Using gas-phase and quantum molecular descriptors to predict dehalogenation rates of chlorinated alkanes by zerovalent iron

Zerovalent iron (Fe0) provides an effective and inexpensive remediation treatment for ground water contaminated with chloroalkanes. Reductive dehalogenation is the most important process associated with a single electron transfer to a chloroalkane molecule at the iron surface, with rates varying considerably among chloroalkanes. In this study, dehalogenation rates were determined under controlled experimental conditions and normalized to the surface area of the Fe0 (KSA). Correlations between KSA's of twelve chloroalkanes with Fe0 and a number of gas-phase molecular properties were analyzed to establish useful linear free energy relationships (LFERs). Four molecular descriptors were selected for LFER development: (i) estimated lowest unoccupied molecular energies (LUMOs), calculated from an ab initio quantum-chemical method (6-31G*), (ii) vertical attachment energies (VAEs), measured by electron transmission spectroscopy (ETS), (iii) thermal electron attachment rate constants, determined by electron beam studies in the gas phase, and (iv) electron capture detector (ECD) response obtained from a commercial gas chromatograph with an ECD detector. Connections between the ECD responses of chloroalkanes and other descriptors were initially investigated. Results showed that the ECD response increased exponentially with increasing attachment rate constants and integrated half-peak cross sections, but decreased with increasing VAEs. ECD measurements were also carried out in monochloroalkanes substituted with unsaturated ethenyl and phenyl moieties, and the response depended strongly on mixing between the unsaturated pi* and the C-Cl sigma* temporary anions, as exhibited by the vertical attachment energies (VAEs) of these states. The results also showed good correlations between ECD responses and the VAEs for the mixed states. Good correlations between KSA's and all descriptors were obtained but a separate trend line was required for chloromethane and the chloro-ethanes/propanes. The best LFER was derived when LUMO energies were used as a chemical descriptor (r2 = 1.0 for methanes, 0.86 for ethanes/propanes). We conclude that LUMO energies, as well as other gas-phase descriptors, may be useful in predicting the dehalogenation rates of chloroalkanes treated with Fe0.