jp9923163_si_001.pdf (290.14 kB)
The Free Energies of Reactions of Chlorinated Methanes with Aqueous Monovalent Anions: Application of Ab Initio Electronic Structure Theory
journal contribution
posted on 1999-12-22, 00:00 authored by Eric J. Bylaska, David A. Dixon, Andrew R. FelmyThe presence of different anionic species in natural waters can significantly alter the degradation rates of
chlorinated methanes and other organic compounds. Favorable reaction energetics is a necessary feature of
these nucleophilic substitution reactions that can result in the degradation of the chlorinated methanes. In this
study, ab initio electronic structure theory is used to evaluate the free energies of reaction of a series of
monovalent anionic species (OH-, SH-, NO3-, HCO3-, HSO3-, HSO4-, H2PO4-, and F-) that can occur in
natural waters with the chlorinated methanes, CCl4, CCl3H, CCl2H2, and CClH3. The results of this investigation
show that nucleophilic substitution reactions of OH-, SH-, HCO3-, and F- are significantly exothermic for
chlorine displacement, NO3- reactions are slightly exothermic to thermoneutral, HSO3- reactions are slightly
endothermic to thermoneutral and HSO4-, and H2PO4- reactions are significantly endothermic. In the case of
OH-, SH-, and F- where there are limited experimental data, these results agree well with experiment. The
results for HCO3- are potentially important given the near ubiquitous occurrence of carbonate species in
natural waters. The calculations reveal that the degree of chlorination, with the exception of substitution of
OH-, does not have a large effect on the Gibbs free energies of the substitution reactions. These results
demonstrate that ab initio electronic structure methods can be used to calculate the reaction energetics of a
potentially large number of organic compounds with other aqueous species in natural waters and can be used
to help identify the potentially important environmental degradation reactions.