Pecher Lisa, Schober Christoph, Tonner Ralf
Faculty of Chemistry and Material Sciences Center, Philipps-Universität Marburg, Hans-Meerwein-Str. 4, 35032, Marburg, Germany.
Current address: Fakultät Chemie, TU München, Lichtenbergstr. 4, 85747, Garching, Germany.
Chemistry. 2017 Apr 24;23(23):5459-5466. doi: 10.1002/chem.201605952. Epub 2017 Feb 16.
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.
利用密度泛函理论分析了一种很有前景的有机 - 无机杂化界面体系——环辛炔在Si(001)上的吸附特性。结果表明，化学吸附的“顶位”构型最为稳定，在该构型中，环三键与表面二聚体之间形成环加合物。“桥位”和“亚层”模式（具有两个分子 - 表面键）以及具有四个键的“基座”模式稳定性较差。利用我们最近提出的周期性能量分解分析（pEDA）进行的研究表明，四键构型因分子和表面内部所需的大变形能以及相当弱的分子 - 表面键而不稳定。色散相互作用对构型的能量和结构有显著影响，导致相当柔性的分子弯曲增加。因此，先前扫描隧道显微镜实验中发现的特征可以用弯曲的“顶位”构型得到确凿解释，并且可以排除“基座”模式。与乙炔的比较表明，环结构以及由此产生的环辛炔应变导致较大吸附质的反应性增加，这是由于环三键为表面键合而预先形成。相比之下，环应变对吸附质 - 表面键的电子效应仅可忽略不计。这些计算突出了深入理论分析对于理解大的柔性分子吸附特性的必要性。
