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, α and β; α-endosulfan exists as two asymmetrical, twist-chair enantiomers which interchange, while β-endosulfan has a symmetrical-chair conformation. β-Endosulfan has been shown to isomerize to α-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 β to α. These movements cannot occur simultaneously nor symmetrically, precluding conversion of α-endosulfan to β-endosulfan.