Thermodynamic, Spectroscopic, and Computational Evidence for the Irreversible Conversion of β- to α-Endosulfan

Previous studies have provided unequivocal evidence for the symmetry of β-endosulfan and the corresponding asymmetry of α-endosulfan; the conversion of β-endosulfan to α-endosulfan was identified. In this study, evidence from differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) experiments combined with computational chemistry calculations was used to propose a molecular mechanism for the corresponding conformational changes that occur in this process. DSC and NMR data of mixtures indicated that both isomers can influence the conformer populations in the solid, solution, and vapor phase. Computational chemistry demonstrated that the relative SO configuration between α- and β-isomers can be the intermediate state through which the conformations of α- and β-isomers affect each other. Furthermore, calculations for mixtures indicated that the asymmetrical conformation of the sulfite in α-endosulfan can induce asymmetry in β-endosulfan, and conversion to α-endosulfan occurs from this transition state. Keywords: α- and β-endosulfan; conformation; NMR; volatility; eutectic mixture; computational chemistry