of the hydrated electron in water clusters, which started 1988 with a paper by Uzi Landman and Joshua Jortner [J. Chem. Phys. 88 (1988) 4429−4447].

The symmetric stretching mode of CO2-(H2O)n was probed by infrared spectroscopy [Chem. Eur. J. 25 (2019) 10165 – 10171]. The absorption shifts to the blue with increasing cluster size, reaching the bulk value known from Raman spectroscopy already for n = 20. This study is important for the general idea of the proposal, since it shows that CO2- in the cluster environment is very similar to the bulk.

A second highlight of the project is the infrared study on Mg+(CO2)(H2O)n, with n = 0 – 8. Here we showed that the antisymmetric stretching mode undergoes a significant shift upon going from n = 2 to n = 3, clear evidence that formation of the carbon dioxide radical anion in this system requires three water molecules. Further hydration leads to a gradual weakening of the Mg2+…CO2- interaction in the cluster [Angew. Chem. Int. Ed. 59 (2020) 7467 –7471].

UV-VIS irradiation of the MgCO2+ complex leads to C-O bond photolysis in a two-photon process at photon energies above 4.75 eV [Theor. Chem. Acc. 139 (2020) 127]. In this complex, CO2 is hardly activated, and yet photolysis of the C-O bond is observed
with high yields. This metal-assisted C-O bond photolysis could provide a mechanistically simple way for light harvesting, driven by the formation of MgO+ which lowers the required photon energy. Further studies with other metals, in particular transition metals that offer a wider range of accessible states, will be highly desirable.
Infrared multiple photon dissociation (IRMPD) of CoCO2(H2O)n, n = 1−10, showed a complex, size-dependent activation of CO2 and H2O at the negatively charged cobalt center [Chem. Eur. J. 26 (2020) 1074-1081], including formation of CO and HCOO- ligands. This study was designated a Very Important Paper by the reviewers.
Anionic copper formate clusters turned out to be a highly meaningful model system for the catalytic conversion of CO2 and H2 to HCOOH on copper catalysts. Heating the clusters with tunable infrared light led to elimination of CO2. Quantum chemical calculations confirmed that this CO2 elimination is endothermic, and that the reverse reaction should proceed without barrier, thus CO2 reacts with copper hydride species to form HCOO- ligands. Moreover, elimination of neutral HCOOH, the desired product of the catalytic reaction, can occur in several mechanistically distinct ways from HCOO- and H-, with copper centers accepting the two extra electrons. Naively, one would expect that formic acid is eliminated following proton transfer, but in the present case, it is actually hydride species that afford the final product formation [ChemPhysChem 20 (2019) 1420-1424; ChemistryOpen 8 (2019) 1453-1459]. IRMPD studies of copper formate clusters revealed very rich Fermi resonances in the C-H stretch region of the formate ligand. Deuteration removed these features, providing a clean view on the spectral characteristics of copper formate. The observed bands correlate closely to those identified in surface studies, which clearly shows that these clusters closely resemble bulk surface species [J. Chem. Phys. in print; DOI: 10.1063/5.0030034].
UV-VIS irradiation of copper formate clusters reveals broad bands corresponding to d-s/p transitions for Cu(I), while Cu(II) species exhibit a weak band of d-d transitions around 1.7 eV, and strong ligand-to-metal charge transfer bands above 3.5 eV [Chem. Eur. J. 26 (2020) 8286–8295]. The associated photochemistry is very complex for Cu(I) species. E.g. Cu(I)2(HCOO)3- can undergo a photochemical disproportionation to Cu(II)(HCOO)3- and Cu(0). Excited state calculations indicate that Cu(I) species feature high-lying conical intersections, while Cu(II) species readily relax radiationless to the ground state. Even the smallest systems are very difficult to describe computationally. Nevertheless, the observed photochemical products can be rationalized, and plausible pathways along the excited states potential energy surfaces can be identified.