Outer-sphere electron transfer from carbazoles to halomethanes. Reduction potentials of halomethanes measured by fluorescence quenching experiments

Abstract

Kinetic studies on the photoinduced electron transfer reduction of a variety of halomethanes in acetonitrile and ethanol at 298 K are reported in terms of the quenching rate constants (k_q) with a series of reductants (carbazoles and anthracenes) whose one-electron oxidation potentials (E_ox) have been measured again in the present work vs. SCE and independently vs. ferrocene. The Rehm–Weller Gibbs energy relationship is applied to determine the fundamental parameters for the one-electron reduction, i.e., the one-electron reduction potentials (E_RX/RX⁻˙) of the halomethanes and the intrinsic barrier for the electron transfer reduction (ΔG^G‡_o). The E_RX/RX⁻˙ values obtained were related to the concerted electron transfer–bond breaking reduction potentials (E_{RX/R˙ + X⁻}) and the standard free enthalpies of dissociation of RX⁻˙ (ΔG^o, diss_RX⁻˙) were estimated in each solvent. Additionally, the one-electron reduction potential (E_RX/RX⁻˙) values estimated in acetonitrile were also related to different thermodynamic parameters such as electron affinity (E_A), LUMO–HOMO energy differences (ΔE) and the bond dissociation energy (D_RX). Optimized geometry, E_A and ΔE for halomethanes were calculated by an ab initio method at the B3LYP level using 3-21G, 6-31+G(d,p) and G-311+G(3df,2pd) basis sets. In all these cases good linear correlations were obtained. The ΔG^G‡_o values obtained are compared with those calculated using the equation ΔG^G‡_o = λ/4 with λ = λ_i + λ_o where the solvent reorganization energy (λ_o) and the inner-sphere reorganization energies (λ_i) associated with the structural change upon the electron-transfer process were calculated, the former by using the Marcus–Hush model and the latter by using semiempirical and ab initio molecular modeling and QSAR properties. Results obtained from the preparative irradiation of carbazoles in the presence of halomethanes, which are consistent with a photoinduced electron transfer mechanism are also discussed.