Reaction Mechanism of Nerve-Agent Hydrolysis with the Cs₈Nb₆O₁₉ Lindqvist Hexaniobate Catalyst

Abstract

We present a detailed mechanism for the hydrolysis of Sarin catalyzed by Cs 8 Nb 6 O 19 obtained using electronic structure calculations. The initial steps of the reaction involve the adsorption of water and Sarin on the hexaniobate catalyst via nonbonding interactions. Dissociation of the coordinated water molecule generates a hydroxide ion that adds nucleophilically to the coadsorbed Sarin molecule in a concerted manner, following a general base catalysis mechanism. The addition of OH − to the nerve agent generates a trigonal bipyramidal pentacoordinated phosphorus intermediate that subsequently undergoes facile dissociation forming either HF or isopropanol and a corresponding phosphonic acid. The rate-determining step of the overall reaction is found to be the dissociation of water on the catalyst in concert with the nucleophilic addition of the nascent OH − to the nerve agent. The calculated barrier for this step is considerably smaller than that measured for bulk base hydrolysis. This work represents a blueprint for future studies aimed to optimize catalysts for base hydrolysis of nerve agents at the gas−surface interface.

Atomic-Level Structural Dynamics of Polyoxoniobates during DMMP Decomposition

“…As such, they are attractive candidates for OP decontamination, as noted in two recent studies that have used polymeric niobates 20, 21 . The experimental work with Cs 8 [Nb 6 O 19 ] 21 has also been recently augmented by a theoretical study of the reaction mechanism at the gas-surface interface involving Sarin 25 . While previous experiments appear to support the prediction that the mechanism follows general base hydrolysis, many atomistic details of the reaction, including the nature of the active sites, their dynamic changes during reaction, and the characterization of intermediates or products, have yet to be experimentally investigated.…”

Impact of ambient gases on the mechanism of [Cs8Nb6O19]-promoted nerve-agent decomposition

“…As we have shown previously 24 and have briefly discussed above (see Scheme 1 ), Sarin hydrolysis by Cs 8 Nb 6 O 19 is a multistep process, with coordination of a water molecule to the catalyst as the first step. The calculations (see Table 1 ) show that coordination of H 2 O to Cs 8 Nb 6 O 19 to form Cs 8 Nb 6 O 19 /H 2 O is exothermic/exergonic by (presented as Δ H /Δ G ) 23.6/17.3 kcal mol –1 .…”

“…6 and below as O 5 and O 4 , respectively). 24 The additional stabilization of the complex arises from the long-range O 4 ···Cs 3 ionic interaction. Thus, for the purposes of modeling the decomposition of Sarin in the presence of carbon and sulfur dioxide, it is sufficient to examine the most energetically favorable pathway, similar to that previously reported for the case with no ambient gas molecules.…”

“…In our previous paper, we showed that this intermediate exhibits multiple isomeric forms. 24 Here, we discuss only the energetically most favorable form, which is directly connected to the transition state TS-F_X. For example, as seen in Fig.…”

“… 12 Our subsequent computational study on the mechanism of decomposition of GB by a Cs-salt of PONb, Cs 8 Nb 6 O 19 (or CsPONb), confirmed the general base hydrolysis mechanism of this reaction at the gas–surface interface. 24 …”