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

Polyoxoniobate catalyst, nerve agent decomposition, reaction mechanism, impact of ambient gases on the stability and reactivity of the polyoxoniobate.


Introduction
The design of materials that can rapidly, fully, and catalytically decontaminate chemical warfare agents (CWAs) and other toxic compounds is an increasingly active area of research and one that presents some questions in fundamental chemistry. 1-5 As suggested by enzymatic chemistry, some of the most effective strategies for CWA destruction involve catalyzed hydrolysis reactions. 4 Specically, it is well established that the P-X bonds (X ¼ F, CN, SR, etc.) of organophosphorus (OP) nerve agents rapidly inactivate acetylcholinesterase (a serine hydrolase), the enzyme that facilitates hydrolysis of the neurotransmitter acetylcholine in the nervous system. This inactivation occurs through rapid nucleophilic addition and irreversible binding of the serine OH to the phosphorus atom of the nerve agent. Thus, an atomistic/molecular level understanding of the hydrolysis of OP compounds by nucleophilic addition and other processes may lead to the development of more effective materials and catalysts for nerve agent decontamination. Ongoing research efforts have identied several organic and inorganic materials, including metal-organic frameworks (MOFs, especially UiO-66, NU-1000 and MOF-808), 5-10 polyoxometalates (POMs), [11][12][13][14] MOF/ POM hybrid materials, 15,16 zirconium hydroxide, 17 zeolites, 18 and organic polymers, 19 as effective OP hydrolysis materials.
Recently, the use of POMs to catalyze nerve agent decontamination has attracted wide attention, in part because POMs are molecular representations of metal oxides and are thus far more amenable than the latter to extensive synthetic compositional alteration and characterization at the molecular level. 20,21 Polyoxoniobates (PONbs), including [Nb 6 O 19 ] 8À , are effective OP nerve agent hydrolysis compounds because their high negative charge densities (negative charge per polyanion oxygen) render them highly basic and nucleophilic. Thus, it is not surprising that the synthesis and in-depth analysis of the structures and reactivities of various (alkali and organic) salts of PONbs continue to be the focus of extensive studies. 22, 23 These studies show that the structures and, consequently, the catalytic activities of these materials for nerve agent decontamination depend on many factors, including (but not limited to) the nature of the counter-cation, the pH of the solution, the aggregate state (powder or solid-state material) of the catalyst, the real-time environmental conditions, and the nature and concentration of ambient gas molecules.
Earlier research on Lindqvist hexaniobate alkali salts (M 8 Nb 6 O 19 , M ¼ Li, K, Cs) reported rapid hydrolysis of the OP agent Sarin (GB, propan-2-yl methylphosphonouoridate, see Scheme 1) both in aqueous solution and at the gas-surface interface. 11 Small-angle X-ray scattering (SAXS) measurements showed aggregation of the OP compounds on the polyoxoniobate (PONb), which led to the suggestion that the reaction follows a general base hydrolysis mechanism. 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), conrmed the general base hydrolysis mechanism of this reaction at the gas-surface interface. 24 Briey, we have found that GB degradation by Cs 8 Nb 6 O 19 includes the following elementary steps (see Scheme 1): (a) the adsorption of water and the nerve agent on the Cs 8 Nb 6 O 19 species, (b) concerted dissociation of the adsorbed water molecule on a basic oxygen atom of the polyoxoniobate and nucleophilic addition of the nascent OH group to the phosphorus center of the nerve agent, (c) rapid reorganization of the resulting pentacoordinated phosphorus intermediate by dissociation of either HF or isopropanol, and formation of POMbound isopropyl methyl phosphonic acid (i-MPA) or methyl phosphonouoridic acid (MPFA), respectively. The calculations showed that