Dehalogenation is among the most important processes involved in contaminant fate, but despite all the work that has been done on the kinetics of dehalogenation, there are few linear free energy relationships (LFERs) that can be used to explain or predict rates of dehalogenation by environmental reductants. Previously, we summarized kinetic data for dehalogenation of chlorinated alkanes and alkenes by zero-valent iron (Fe0) and showed that correlation analysis of these data with published two-electron reduction potentials did not give a simple relationship. In this study, we report successful LFERs based on estimated lowest unoccupied molecular orbital (LUMO) energies calculated from semiempirical (AM1 and PM3) and ab initio methods (6-31G*) and one-electron reduction potentials. Solvation effects can be modeled with COSMO and incorporated into semiempirical estimates of E LUMO, but this did not improve the correlation with k. The best LFER (log k = −5.7−1.5 E LUMO) explains 83% of the variability in surface area-normalized rate constants (k) with ab initio LUMO energies. The LFER is improved by correcting for statistical bias introduced by back transformation from log-linear regression models. New kinetic data for six compounds are compared with rate constants predicted using the unbiased LFER.

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While reduction of chlorinated hydrocarbons by zero-valent iron in water is strongly influenced by the oxide layer at the metal−water interface, the role of the oxide in the dechlorination mechanism has not been fully characterized. In this paper, we investigate the semiconducting properties of the oxide layer on granular iron and show how the electronic properties of the oxide affect electron transfer to aqueous CCl4. Specifically, we determine whether conduction-band electrons contribute to the reduction of CCl4 by using light to increase the number of conduction-band electrons at the oxide surface and measuring how this treatment affects the rate and products of CCl4 degradation. We find that photogenerated conduction-band electrons do degrade CCl4 and, more importantly, shift the product distribution to more completely dechlorinated products that are indicative of two-electron transfer with a dichlorocarbene intermediate. Since the photogenerated electrons give different reduction products than the dark reducers, we conclude that the latter must not be conduction-band electrons. Further investigation of the reduction with photogenerated electrons is carried out by adding hole scavengers to the system. Isopropyl alcohol reacts with photogenerated holes to yield the α-hydroxyalkyl radical, which is known to reduce CCl4. With isopropyl alcohol present, we observe faster degradation of CCl4 with higher light intensity. Since no such increase is seen without isopropyl alcohol, the rate of CCl4 degradation by conduction-band electrons in water must not be limited by the number of photogenerated electron−hole pairs but rather by electron transfer from the oxide conduction band to CCl4.

The product translational energy distribution P(ET) for acetylene photodissociation at 193 nm was obtained from the time-of-flight spectrum of the H atom fragments. The P(ET) shows resolved structure from the vibrational and electronic excitation of the C2H fragment; comparison of the translational energy release for given excited states of C2H with the known energy levels of these states gives D0(HCC–H)=131.4±0.5 kcal/mol. This value is in agreement with that determined previously in this group from analogous studies of the C2H fragment and with the latest experimental and theoretical work. The high resolution of the experiment also reveals the nature of C2H internal excitation. A significant fraction of the H atoms detected at moderate laser power were from the secondary dissociation of C2H. The P(ET) derived for this channel indicates that most of the C2 is produced in excited electronic states.

Angle resolved time of flight (TOF) measurements of the fragments produced when allene is photolyzed at 193 nm are described. The two primary processes that have been identified from these measurements are the H+C3H3 and the H2+C3H2 channels. The quantum yields for these first steps are 0.89 and 0.11, respectively. Subsequent photolysis of the C3H3 radical produces H2+C3H, C3H2+H, and C2H2+CH, while the C3H2 produces C3+H2, C2H+CH, and C2H2+C. The translational energy distributions for each one of these steps have been derived using the forward convolution technique. These energy distributions reveal the exit barriers and other constraints on the potential energy surfaces that lead to the above stated products.