Impact of ambient gases on the mechanism of [Cs 8 Nb 6 O 19 ]-promoted nerve-agent decomposition.

The impact of ambient gas molecules (X), NO2 , CO2 and SO2 on the structure, stability and decontamination activity of Cs8 Nb6 O19 polyoxometalate was studied computationally and experimentally. It was found that Cs8 Nb6 O19 absorbs these molecules more strongly than it adsorbs water and Sarin (GB) and that these interactions hinder nerve agent decontamination. The impacts of diamagnetic CO2 and SO2 molecules on polyoxoniobate Cs8 Nb6 O19 were fundamentally different from that of NO2 radical. At ambient temperatures, weak coordination of the first NO2 radical to Cs8 Nb6 O19 conferred partial radical character on the polyoxoniobate and promoted stronger coordination of the second NO2 adsorbent to form a stable diamagnetic Cs8 Nb6 O19 /(NO2 )2 species. Moreover, at low temperatures, NO2 radicals formed stable dinitrogen tetraoxide (N2 O4 ) that weakly interacted with Cs8 Nb6 O19 . It was found that both in the absence and presence of ambient gas molecules, GB decontamination by the Cs8 Nb6 O19 species proceeds via general base hydrolysis involving: (a) the adsorption of water and the nerve agent on Cs8 Nb6 O19 /(X), (b) concerted hydrolysis of a water molecule on a basic oxygen atom of the polyoxoniobate and nucleophilic addition of the nascent OH group to the phosphorus center of Sarin, and (c) rapid reorganization of the formed pentacoordinated-phosphorus intermediate, followed by dissociation of either HF or isopropanol and formation of POM-bound isopropyl methyl phosphonic acid (i-MPA) or methyl phosphonofluoridic acid (MPFA), respectively. The presence of the ambient gas molecules increases the energy of the intermediate stationary points relative to the asymptote of the reactants and slightly increases the hydrolysis barrier. These changes closely correlate with the Cs8 Nb6 O19 -X complexation energy. The most energetically stable intermediates of the GB hydrolysis and decontamination reaction were found to be Cs8 Nb6 O19 /X-MPFA-(i-POH) and Cs8 Nb6 O19 /X-(i-MPA)-HF both in the absence and presence of ambient gas molecules. The high stability of these intermediates is due to, in part, the strong hydrogen bonding between the adsorbates and the protonated [Cs8 Nb6 O19 /X/H]+ -core. Desorption of HF or/and (i-POH) and regeneration of the catalyst required deprotonation of the [Cs8 Nb6 O19 /X/H]+ -core and protonation of the phosphonic acids i-MPA and MPFA. This catalyst regeneration is shown to be a highly endothermic process, which is the rate-limiting step of the GB hydrolysis and decontamination reaction both in the absence and presence of ambient gas molecules.