The mechanisms of the photochemical carbon-halogen bond activation reactions were investigated theoretically using a model system of hexahalobenzene C6X6 (X= F, Cl, and Br) and phenyl isonitrile (PhN= C:), with the M06-2X method and the 6-311G(d) basis set. The theoretical findings suggest the photoactivation mechanism should proceed as follows: Rea-X-S0 + hv -> FC-X-T1 -> PhNC-T1 + C6X6 -> Int-X-T1 -> TS-X-T1 -> T1/S0-X -> Pro-X-S0. The present theoretical evidence confirms the reported results by Studer and co-workers that using UV irradiation, a simple and efficient transition-metal-free C-F activation in various perfluoroarenes with aryl and alkyl isonitriles obtain imidoyl fluorides. Nevertheless, due to the highly vertical Frank-Condon excitation energy, our theoretical study demonstrates N-heterocyclic carbene cannot undergo the C-F bond activation reaction with hexafluorobenzene under photoirradiation, which closely agrees with the available experimental observations. The present theoretical research reveals the spin crossover processes, which lead to a concerted reaction pathway, play a key role in the re-gioselecitivities of the final photoinsertion products.
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