The Free Energies of Reactions of Chlorinated Methanes with Aqueous Monovalent Anions:  Application of Ab Initio Electronic Structure Theory

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.