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

摘要

Endosulfan (6,7,8, 9,10,10-hexachloro-l,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, a 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 a. These movements cannot occur simultaneously nor symmetrically, precluding conversion of a-endosulfan to β-endosulfan. Kendosulfan (6,7,8,9,10,10-hexachloro-l,S,Sa,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepine-3-oxide);; 1,3-dioxepane;; temperature-dependent Raman (TDR) spectroscopy;; 2D-TDR spectra;; Raman ring-bending frequency;; temperature-dependent conformational changes