Temperature-Dependent Raman Spectroscopic Evidence of and Molecular Mechanism for Irreversible Isomerization of β-Endosulfan to α-Endosulfan

Endosulfan (6,7,8, 9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepine-3-oxide) is a broad-spectrum, organochlorine insecticide used on numerous crops since the 1950s. It is has been identified as a persistent organic pollutant (POP) due to its persistence, bioaccumulation, long-range transport, and adverse effects to human health and aquatic ecosystems; it will be phased out in the United States in 2016. Endosulfan consists of two diastereomers, alpha and beta; alpha-endosulfan exists as two asymmetrical, twist-chair enantiomers which interchange, while beta-endosulfan has a symmetrical-chair conformation. beta-Endosulfan has been shown to isomerize to a-endosulfan. Here we document the previously proposed isomerization mechanism using temperature-dependent Raman (TDR) spectroscopy. The bending frequencies in the fingerprint region were assigned to specific bonds. Changes in the signal intensity as a function of temperature were used to identify detailed ring movements and thus conversion of beta to alpha. These movements cannot occur simultaneously nor symmetrically, precluding conversion of alpha-endosulfan to beta-endosulfan.