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.
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.
The photodissociation of 1,1-and 1,2-difluoroethylene (DFE) at 193 nm was studied by measuring product translational energy distributions, P(E T ), for the various product channels. The P(E T )'s are used to obtain information on the exit barriers, product internal energy, transition states, and the stability of intermediates in many of these channels. Significant differences in the P(E T )'s for three-centered elimination of HF to produce :CdCHF and four-centered elimination of HF to give HCtCF were observed. These were attributed to differences in the exit barriers and transition states for the two types of elimination. This is the first reported study of the three-and four-centered H 2 elimination pathways producing :CdCF 2 and FCtCF, respectively. Both reactions showed the presence of a small exit barrier. This work also gives the first detailed description of the H and F atomic elimination channels. The P(E T ) for primary H atom elimination indicates a simple bond rupture mechanism; the P(E T ) for secondary H atom elimination suggests that triplet product is formed. The P(E T )'s for F atom elimination indicate that • CHdCHF is more stable than • CFdCH 2 . Where appropriate, comparisons of the various DFE and ethylene photodissociation channels were made.
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