Fluorine-substitution induced switching of dissociation patterns of C6H4˙+ produced by photoelimination of MgF2 from the complexes of Mg˙+(multifluorobenzene)

Abstract

Complexes of fluorinated benzenes (o-C₆H_4−nF_2+n) and Mg˙⁺ are subjected to ultraviolet photodissociation (260–340 nm), producing efficiently benzyne radical cations (C₆H_4−nF_n˙⁺) besides Mg˙⁺ and MgF⁺. We show that the consecutive dissociation of C₆H_4−nF_n˙⁺ follows the [C4⁺ + C2] pattern exclusively for n ≤ 2 after the parent complexes absorb one or two photons. However, the dissociation pattern is switched to [C5⁺ + C1] and [C1⁺ + C5] for n ≥ 3. In particular, upon two-photon absorption at 340 nm by the complexes of Mg˙⁺(C₆HF₅) (1) and Mg˙⁺(C₆F₆) (2), photoproducts of CF⁺, C₅H⁺, and C₅HF˙⁺ from C₆HF₃˙⁺ and CF⁺, C₅F⁺, C₅F₂˙⁺, and C₅F₃⁺ from C₆F₄˙⁺ are detected, respectively. Theoretical calculations are used to explain the switching of the dissociation patterns induced by the fluorine substitutions. It was found that the formation of C5⁺ + C1 is energetically more favorable than that of C4⁺ + C2 from C₆HF₃˙⁺ and C₆F₄˙⁺ and of C1⁺ + C5. Except for C₅H₂F⁺ + CF, all the channels of [C5⁺ + C1] and [C1⁺ + C5] are energetically less favorable than those of [C4⁺ + C2] from C₆H₃F˙⁺ and C₆H₂F₂˙⁺. In most cases, the calculated results agree well with the experimental observations.