Can a Three-Carbon Olefin Close a Five-Membered Ring on Reaction with the Silicon Nitride Radical (SiN, X ² Σ ⁺ )? A Crossed Molecular Beams and Ab Initio Study

Substituting silicon for carbon in reactive molecular frameworks profoundly influences bonding characteristics, electronic structure, and reaction pathways, making silicon-containing systems a topic of sustained interest in fundamental and applied chemistry. Elucidating how silicon incorporation modulates elementary reaction dynamics is essential for establishing predictive principles relevant to organosilicon synthesis, materials chemistry, and heterocycle design. In this context, the reaction of the silicon nitride radical (SiN) with propene (C₃H₆) serves as an ideal model system to probe these effects. Previous studies have shown that reactions of SiN with two-carbon unsaturated hydrocarbons such as ethylene (C₂H₄) and acetylene (C₂H₂) predominantly yield acyclic silaisonitrile derivatives, whereas reactions with four-carbon systems, exemplified by 1,3-butadiene (C₄H₆), favor the formation of cyclic products. As a three-carbon unsaturated hydrocarbon, propene (C₃H₆) occupies a critical borderline position, offering a unique opportunity to determine whether silicon nitride reactivity preferentially promotes cyclic or acyclic product formation in this borderline case. Crossed molecular beam experiments, combined with high-level electronic structure calculations and Rice-Ramsperger-Kassel-Marcus (RRKM) statistical analysis, reveal that the reaction proceeds via indirect dynamics involving long-lived intermediates and tight exit transition states. Although the potential energy surface features multiple competing pathways, reaction energetics, and barrier heights strongly favor the formation of acyclic products, while cyclic Si-N heterocycles emerge only as minor channels. Together, these results provide fundamental insight into how main-group substitution governs reaction selectivity and pathway control, establishing general principles for silicon-centered reaction dynamics and expanding the conceptual framework of silicon-nitrogen chemistry.