A laser flash photolysis and theoretical study of hydrogen abstraction from phenols by triplet α-naphthoflavone

Abstract

The hydrogen abstraction (HA) reaction by the triplet of α-naphthoflavone (1) has been investigated experimentally by the use of laser flash photolysis (LFP) and theoretically with density functional theory (DFT) and atoms in molecules (AIM). The triplet excited state of 1, in acetonitrile, has an absorption maximum at 430 nm and lifetime of 10 μs. The quenching rate constants for the triplet of 1 with 1,4-cyclohexadiene, substituted phenols and amines were determined. The low reactivity of this ketone with respect to HA from 1,4-cyclohexadiene is in accord with a π,π* excited state. HA from phenols in acetonitrile is proposed to occur in a diffusion controlled reaction from free phenol based upon the determination of the Abraham β^H₂ value for acetonitrile and correction of the quenching rate constants for hydrogen bonding of the phenols to acetonitrile. A molecular orbital analysis of the triplet (SOMO and SOMO-1) of 1 reveals contributions from the carbonyl oxygen atom, but principally from the α-carbon and the associated π-bond network, consistent with a π,π* excited state. From a thermodynamic point of view, the triplet HA from phenol to oxygen of the carbonyl group is 17 kcal mol⁻¹ less demanding than the transfer to the α-carbon, consistent with the acidic nature of the phenolic hydrogen atom. DFT and AIM analysis of the hydrogen abstraction reaction reveals that the transition state (TS) is pseudo-symmetrically polarized and that HA in the hydrogen bonded exciplex occurs in a concerted manner but not necessarily by simultaneous electron and proton transfer.