Temperature dependent Raman Spectroscopic evidence of and molecular mechanism for thermal transfer between Beta-Endosulfan and Alpha-Endosulfan

Authors: SCHMIDT, WALTER - US Department Of Agriculture (USDA); Hapeman, Cathleen; McConnell, Laura; MOOKHERJI, SWATI - US Department Of Agriculture (USDA); NGUYEN, JULIE - US Department Of Agriculture (USDA); QIN, JIANWEI - US Department Of Agriculture (USDA); LEE, HOYOUNG - US Department Of Agriculture (USDA); CHAO, KEVIN - US Department Of Agriculture (USDA); KIM, MOON - US Department Of Agriculture (USDA)

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2014
Publication Date: 2/3/2014
Publication URL: http://handle.nal.usda.gov/10113/59807
Citation: Schmidt, W., Hapeman, C.J., McConnell, L.L., Mookherji, S., Nguyen, J.K., Qin, J., Lee, H., Chao, K., Kim, M. 2014. Temperature dependent Raman Spectroscopic evidence of and molecular mechanism for thermal transfer between Beta-Endosulfan and Alpha-Endosulfan. Journal of Agricultural and Food Chemistry. 62:2023-2030.

Interpretive Summary: Endosulfan is a broad-spectrum, organochlorine insecticide used on numerous crops since the 1950's. It is a persistent organic pollutant (POP) due to its stability, bioaccumulation, long-range transport in the atmosphere, and adverse effects to aquatic ecosystems and to human health. Endosulfan is scheduled to be phased out in 2016 in the United States, but due to its persistence in soil and its long-range transport, it will remain a global issue long after its use has ceased. Endosulfan is a mixture of two chemicals, alpha-endosulfan and beta-endosulfan, and they move in the environmental differently. In addition, earlier research has shown that beta-endosulfan can change into alpha-endosulfan. This was an important finding since beta-endosulfan is thought to degrade faster than alpha-endosulfan. Simulations experiments and chemical calculations were carried out to explain this conversion. In this study, a newly-developed technique called temperature-dependent Raman (TDR) spectroscopy was used to document the actual changes and movements of the bonds involved when beta-endosulfan changes to alpha-endosulfan. These experiments also provide the first tangible evidence for why alpha-endosulfan cannot convert to beta-endosulfan. These results are important because alpha-endosulfan volatilizes more easily than beta-endosulfan and it is easily transported to remote areas, such as the Arctic Circle.

Technical Abstract: 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 1950's. It 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 and will be phased-out in the United States in 2016. Endosulfan consists of two diastereomers, alpha and beta, and while the alpha-isomer exists as two asymmetrical, twist-chair enantiomers which interchange, the beta-isomer is a symmetrical-chair conformation. Furthermore, the beta-isomer has been observed to isomerize to the alpha-isomer. 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 at certain frequencies as a function of temperature were used to identify detailed ring movements, and thus the isomerization mechanism. These movements cannot occur simultaneously nor symmetrically, and thereby preclude conversion of the alpha-isomer to the beta-isomer. These results will be of interest to other scientists and regulatory agencies.