Using torsional forces to explain the gradient temperature Raman spectra of endosulfan isomers and its irreversible isomerization

Authors: Schmidt, Walter; Hapeman, Cathleen; MCCONNELL, LAURA - University Of Maryland; Mookherji, Swati; Rice, Clifford; Nguyen, Julie; Qin, Jianwei - Tony Qin; Chao, Kuanglin - Kevin Chao; Kim, Moon; BROADHURST, CATHERINE - University Of Maryland; Shelton, Daniel

Submitted to: Journal of Molecular Spectroscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2017
Publication Date: 3/23/2017
Citation: Schmidt, W.F., Hapeman, C.J., McConnell, L., Mookherji, S., Rice, C., Nguyen, J.K., Qin, J., Chao, K., Kim, M.S., Broadhurst, C.L., Shelton, D.R. 2017. Using torsional forces to explain the gradient temperature Raman spectra of endosulfan isomers and its irreversible isomerization. Journal of Molecular Spectroscopy. 1139:43-51.

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 persistence, bioaccumulation, long-range transport in the atmosphere, and adverse effects to aquatic ecosystems and to human health. In 2016, the last uses of endosulfan were phased out in the United States, but due to its persistence in soil and its long-range transport, it will remain a global issue for many more years. 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 was thought to degrade faster than alpha-endosulfan. Thus, experiments using Gradient Temperature Raman Spectroscopy (GTRS) and chemical calculations were carried out to explain this conversion and to document the actual changes and movements of the bonds involved when beta-endosulfan changes to alpha-endosulfan. In the current analysis, other forces (torsional) were considered and provided additional evidence to the irreversibility of the conversion. These results explain why beta-endosulfan is rarely found in the atmosphere and why alpha-endosulfan is easily transported to remote areas, such as the Arctic Circle. This investigation findings provides insightful information to environmental regulators and researchers.

Technical Abstract: Since the 1950's, the broad-spectrum, organochlorine insecticide endosulfan (6,7,8, 9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepine-3-oxide) has been used on numerous crops. Due to its persistence, bioaccumulation, long-range transport, and adverse effects to human health and ecosystems, it was officially identified as a persistent organic pollutant (POP) in 2011. The last uses in the United States were phased out 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. In addition, the beta-isomer was found to isomerize to the alpha-isomer. Previous studies using Gradient Temperature Raman Spectroscopy (GTRS) and chemical calculations afforded evidence for specific bond movements and the irreversibility of the isomerization mechanism. However, not all of the peaks observed in the spectra could be explained. Thus, new analyses of the GTRS data were conducted to examine the effects of torsional forces on the bond movement, which allowed for the identification of all the peaks. These newly-identified torsional forces provide further confirmation of the isomerization mechanism and its irreversibility. Finally, this isomerization explains why beta-endosulfan is rarely detected in the atmosphere.