Topological Woodward-Hoffmann classification for cycloadditions in polycyclic aromatic azomethine ylides
arxiv(2024)
摘要
The study of cycloaddition mechanisms is central to the fabrication of
extended sp2 carbon nanostructures. In the context of polycyclic aromatic
hydrocarbons (PAHs), the reaction modeling of these compounds has focused
mostly on putative, energetically preferred, exothermic products with limited
consideration for symmetry allowed or forbidden mechanistic effects. Here, we
introduce a scheme for formally classifying symmetry-forbidden reaction
coordinates in Woodward-Hoffmann correlation diagrams. Topological classifiers
grant access to the study of reaction pathways and correlation diagrams in the
same footing, and are employed to investigate cycloaddition reactions of
polycyclic aromatic azomethine ylide (PAMY) yielding PAHs with a
tetracenoisoindole core by means of tight-binding modeling and density
functional theory (DFT). Our scheme provides new insights into cycloadditions
of PAMY with pentacene, as observed by mass spectrometry and scanning tunneling
microscopy, resulting in topologically-allowed pathways if the product is
endothermic, and topologically-forbidden if the product is exothermic. Using
this approach, we find that an endothermic, multi-step mechanism is in much
better agreement with the experiment than a putative exothermic mechanism. Our
work unveils topological classification as a crucial element for reaction
modeling, and highlights its fundamental role in the design of cycloadditions
in on-surface and solid-state chemical reactions, while underscoring exothermic
pathways which are topologically-forbidden.
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