Calculated One- and Two-Electron Reduction Potentials and Related Molecular Descriptors for Reduction of Alkyl and Vinyl Halides in Water

Abstract

One- and two-electron reduction potentials (E₁ and E₂ values) were calculated from published thermodynamic data for 39 halogenated C₁ and C₂ compounds, including many commonly encountered groundwater contaminants. Because reductive dehalogenation is an important pathway for their destruction under anaerobic conditions, information concerning the relevant reduction potentials may be useful for assessing the thermodynamic feasibility of a particular reaction, as well as in developing linear free energy relationships (LFERs) or other quantitative structure-activity relationships (QS ARs) that may enable prediction of rates of transformation. E₁ values were calculated assuming a stoichiometry corresponding to dissociative electron transfer, which produces a carbon-centered radical and a halide ion. E₂ values were calculated for both hydrogenolysis and reductive β-elimination reactions. Uncertainties in the thermodynamic data for the organohalides under consideration may introduce substantial uncertainty in the resulting E₁ values. Hence, relationships between calculated E1 values and various surrogate parameters were also investigated. E₁ values were correlated with lowest unoccupied molecular orbital (LUMO) energies and carbon-halogen homolytic bond dissociation energies (BDE values), which were computed via density functional theory. Correlations were also attempted between E₁ values and vertical attachment energies (VAE values), the latter representing experimental measures of the ease of reduction of a molecule to a radical anion in the gas phase. These alternate descriptors may provide a means for estimating E₁. Additional studies will need to be undertaken to establish which descriptor best correlates with reactivity in environmental reductive dehalogenation.

Keywords:

• reductive dehalogenation
• reductive dechlorination
• dissociative electron transfer
• anaerobic
• halogenated alkanes
• chlorinated ethylenes
• biodegradation
• enthalpies of formation
• computational chemistry
• density functional theory
• QSARs
• LFERs.