RESUMEN
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.
RESUMEN
Computational modeling of organic interface formation on semiconductors poses a challenge to a density functional theory-based description due to structural and chemical complexity. A hierarchical approach is presented, where parts of the interface are successively removed in order to increase computational efficiency while maintaining the necessary accuracy. First, a benchmark is performed to probe the validity of this approach for three model reactions and five dispersion corrected density functionals. Reaction energies are generally well reproduced by generalized gradient approximation-type functionals but accurate reaction barriers require the use of hybrid functionals. Best performance is found for the model system that does not explicitly consider the substrate but includes its templating effects. Finally, this efficient model is used to provide coverage dependent reaction energies and suggest synthetic principles for the prevention of unwanted growth termination reactions for organic layers on semiconductor surfaces.
RESUMEN
The chemistry of organic adsorbates on surfaces is often discussed in terms of Pauli repulsion as limiting factor regarding the packing of molecules. Here we show that the attractive part of the van der Waals potential can be similarly decisive. For the semiconductor surface Si(001), an already covalently bonded molecule of cyclooctyne steers a second incoming molecule via dispersion interactions onto the neighbouring adsorption site. This helps in understanding the nonstatistical pattern formation for this surface-adsorbate system and hints toward an inclusion of dispersion attraction as another determining factor for surface adsorption.
RESUMEN
By using computational chemistry it has been shown that the adsorption of ether molecules on Si(001) under ultrahigh vacuum conditions can be understood with classical concepts of organic chemistry. Detailed analysis of the two-step reaction mechanism-1)â formation of a dative bond between the ether oxygen atom and a Lewis acidic surface atom and 2)â nucleophilic attack of a nearby Lewis basic surface atom-shows that it mirrors acid-catalyzed ether cleavage in solution. The O-Si dative bond is the strongest of its kind, and the reactivity in stepâ 2 defies the Bell-Evans-Polanyi principle. Electron rearrangement during C-O bond cleavage has been visualized with a newly developed method for analyzing bonding, which shows that the mechanism of nucleophilic substitutions on semiconductor surfaces is identical to molecular SN 2 reactions. Our findings illustrate how surface science and molecular chemistry can mutually benefit from each other and unexpected insight can be gained.
RESUMEN
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.