Determination of absolute rate constant for the phenolic hydrogen abstraction reaction by the triplet excited state of xanthone in acetonitrile and in ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [bmim.PF_6]

摘要

The triplet excited state of xanthone was generated and characterized by laser flash photolysis in acetonitrile (λ_(max)=620 nm; τ=1.8 μs) and in ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [bmim.PF_6] (λ_(max)=620 nm; τ=3.0 μ_s). It reacts with phenols yielding the corresponding xanthone ketyl radical. Stern-Volmer plots for the reaction of triplet xanthone with phenols led to the determination of absolute rate constants for phenolic hydrogen abstraction in the order of ?10~9 Lmol~(-1)s~(-1) in acetonitrile and ?10~8 Lmol~(-1)s~(-1) in [bmim.PF_6]. The lower diffusioncontrolled rate constant for [bmim.PF_6] is responsible for the difference in the phenolic hydrogen abstraction rate constants in this solvent.