Determination of the Surface Effects on Sarin Degradation

Abstract

While it is well established that the dominant degradation mechanism for the chemical warfare agent, sarin, is hydrolysis via an SN2 mechanism in aqueous environments, the same cannot be said when hydrophilic or hydrophobic surfaces are present. Utilizing first-principles molecular dynamics simulations based on density functional theory, potential degradation pathways for sarin were investigated in the vicinity of both hydrophilic and hydrophobic surfaces. In an idealized situation where sarin is within hydrogen-bonding distance to a hydrophilic surface, the most probable degradation occurred via an SN2 mechanism, but with a lowering of the reaction barrier (ΔΔG⧧) of ∼15 kcal/mol as compared with bulk aqueous SN2 decomposition. However, in the presence of an idealized hydrophobic surface, it was found that the degradation mechanism for sarin proceeded via an SN1-type mechanism where the phosphorus–fluorine bond breaks first, followed by a second step in which water acts as a nucleophile. The reaction barrier associated with such a reaction is ∼10 kcal/mol higher than the SN2 degradation path found in bulk aqueous environments. The results found here clearly demonstrate how surfaces can significantly alter sarin decomposition.