Chlorination pattern effect on thermodynamic parameters and environmental degradability for C10-SCCPs: Quantum chemical calculation based on virtual combinational library

Abstract

Short-chain chlorinated paraffins (SCCPs) are still controversial candidates for inclusion in the Stockholm Convention. The inherent mixture nature of SCCPs makes it rather difficult to explore their environmental behaviors. A virtual molecule library of 42,720 C10-SCCP congeners covering the full structure spectrum was constructed. We explored the structural effects on the thermodynamic parameters and environmental degradability of C10-SCCPs through semi-empirical quantum chemical calculations. The thermodynamic properties were acquired using the AM1 method, and frontier molecular orbital analysis was carried out to obtain the EHOMO, ELUMO and ELUMO–EHOMO for degradability exploration at the same level. The influence of the chlorination degree (NCl) on the relative stability and environmental degradation was elucidated. A novel structural descriptor, μ, was proposed to measure the dispersion of the chlorine atoms within a molecule. There were significant correlations between thermodynamic values and NCl, while the reported NCl-dependent pollution profile of C10-SCCPs in environmental samples was basically consistent with the predicted order of formation stability of C10-SCCP congeners. In addition, isomers with large μ showed higher relative stability than those with small μ. This could be further verified by the relationship between μ and the reactivity of nucleophilic substitution and OH attack respectively. The C10-SCCP congeners with less Cl substitution and lower dispersion degree are susceptible to environmental degradation via nucleophilic substitution and hydroxyl radical attack, while direct photolysis of C10-SCCP congeners cannot readily occur due to the large ELUMO–EHOMO values. The chlorination effect and the conclusions were further checked with appropriate density functional theory (DFT) calculations.