Quantum chemical examination of the sequential halogen incorporation scheme for the modeling of speciation of I/Br/Cl-containing trihalomethanes.

The recently developed three-step ternary halogenation model interprets the incorporation of chlorine, bromine and iodine ions into natural organic matter (NOM) and formation of iodine-, bromine-, and chlorine-containing trihalomethanes (THMs) based on the competition of iodine, bromine and chlorine species at each node of the halogenation sequence. This competition is accounted for using the dimensionless ratios (denoted as ) of kinetic rates of reactions of the initial attack sites or halogenated intermediates with chlorine, bromine and iodine ions. However, correlations between the model predictions made and mechanistic aspects of the incorporation of halogen species need to be ascertained in more detail. In this study, quantum chemistry calculations were firstly used to probe the formation mechanism of ten species of Cl-/Br-/I- THMs. The HOMO energy (EHOMO) of each mono-, bi-, or trihalomethanes were calculated by B3LYP method in Gaussian 09 software. Linear correlations were found to exist between the logarithms of experimentally determined kinetic preference coefficients  reported in prior research and, on the other hand, differences of EHOMO values between brominated/iodinated and chlorinated halomethanes. One notable exception from this trend was that observed for the incorporation of iodine into mono- and di-iodinated intermediates. These observations confirm the three-step halogen incorporation sequence and the factor γ in the statistical model. The combined use of quantum chemistry calculations and the ternary sequential halogenation model provides a new insight into the microscopic nature of NOM-halogen interactions and the trends seen in the behavior of γ factors incorporated in the THM speciation models.