Are Chlorine Isotopologues of Polychlorinated Organic Compounds Exactly Binomially Distributed? A Theoretical Study and Implications to Experiments

Abstract

Chlorine isotopologues of polychlorinated organic compounds are usually recognized to follow binomial distribution. Is this recognition exactly true? This study presents a solid theoretical derivation to prove whether the isotopologue distributions of organochlorines are binomial or not, and investigates the implications of the distributions to relevant experimental studies. During synthetic reactions, the C-Cl bonds with stronger strengths are more readily to be formed with heavy isotopes. The chlorine KIEs are higher during the breakages of the stronger C-Cl bonds than during the breaking of the weaker ones. For a synthetic organochlorine, if the rate-limiting step during a chlorination reaction is the formation of C-Cl bonds, then chlorine KIE takes effect in the reaction, resulting in always higher chlorine isotope ratio on the reaction position than that of the initial chlorine atoms from the chlorine source. Chlorine KIEs vary during different chlorination reactions, leading to inconsistent chlorine isotope ratios on different reaction positions of asymmetric reaction intermediates and final products. If the rate-determining step is the generation of reactive chlorine atoms and the initial isotope ratio of the chlorine source is constant, then the chlorine isotope ratios on the reaction positions of all intermediates and final products are equivalent, demonstrating that the isotopologues follow binomial distribution. After physical changes and dechlorination reactions, organochlorines in the environment are unlikely binomially distributed. Experimental data show that the detected isotopologues of all the investigated organochlorines disobeyed binomial distribution. This study may bring a new perspective about the chlorine isotopologue distributions of organochlorines, and the proposed solutions may help to obtain better data for quantification and CSIA-Cl of organochlorines.