Ionization of Aromatic Sulfides in Nonpolar Media: Free vs Reaction-Controlled Electron Transfer

Abstract

The primary products of the bimolecular free electron transfer (FET) from aromatic sulfides (PhSCH2Ph, PhSCHPh2, PhSCPh3) to n-butyl chloride radical cations are two radical cation conformers: a dissociative and a metastable one. In analogy with formerly studied donor systems, this result seems to reflect femtosecond oscillations in the ground state of the sulfides such as torsion motions around the Ar-S bond. This motion is accompanied by a marked electron fluctuation within the HOMO (or the n) orbitals. The FET products observed in the nanosecond time scale such as the metastable sulfide radical cations (Ar-S-CR3*+), the dissociation products R3C+; and R3C*, and their (experimentally) nondetectable counterparts Ar-S* as well as Ar-S+ can be understood with the simplified assumption of two extreme conformations, namely a planar and a twisted donor molecule. Using mediator radical cations (benzene, butylbenzene, biphenyl), the stepwise reduction of the free energy of the electron transfer from -DeltaH = 2.5 to <or=0.5 eV changes the dissociation pattern of the sulfide radical cations toward a uniform product such as the metastable sulfide radical cation. From that, we tentatively interpret the mechanism of the studied electron transfer as diffusion-controlled (collision-determined FET) at higher -DeltaH values and as reaction-controlled at -DeltaH below 0.5 eV.