Chem Sci. 2025 Oct 29. doi: 10.1039/d5sc05991g. Online ahead of print.
ABSTRACT
Exotic cycloaddition entrance channels were discovered for the bimolecular gas-phase reactions of the phenylethynyl radical (C6H5CC, X2A1) with ethylene-d4 (C2D4) and propylene (C3H6) as explored under single-collision conditions utilizing the crossed molecular beams technique combined with electronic structure and statistical calculations. Connecting the concepts of barrierless entrance channels, excited states, and facile non-photochemically activated cycloadditions, the reaction pathway features an unconventional thermal [2 + 2] cycloaddition forming a four-membered ring collision complex followed by multiple isomerizations prior to unimolecular decomposition via atomic hydrogen loss to (un)substituted naphthalenes-naphthalene-d4 (C10H4D4) and 1-/2-methylnaphthalene (C11H10). The small energy gap between the singly-occupied a1 highest occupied molecular orbital (HOMO) with a σ-character and the underlying doubly-occupied b1 molecular orbital with a π-character allows a facile promotion of an electron. This in turn enables a versatile low-temperature reactivity of phenylethynyl, where the end-on and side-on barrierless approaches of ethylene are due to its interaction with the σ and π orbitals, respectively, thus suggesting this mechanism as a possible method for tuning substituents in polycyclic aromatic hydrocarbon (PAH) formation and highlighting its versatility as a probe of fundamental carbon chemistry via counterintuitive cycloaddition reactions under single-collision conditions.
PMID:41170523 | PMC:PMC12569633 | DOI:10.1039/d5sc05991g
