The importance of electronically excited states and of the electron spin on the reactivity of metal dimers and small clusters: matrix isolation experiments and quantum chemical calculations

• Applicant: Professor Dr. Hans-Jörg Himmel
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2014 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 260579771

Metal and metaloxide cluster models are extremely important to develop mechanistic concepts of heterogenous catalytic reactions. The large number of eneregtically low-lying electronically excited states, that are populated already at room temperature, are responsible for the high reactivity and selectivity of cluster reactions. It allows the cluster to flexibly adjust its electronic structure so that it ideally fits to the reaction with a substrate molecule. Besides gas-phase studies, measurements with the help of the matrix-isolation technique provide significant contributions to this research theme. The matrix-isolation technique is ideally suited to study the reactivity of electronically excited states of these clusters. The low temperatures (4 - 40 K) of the matrix garantee that all clusters are first in their electronic ground state. By light irradiation they can be selectively excited. The Heidelberg matrix apparatus (self-made) is especially designed for such experiments. It allows absorption measurements (IR and vis-NIR) with high resolution in the range 10 - 25000 cm-1, as well as emission measurements (Raman and fluorescence). The clusters can be excited with a laser of narrow bandwidth.While a major part of the work during the first funding period was concerned with the characterization of clusters in their electronic ground state and in energetically low-lying excited states, the work in the second period concentrates almost exclusively on the reactivities of the clusters in dependence of their electronic state. The results from the first funding period and other results from our group provide the basis for these studies. The objects are vanadium- and vanadium oxide molecules (subproject A) and titanium- and titanium oxide molecules (subproject B). The electronic states of these molecules were already studied in detail. The experimental work is accompanied by quantum chemical calculations. Sophisticated and time-consuming studies are planned, but all objectives are achievable.

• DFG Programme: Research Grants