Predicting aqueous speciation of environmentally relevant halogen oxidants: chloramines, bromamines, and bromochloramines

Chloramines, bromamines, and bromochloramines are halogen-containing oxidants that arise from the reaction between ammonia and hypohalous acids during the disinfection of drinking and wastewaters. Although involved in the formation of toxic disinfection byproducts, these molecules are difficult to study by experimental means, and their speciation remains poorly defined. To bridge this gap, we successfully designed a benchmark level computational method, called TA14, adapted from the Weizmann theory and the Feller-Peterson-Dixon procedures, able to establish the thermodynamics of these species in gas phase. Our composite method combines a systematic sequence of wavefunction theory calculations (up to CCSDTQ), relativistic effects, diagonal Born-Oppenheimer corrections, and anharmonic vibrational modes. Free energies of aqueous solvation were modeled with the SMD implicit solvation model together with the cluster-continuum approach developed by Bryantsev in 2008. This combined solvation model allowed us to evaluate the effects of aqueous solvation on halamine formation and decomposition. These gas phase and solution phase thermodynamic data are used to predict aqueous equilibrium constants describing the formation reactions of chloramines, bromamines, and bromochloramines. Based on the comparisons with the available experimental data, we propose uncertainties ranging from 1 to 2 logarithm units for computed aqueous equilibrium constants. These newly reported estimates of reaction thermodynamics will enable us to assess the formation rate constants of these reactive halogenated species, leading to more accurate predictions of speciation of halamines during water treatment.