The Free Energies of Reactions of Chlorinated Methanes with Aqueous Monovalent
Anions: Application of Ab Initio Electronic Structure Theory
Eric J. Bylaska
David A. Dixon
Andrew R. Felmy
10.1021/jp9923163.s001
https://acs.figshare.com/articles/journal_contribution/The_Free_Energies_of_Reactions_of_Chlorinated_Methanes_with_Aqueous_Monovalent_Anions_Application_of_Ab_Initio_Electronic_Structure_Theory/3728202
The 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<sup>-</sup>, SH<sup>-</sup>, NO<sub>3</sub><sup>-</sup>, HCO<sub>3</sub><sup>-</sup>, HSO<sub>3</sub><sup>-</sup>, HSO<sub>4</sub><sup>-</sup>, H<sub>2</sub>PO<sub>4</sub><sup>-</sup>, and F<sup>-</sup>) that can occur in
natural waters with the chlorinated methanes, CCl<sub>4</sub>, CCl<sub>3</sub>H, CCl<sub>2</sub>H<sub>2</sub>, and CClH<sub>3</sub>. The results of this investigation
show that nucleophilic substitution reactions of OH<sup>-</sup>, SH<sup>-</sup>, HCO<sub>3</sub><sup>-</sup>, and F<sup>-</sup> are significantly exothermic for
chlorine displacement, NO<sub>3</sub><sup>-</sup> reactions are slightly exothermic to thermoneutral, HSO<sub>3</sub><sup>-</sup> reactions are slightly
endothermic to thermoneutral and HSO<sub>4</sub><sup>-</sup>, and H<sub>2</sub>PO<sub>4</sub><sup>-</sup> reactions are significantly endothermic. In the case of
OH<sup>-</sup>, SH<sup>-</sup>, and F<sup>-</sup> where there are limited experimental data, these results agree well with experiment. The
results for HCO<sub>3</sub><sup>-</sup> 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<sup>-</sup>, 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.
1999-12-22 00:00:00
OH
H 2 PO 4
HSO 3
HSO 4
CCl 2 H 2
CCl 3 H
HCO 3
nucleophilic substitution reactions
ab initio
Ab Initio Electronic Structure Theory
SH
species
Favorable reaction energetics
chlorinated methanes