the phosphonic acids i-MPA and MPFA are strongly bound to the protonated [Cs 8 Nb 6 O 19 H] + -core through hydrogen bonds and electrostatic interactions with the Cs counter-ions, suggesting that full catalyst regeneration may require additional treatment and depends on the nature of the countercations as well as the real-time (ambient) experimental conditions. Although PONb catalysts have been shown to react with CWAs, the chemistry has yet to be characterized in the presence of ambient gases (for example, NO 2 , CO 2 and SO 2 ), which may affect the stabilities, structural motifs, and activities of the PONb catalysts. Because this issue is vital to the application of PONbs as decontamination catalysts in real conditions, this paper probes the impact of the common battleeld contaminants NO 2 , CO 2 and SO 2 on the structure, stability and decontamination activity of the exemplary PONb species Cs 8 Nb 6 O 19 . This study addresses in depth the effects of these ambient gases on the structures of the catalysts and the base hydrolysis mechanism for Sarin degradation using density functional theory (DFT) calculations and infrared (IR) spectroscopy.  Fig. 2, i) at 1659 cm À1 , 1290 cm À1 and 1229 cm À1 . Interestingly, we observed no feature in the infrared spectrum around 2300 cm À1 , which would be associated with a linear CO 2 molecule bound to the POM surface. The 1229 cm À1 and 1290 cm À1 bands are consistent with the IR-inactive symmetric stretch from the gas-phase (n 1 (C-O)), which becomes IR active upon binding to the POM. The 1659 cm À1 band is likely related to the antisymmetric n 3 (C-O) stretch of CO 2 /CO 3 (occurs at 2349 cm À1 in the gas-phase 25 and signicantly redshis upon adsorption). The presence of both the n 1 and n 3 bands in the infrared spectrum indicates a bent structure of the adsorbate at the surface. Upon evacuation of CO 2 from the chamber, the 1229 cm À1 spectroscopic feature diminishes, indicating that this band corresponds to the vibrational motion of weakly bound CO 2 species on the surface (Fig. 2, ii); however, the two other features (1659 and 1290 cm À1 ) persist until annealing the Cs 8 Nb 6 O 19 sample at 423 K (Fig. 2, iii). The elevated much more strongly, by nearly a factor of two, than CO 2 . However, both molecules clearly persist on the POM at ambient and well above ambient temperatures.

Results and discussion
Further calculations showed that single Cs 8 Nb 6 O 19 species can bind several CO 2 and SO 2 molecules. As seen in Table 2, where we have summarized the adsorption energies as a function of the number of molecules adsorbed at the six O t sites, there is a pronounced monotonic convergence of the electronic and enthalpy binding energies up to n ¼ 6. The free energies, on the other hand, reveal thermodynamic instability for CO 2 adsorption at larger values of n, suggesting that only the Cs 8 Nb 6 O 19 /(CO 2 ) n species with n # 4 are viable. The Cs 8 Nb 6 O 19 / (SO 2 ) n species may still be stable for n > 6.
A2. Structure of Cs 8 Nb 6 O 19 in the presence of NO 2 radicals. As one might expect, the interaction of Cs 8 Nb 6 O 19 with NO 2 radicals is conceptually different than those discussed above for the diamagnetic CO 2 and SO 2 molecules. Indeed, at rst, in the gas-phase and at low temperatures, NO 2 radicals are in equilibrium with dinitrogen tetraoxide, N 2 O 4 , while higher temperatures shi the equilibrium towards nitrogen dioxide. 26 The calculations show that N 2 O 4 is planar, with an N-N bond distance of 1.85Å, which is signicantly longer than the average N-N single bond length of 1.45Å; this species has a dimerization energy of DE/DH/DG ¼ 19. 8 Table 1). To summarize, a relatively weak coordination of the rst NO 2 radical to Cs 8 Nb 6 O 19 at the Cs sites promotes stronger coordination of the second NO 2 molecule at the O t site of Cs 8 Nb 6 O 19 . As a result, the coordination of the NO 2 radicals to Cs 8 Nb 6 O 19 is substantially stronger than that of CO 2 , yet slightly weaker than that of one SO 2 molecule.
