One-electron reduction of substituted chlorinated methanes as determined from ab initio electronic structure theory

Substituted chloromethyl radicals and anions are potential intermediates in the reduction of substituted chlorinated methanes (CH_xCl_3-xL, with L^- = F^-, OH^-, SH^-, NO₃^-, HCO₃^- and x = 0-3). Thermochemical properties, ΔH_f^°(298.15 K), S^°(298.15 K,1 bar), and ΔGs(298.15 K, 1 bar), were calculated by using ab initio electronic structure methods for the substituted chloromethyl radicals and anions: CH_yCl_2-yL· and CH_yCl_2-yL^-, for y = 0-2. In addition, thermochemical properties were calculated for the aldehyde, ClHCO, and the gem-chlorohydrin anions, CCl₃O^-, CHCl₂O^-, and CH₂ClO^-. The thermochemical properties of these additional compounds were calculated because the nitrate-substituted compounds, CH_yCl_2-y(NO₃)· and CH_yCl_2-y(NO₃)^-, were not stable, with all levels of ab initio theory leading to highly dissociated complexes. On the basis of these thermochemical estimates, the overall reaction energetics (in the gas phase and aqueous phase) for several mechanisms of the first electron reduction of the substituted chlorinated methanes were predicted. In almost all of the cases, the thermodynamically most favorable pathway resulted in loss of Cl^-. The exception was for the reduction of the nitrate-substituted chlorinated methanes CH_xCl_3-x(NO₃). On reduction, these compounds were shown to readily decompose into a Cl^- anion, NO₂· gas, and an aldehyde. In addition, the results of this study suggest that a higher degree of chlorination corresponds to a more favorable reduction. Relative to the nonsubstituted chlorinated methanes, the thermodynamic results suggest the CH_xCl_3-xF, CH_xCl_3-xOH, and CH_xCl_3-x(HCO₃) compounds are moderately more difficult to reduce, the CH_xCl_3-xSH compounds are moderately less difficult to reduce, and the CH_xCl_3-x(NO₃) compounds are substantially more favorable to reduce. These results demonstrate that ab initio electronic structure methods can be used to calculate the reduction potentials of organic compounds to help identify the potentially important environmental degradation reactions.