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To better support researchers and contribute constructively to international scientific discourse, Angewandte Chemie has launched two opinion-based article categories with the support of the German Chemical Society (GDCh) and the journal's governing bodies. Scientific Perspectives outline best practice in the chemical sciences while Viewpoint Articles address the culture of the chemical community.

Wir zeigen mit quantenchemischen Methoden, dass sich die Adsorption von Ether-Molekulen auf Si(001) im Ultrahochvakuum durch klassische Konzepte der organischen Molekulchemie verstehen lasst. Wie unsere detaillierte Analyse zeigt, spiegelt der zweistufige Mechanismus – 1) Ausbildung einer dativen Bindung zwischen dem Sauerstoffatom am Ether und einem Lewis-sauren Oberflachenatom, 2) nukleophiler Angriff eines nahegelegenen Lewis-basischen Oberflachenatoms – die saurekatalysierte Etherspaltung in Losung wider. Zudem ist die dative O-Si-Bindung die starkste ihrer Art und die Reaktivitat in Schritt 2 verlauft entgegen dem Bell-Evans-Polanyi-Prinzip. Mithilfe einer neuartigen Methode der Bindungsanalyse wird auserdem die Umverteilung von Elektronendichte wahrend des C-O-Bindungsbruchs visualisiert. Hierbei wird deutlich, dass der Mechanismus nukleophiler Substitutionen auf Halbleiteroberflachen identisch zu molekularen SN2-Reaktionen ist. Unsere Untersuchungen verdeutlichen, wie die Forschungsfelder der Oberflachenwissenschaften und Molekulchemie voneinander profitieren konnen und so unerwartete Einblicke ermoglichen.

Using computational chemistry, we show that the adsorption of ether molecules on Si(001) under ultra-high vacuum conditions can be understood with textbook organic chemistry. The two-step reaction mechanism of (1) dative bond formation between the ether oxygen and a Lewis acidic surface atom and (2) a nucleophilic attack of a nearby Lewis basic surface atom is analysed in detail and found to mirror the acid-catalysed ether cleavage in solution. The O-Si dative bond is found to be the strongest of its kind and reactivity from this state defies the Bell-Evans-Polanyi principle. Electron rearrangement during the C-O bond cleavage is visualized using a newly developed bonding analysis method, which shows that the mechanism of nucleophilic substitutions on semiconductor surfaces is identical to molecular chemistry SN2 reactions. Our findings thus illustrate how the fields of surface science and molecular chemistry can mutually benefit and unexpected insight can be gained.

The adsorption characteristics of a promising system for hybrid organic-inorganic interfaces, cyclooctyne on Si(001), is analyzed using density functional theory. The chemisorbed 'on-top' configuration, where a cycloadduct is formed between the ring triple bond and a surface dimer, is shown to be most stable. Less stable are 'bridge' and 'sublayer' modes featuring two molecule-surface bonds and the 'pedestal' mode with four bonds. Investigations with our recently proposed periodic energy decomposition analysis (pEDA) reveal that the four-bond configuration is destabilized by large deformation energies needed within molecule and surface as well as rather weak molecule-surface bonds. Dispersion interactions show significant influence on energy and structure of the configurations leading to an increased bending of the rather flexible molecules. Thus, features found in previous scanning tunneling microscopy experiments are conclusively explained with bent 'on-top' configurations and the 'pedestal' mode can be ruled out. A comparison to acetylene shows that the ring structure and the resulting strain of cyclooctyne are responsible for an increased reactivity of the larger adsorbate due to a pre-forming of the ring triple bond for surface bonding. In contrast, ring strain leads only to negligible electronic effects on the adsorbate-surface bonds. The computations highlight the need for in-depth theoretical analysis to understand adsorption characteristics of large, flexible molecules.

Intermediate states to covalent attachment of molecules on surfaces, so called precursors, are usually considered to be physisorbed and mobile. We show that this view should be reconsidered and provide evidence for a chemisorbed precursor for ethylene on Si(001). The character of the molecule-surface bond as a π complex is determined and quantified using our recently developed method for energy and charge analysis in extended systems. In contrast to previous assumptions, the precursor should thus be immobile, which is underlined by computation of high diffusion energy barriers. This has important implications for understanding and modelling of adsorption kinetics. Our analysis highlights that taking the viewpoint of molecular chemistry helps uncover important aspects in the adsorption process on surfaces. Previous experimental results that appear to be in contrast to our model are examined and reinterpreted.