The reported highly stable Cs 8 Nb 6 O 19 /[NO 2 (Cs N )NO 2 (O t )] complex with two NO 2 fragments can also be formed via N-N bond activation of the coordinated N 2 O 4 molecule by the polyoxometalate catalyst. The former pathway (i.e. stepwise addition of two NO 2 radicals to polyoxoniobate) may be valid at ambient temperature, while the latter process may occur at low temperature. In this paper, we did not study the N-N activation barrier; however, we found that the complex Cs 8  )], the coordination of the NO 2 radical positioned at O t to Cs 8 Nb 6 O 19 is substantially stronger than that of CO 2 , yet is slightly weaker than that of one SO 2 molecule.
These conclusions from our computations are fully supported by experiments. As with CO 2 , the adsorption of NO 2 onto Cs 8 Nb 6 O 19 was probed with the use of infrared spectroscopy. Fig. 4 shows spectra for the interaction between NO 2 and Cs 8 Nb 6 O 19 . The adsorption of NO 2 on the surface resulted in two infrared vibrational featuresone at 1668 cm À1 and another at 1240 cm À1 (Fig. 4, i). These features are attributed to adsorbate N-O stretches as opposed to POM motions, which appear below 1000 cm À1 . Analysis of the calculated [Cs 8 Nb 6 O 19 ]-NO 2 complexes suggests that the 1668 cm À1 feature is likely due to an asymmetric NO stretch originating from the Cs 8 Nb 6 O 19 /N 2 O 4 complex, which the calculations (unscaled) show to be at 1813 cm À1 . For the other NO 2 complexes, the highest frequency NO stretch is found below 1600 cm À1 , i.e. at a lower frequency relative to the experimental peak. Taking into account the usual anharmonic correction, these modes will be found to be even further redshied to lower frequencies. However, if we consider that the calculated asymmetric (strongly IR active) NO stretch of a free NO 2 radical is 1752 cm À1 , and the known experimental value 25 is 1618 cm À1 , the resulting frequency scale factor of 0.92 brings the asymmetric NO stretch of the Cs 8 Nb 6 O 19 /N 2 O 4 complex to exactly 1668 cm À1 . Moreover, if the measured 1668 and 1240 cm À1 peaks originate from the same thermal mixture of diamagnetic complexes, we expect that the 1240 cm À1 peak, which persists at higher temperatures (Fig. 4, iii), is due to the Cs 8   Thus, we present two spectra (shown in Fig. 5); one corresponds to low temperature, Fig. 5(A), which is the sum of Cs 8 Fig. 5(B), which is pure Cs 8 Nb 6 O 19 /[NO 2 (Cs N )NO 2 (O t )]. The frequency axis is scaled by the same factor of 0.92. The calculation is consistent with experiments with regard to the disappearance of the 1668 cm À1 peak and the persistence of the 1240 cm À1 peak. However, the peak at 1470 cm À1 (aer scaling) in the calculated spectra, which is attributed to a local NO stretch of the NO 3 unit in Cs 8 Nb 6 O 19 /[NO 2 (Cs N )NO 2 (O t )], is absent in the roomtemperature experiments. This suggests that the formation of NO 3 requires a signicant amount of thermal energy.
Furthermore, experiments clearly show that NO 2 is strongly bound to the POM, i.e. the 1240 cm À1 peak is present even aer gas phase evacuation (Fig. 4, ii) and upon heating the POM to 423 K (Fig. 4, iii). In fact, thermal treatment up to 600 K was required to fully desorb the species, i.e. to remove NO 2 radicals. This suggests that NO 2 binds more strongly to the POM than CO 2 , which is consistent with the computational data for the Cs 8 Nb 6 O 19 /[NO 2 (Cs N )NO 2 (O t )] complex presented above (see Table 1 and the ESI ‡).
Thus, the data presented above show that in the presence of ambient gas molecules of CO 2 , NO 2 and SO 2 , Cs 8 Nb 6 O 19 will absorb these molecules more strongly than the water and GB molecules required for hydrolysis of Sarin (see Table 1). This is expected to impact the hydrolysis of Sarin by Cs 8 Nb 6 O 19 in the following two ways: rst, because the ambient gas molecules coordinate to catalytically active O t -centers, they block these catalytically active centers, hindering water and Sarin coordination, and may alter the previously reported mechanism of Sarin hydrolysis by Cs 8 Nb 6 O 19 . Second, the interaction of an ambient gas molecule with Cs 8 Nb 6 O 19 may change the electronic properties of the polyoxoniobate: this is expected to only impact the calculated energetics of the Sarin hydrolysis and to not signicantly change the nature of the previously reported intermediates and transition state structures.
