Electron transfer from phenolic compounds to parent n-butyl chloride radical cations—a quantum chemical study of product transient formation and stability

Abstract

The Density Functional Theory (DFT) B3LYP and the semiempirical PM3 quantum chemical methods were used to describe the mechanism of electron transfer from aromatic solutes to n-butyl chloride radical cations. The influence of the electronic parameters, the distribution of the Mulliken charges for the radical cations, and the equilibrium geometries of the ground state and the radical cation on the chemical reactivity were analyzed. Pulse radiolysis experiments had shown that phenol radical cations and phenoxyl radicals appear as direct products of the electron transfer, indicating a product distribution determined by encounter geometry. The quantum chemical approaches introduced enabled both the geometry, the energy and the molecular orbitals of an encounter complex to be calculated, explaining the rapid phenoxyl radical formation by a second reaction channel. The experimentally observed lifetimes of the phenol radical cations were correlated with quantum chemical calculated spin and Mulliken charge distributions. It was found that the influence of the solvent type approximated by the Onsager reaction field model and the DFT method showed no essential differences in the atomic spin and Mulliken charge distributions. The semiempirical PM3-calculated parameters tally well with those observed at the B3LYP/6-31G(d) level. Furthermore, a reliable trend correlation between the atomic spin density on the oxygen atom and the lifetime of the radical cations was found. The configuration interaction with single excitation approximation at the semiempirical PM3 level was applied to calculate the low-energy electronic transitions of the aromatic transients. Finally, the electronic transitions are calculated for both the radical cations and the deprotonated neutral radicals.