Essentials of Computational Chemistry

422 11 IMPLICIT MODELS FOR CONDENSED PHASES
different from the solvent is represented in both the electrostatic and non-electrostatic terms
(Spassov, Yan, and Szalma 2002).
Still faster timescales are associated with phenomena like electron transfer (i.e., redox
reactions) and photon absorption/emission and possible associated electronic excitation.
Since these processes occur on the timescale of electronic motion, the surrounding solvent
molecules may be regarded as frozen in place during the reaction, and clearly an equilibrium
view of the instantaneous solvation is incorrect.
These issues will be addressed in more detail in Chapters 14 and 15. Here, we will simply
note that successful extensions of continuum models to such ultrafast processes as electron
transfer and photoexcitation requires that the response properties of the solvent be explicitly
separated into slow and fast parts that interact with the solute over the appropriate timescales
(see, for instance, Cossi and Barone 2000). This approach can also be used for dealing with
non-equilibrium effects on transition states. However, unless there is a very sudden transfer
of charge that takes place over a significant distance failure to account for non-equilibrium
effects rarely has much consequence in estimating a free energy of activation – errors in
the gas-phase potential energy difference between minima and the TS structure are typically
larger in magnitude. Thus, unmodified continuum solvation models can still be quite useful in
constructing diagrams like that shown in Figure 11.4 for the purpose of describing reactivity
in solution.
11.5 Case Study: Aqueous Reductive Dechlorination
of Hexachloroethane
Synopsis of Patterson, Cramer, and Truhlar (2001) ‘Reductive Dechlorination of Hexachloroethane
in the Environment. Mechanistic Studies via Computational Electrochemistry’.
Halogenated alkanes are very useful as solvents in a variety of industrial processes (at one
time they were the solvents of choice for the dry cleaning of clothes, for example). The scale
of their use is such that their accidental or deliberate discharge into the environment can
lead to long-term contamination problems. As is true for many environmental contaminants,
the molecule originally released may not be a particular danger from an environmental
perspective, but some product into which it is transformed may be considerably more
cause for concern.
An example is hexachloroethane (C2Cl6). In environmental aqueous phases, it typically
undergoes reductive dechlorination relatively rapidly. One product of this dechlorination,
produced in small amounts, is trichloroacetic acid (Cl3CCO2H), which is a regulated
carcinogen in the United States. The authors studied the mechanistic aspects of C2Cl6
reductive dechlorination with both methodological and chemical goals. The methodological
question involved identifying appropriate levels of theory for modeling the relevant
reactions, while the chemical questions to be addressed were associated with identifying the
relevant mechanistic pathways for reduction and any possible explanation for the generation
of Cl3CCO2H as a product.
Various data were available for comparison in order to identify adequate theoretical
levels for application. Table 11.5 illustrates the performance of two different levels of
theory, CCSD(T)/aug-cc-pVDZ//BPW91/aug-cc-pVDZ and BPW91/aug-cc-pVDZ, with