Below, we test the rst hypothesis by (a) studying the full potential energy surfaces of GB hydrolysis by Cs 8 Nb 6 O 19 /X, where X ¼ CO 2 and SO 2 , and (b) comparing these new ndings with our previous results on the same reaction in the absence of ambient gas molecules. For the sake of simplicity, we discuss in detail only the reaction mechanism (as well as the structures of the pre-reaction complexes, intermediates, transition states and products) for X ¼ CO 2 and compare these ndings with those (previously reported) in the absence of ambient gas molecules. In addition, we briey discuss, where appropriate, our ndings for X ¼ SO 2 (full potential energy surfaces are available in the ESI ‡). The reactivities of the NO 2 and other radical species coordinated to Cs 8 Nb 6 O 19 will be reported elsewhere. Water molecule coordination to the adduct formed when CO 2 (or SO 2 ) binds to the O t -center of Cs 8 Nb 6 O 19 is a few kcal mol À1 less than that for "free" Cs 8 Nb 6 O 19 ; the energies are À22.4/À11.9 and À21.0/À9.7 kcal mol À1 for X ¼ CO 2 and SO 2 , respectively. This effect is more pronounced for the Cs 8 Nb 6 O 19 /SO 2 adduct than for Cs 8 Nb 6 O 19 /CO 2 . Furthermore, as seen in Fig. 6 Fig. 6 and below as O 5 and O 4 , respectively). 24 The additional stabilization of the complex arises from the longrange 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 keeping with the previously established shorthand notation, the pre-reaction complex of this reaction pathway is labeled as R-F_X.

B. Hydrolysis of Sarin by
The present calculations show that the coordination of Sarin (GB) to Cs 8  In the next step, hydrolysis of the coordinated water molecule occurs between the coordinated gas molecule X, the bridging oxygen (O m ) of the Cs 8 Nb 6 O 19 -core and the phosphorus center of Sarin. The transition state associated with this process, TS-F_CO 2 , is shown in Fig. 6 (for TS-F_SO 2 , see the ESI ‡). As seen in this gure, at TS-F_CO 2 , the breaking O 3 -H 1 bond of the water molecule is elongated to 1.16Å, and the forming O m -H 1 bond distance becomes 1.25Å. In addition, Nb 1 -O m and Nb 2 -O m bonds are slightly elongated and the Cs 3 -O 4 bond is slightly shortened. Importantly, the Cs 3 -center of the Cs 8 Nb 6 O 19 core also interacts with the oxygen (O 3 ) of water and provides additional support for hydrolysis. Here, the other coordinates of interest are the P-O 3 (H 2 O) bond and the P-F bond.
As seen in Fig. 6, P-O 3 (H 2 O) undergoes a major reduction from 3.12Å in R-F_CO 2 to 1.96Å in TS-F_CO 2 . Its P-F counterpart, located trans to the water-activated molecule, extends from the typical single bond in R-F_CO 2 , with an increase from 1.62Å to 1.71Å in TS-F_CO 2 . Similar geometry changes at the hydrolysis transition state were observed for X ¼ SO 2 (see the ESI ‡). As seen in Fig. 7, the calculated DE/DG barrier heights relative to R-F_X are 7.8/7.5 and 8.4/8.8 kcal mol À1 for X ¼ CO 2 and SO 2 , respectively. These values are slightly larger than the values of 6.8/6.1 kcal mol À1 calculated for the reaction in the absence of these ambient gas molecules; this suggests that common battleeld contaminants may impair the hydrolytic decomposition of nerve agents under operational conditions. The hydrolysis product is a pentacoordinated-phosphorus complex P5-F_X with a trigonal bipyramidal structure around the central phosphorus atom. 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. 6, the formed P-O 3 bond in P5-F_CO 2 contracts to 1.79Å, while its P-F counterpart, located trans to the activated water molecule, extends to 1.76Å. The broken O 3 -H 1 bond is elongated to 1.64Å and the Nb 1 -O m and Nb 2 -O m bonds are elongated to 2.20 and 2.13Å, respectively. Concurrently, the Cs 3 -O 3 bond of 3.11Å is formed to provide additional stabilization to the pentacoordinated-phosphorus complex, similar to that previously reported in P5-F. 24 Based on the calculated Mulliken charge distribution, the resulting P5-F_X complexes can be labeled as a [(GBOH À )-(Cs 8 Nb 6 O 19 H/X) + ] ion-pair system.
