Atmospheric degradation mechanisms of a simulant organophosphorus pesticide isopropyl methyl methylphosphonate: A theoretical consideration

Abstract

[a] Calculations using density functional theory were performed to explore the mechanisms for atmospheric degradation of isopropyl methyl methylphosphonate (IMMP). The potential energy surface profiles for OH-initiated reaction of IMMP were constructed, and all possible degradation channels were considered. Rate constants were further calculated using transition state theory. It was established from these calculations that H-abstractions from alkyl groups have much lower energy barriers than substitutions of alkoxyl groups, and four possible H-abstraction channels are competitive.Investigations into the secondary reactions under the presence of O 2 /NO were also performed. It is shown that O 2 addition, reaction of peroxide radicals with NO to form RO radicals, and removal of ÁRO are the major degradation pathways for alkyl radicals. Four selected products, CH 3 OP(O)(CH 3 )OC(O)CH 3 , CH 3 OP(O)(O)CH 3 , (CH 3 ) 2 CHOP(O)(CH 3 )OH, and (CH 3 ) 2 CHOP(O) (CH 3 )OCH¼ ¼O, are predicted to be the major products in this study.

Oxidative Destruction of Multilayer Diisopropyl Methylphosphonate Films by O(3P) Atomic Oxygen

“…This assertion is reinforced by experimental evidence indicating that reactions between O­( 3 P) and organic or hydrocarbon-based films proceed largely through such abstraction mechanisms. − Second, and perhaps more importantly, while the initial abstraction by O­( 3 P) may be energetically limiting or rate-limiting, once a hydrogen has been abstracted, hydroxyl radicals are produced. These hydroxyl radicals can continue reacting within the film, and the barriers for hydrogen abstraction from DIMP by hydroxyl radicals are significantly lower. , One theoretical study concluded that abstraction from all three possible sites by hydroxyl radicals is facile under atmospheric conditions, with barriers at or below 2kT for our system . These low-barrier, secondary abstractions help explain why all of the DIMP C–H stretching peaks are observed to decrease concurrently despite the disparity in barriers for direct abstraction by O­( 3 P).…”

“…The fact that all three of these modes are observed to decrease in intensity indicates that the system has enough energy to overcome the barriers for reaction at each of the C–H sites visible in the RAIR spectra (P–CH 3 and C–CH 3 ). The tertiary C–H stretch of the isopropoxy group is unresolved in our spectra, but theoretical barriers for hydrogen abstraction are comparable for both possible sites on the isopropoxy groups, so the reaction is likely occurring at the tertiary site as well. ,, …”

“…This work is, in part, an extension of previous research from our lab examining the oxidative destruction of dimethyl methylphosphonate (DMMP) . The mechanistic conclusions presented here for the oxidative reactions of both simulants are additionally informed by theoretical investigations of the interaction between organophosphates and reactive oxygen species. ,− The isopropyl groups in DIMP provide an additional structural similarity with the nerve agent Sarin as well as the possibility of observing alkyl reaction channels unseen in DMMP . Through the use of in situ reflection–absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS), we are able to conclusively demonstrate oxidative destruction between adsorbed DIMP and O­( 3 P) under ultrahigh vacuum (UHV) conditions, and compare that to the analogous reaction with DMMP.…”

“…Understanding the oxidative destruction of organophosphates is increasingly essential for national defense and environmental protection. , Organophosphonate nerve agents like Soman and Sarin remain a significant threat in modern warfare, despite the multinational treaty aimed at preventing their creation and stockpiling . Additionally, less toxic organophosphates are widely used as pesticides and fire-retardant additives. , Whether combating dangerous exposure to nerve agents, modeling the environmental impact of pest control measures, or exploring the efficacy of flame retardant coatings, it is critical to understand both the atmospheric and condensed phase destruction pathways, persistence, and dispersal of these compounds. − …”

“…In studies of both gas phase and surface adsorbed organophosphate reactivity, it has become clear that oxygen plays a crucial role in the effective destruction of these compounds. Under atmospheric conditions, for example, it is widely accepted that reaction with hydroxyl radicals is the dominant loss process for organophosphates. ,− The surface-induced, photocatalytic, or thermal destruction of these compounds has also been extensively studied on metals, − metal oxides, − and nanoparticle assemblies. − While metal oxides in particular are often successful in decomposing organophosphates through coordination to surface-bound hydroxyl groups, many are limited by the number of active sites and significantly decreased activity upon repeat cycles of adsorption and reaction. Recently, these limitations were partially alleviated by the addition of ozone to the reaction, , which again highlights the importance of oxygen in the reliable destruction of these compounds.…”