As shown in Fig. 7, the hydrolysis of Sarin, i.e. the reaction Cs 8 Nb 6 O 19 /X + H 2 O + GB / R-F_X / TS-F_X / P5-F_X for X ¼ none, CO 2 or SO 2 , is exergonic by À50.1/À27.9, À38.8/À11.1 and À40.5/À14.7 kcal mol À1 (presented as DH/DG) and proceeds over energy barriers of 6.8/6.1, 7.5/7.5 and 8.4/8.8 kcal mol À1 (calculated relative to the pre-reaction intermediate R-F_X), respectively. The reaction R-F_X / TS-F_X / P5-F_X is exothermic for X ¼ none and SO 2 by 6.8/6.4 and 0.4/ 1.2 kcal mol À1 , respectively, but is endothermic by 3.4/ 3.2 kcal mol À1 for X ¼ CO 2 . These energy values allow us to conclude that the presence of ambient gas molecules increases the energies of the stationary points relative to the asymptote of the reactants. This is likely the result of a different charge distribution on Cs 8 Nb 6 O 19 /X relative to Cs 8 Nb 6 O 19 , where X acquires a net negative charge of 0.8 to 0.7 |e|, as we showed above. Furthermore, X coordinates to the O t reactive center and disables its hydrolytic activity. The presence of ambient gases increases the hydrolysis barrier by 1.0 to 2.5 kcal mol À1 , and this change in the energy barrier is closely associated with the Cs 8 Nb 6 O 19 -X complexation energy: the stronger the Cs 8 Nb 6 O 19 -X bond, the higher the barrier for Sarin hydrolysis.

C. Pentacoordinated P5-F_X intermediate dissociation
Once the pentacoordinated P5-F_X species is formed, the reaction can proceed along several paths, as discussed in our previous paper. 24 In order to evaluate the role of the catalyst in the course of the reaction, as above, here we discuss in detail only those pathways that directly involve the catalyst. These are the HF and isopropanol elimination and, ultimately, desorption pathways. The accompanying products of these paths are isopropyl methyl phosphonic acid (i-MPA) and methyl phospho-nouoridic acid (MPFA), respectively. It is evident that in order to form HF or isopropanol, protonation of the uoride or oxygen centers of the isopropoxy ligand is required. Furthermore, in order to facilitate regeneration of the catalyst, the ultimate proton source should be the O m H 1 -group of the [Cs 8 Nb 6 O 19 H/X] + cation. However, these processes are expected to be very complex and may proceed via multiple mechanisms. One of these processes could involve any surrounding water molecules, which are expected to be present in real experimental conditions. In this mechanism, a water molecule located close to the uoride or oxygen atoms of the iopropoxy ligand is expected to donate its proton to these groups (to form HF and/or isopropanol, respectively) and compensate by removing the proton from the O m H 1 -group of the catalyst via a H-bonding network. This process depends on multiple factors (including, but not limited to, the concentration of water in the system and the reaction temperature) and was not studied in this paper.
Another possible mechanism of HF and/or isopropanol formation is direct removal of the proton from the O m H 1 -group of the catalyst by the uoride and/or isopropoxide ligands, respectively. As shown previously for the Cs 8 Nb 6 O 19 catalyst (i.e., in the absence of ambient gas molecules), 24 these processes occur with very small energy barriers which have no contribution to the overall outcome of the decontamination reaction but lead to the most energetically stable intermediates, Cs 8 Nb 6 O 19 -(i-MPA)-HF and Cs 8 Nb 6 O 19 -(MPFA)-(i-POH), respectively (see Fig. 8). Here, we performed an extensive search to locate the transition states TS2-F_HF_X and TS2-F_(i-POH)_X that lead to either HF and i-MPA or isopropanol (i-POH) and MPFA from the most stable pentacoordinated intermediate P5-F_X (where X ¼ CO 2 or SO 2 ). Ultimately, we were able to locate only the TS2-F_(i-POH)_X transition state (see Fig. 8). The search for the HF formation transition state TS2-F_HF_X was unsuccessful and always led to either the Cs 8 Nb 6 O 19 /X-(i-MPA)-HF intermediate, its derivative F À /H + /O m intermediate, or the pre-reaction complex P5-F_X. For example, in Fig. 8, we present the intermediates, transition states and products involved in pentacoordinated P5-F_CO 2 intermediate dissociation alone, with their important geometry parameters (for those of X ¼ SO 2 , see the ESI ‡). The relative energies of these species, calculated from the P5-F_X pre-reaction complex, are given in Fig. 9.
As seen in Fig. 8, at the transition state TS2-F_(i-POH)_CO 2 associated with the formation of i-POH and MPFA, the activated P-O 5 (Osp 2 ) bond extends to 2.19Å from 1.68Å in P5-F_CO 2 , with simultaneous formation of a double H-bond network (O 5 -H 1 ¼ 1.63Å and O 1 -H 2 ¼ 1.57Å) as a precursor to the As seen in Fig. 9, the calculated energy barriers from P5-F_X are 7.0/7.6, 5.4/5.4 and 3.6/6.5 kcal mol À1 for X ¼ none, CO 2 and SO 2 . Thus, the presence of these gas molecules in the reaction mixture slightly reduces the pentacoordinated P5-F_X intermediate dissociation barrier (by 2 to 4 kcal mol À1 ). Furthermore, the changes in the i-POH and MPFA formation energy barriers correlate with the Cs 8

D. Catalyst regeneration
Thus, the facile dissociation of the P5_F_X species, as reported previously for P5_F, yields HF and i-MPA and/or i-POH and MPFA products. All these species are initially bound to Cs 8 Nb 6 O 19 , as shown in Fig. 8  As seen in Fig. 9, the formed Cs 8 Nb 6 O 19 /X-MPFA-(i-POH) is the energetically lowest structure on the potential energy surface of the entire GB hydrolysis and decontamination reaction by Cs 8 Nb 6 O 19 both in the absence and presence of ambient gas molecules. The intermediate Cs 8 Nb 6 O 19 /X-(i-MPA)-HF is found to be only slightly higher in energy. As we mentioned previously, 24 the high stability of these intermediates is due in part to the strong hydrogen bonds between the adsorbates and the Cs 8 Nb 6 O 19 /X-core; however, it is also due to the additional stabilizing interactions between the Cs counter-ions and the electronegative atoms of the nerve-agent fragments.
Desorption This step of the reaction, which forms a nal decontaminated form of the GB and re-generated catalyst, is found to be highly endothermic/endergonic, i.e. 6.4/6.5 and 2.8/1.9 kcal mol À1 for i-MPAH and MPFAH formation, respectively. Overall, the last steps of the reaction, i.e. the reactions Cs 8 Nb 6 O 19 /X-(i-MPA)-HF / Cs 8 Nb 6 O 19 /X + HF + (i-MPAH) and Cs 8 Nb 6 O 19 /X-MPFA-(i-POH) / Cs 8 Nb 6 O 19 /X + (i-POH) + MPFAH, are highly prohibitive and require energies of 80.3/60.2 (X ¼ none), 64.4/36.3 (X ¼ CO 2 ), and 66.0/39.4 (X ¼ SO 2 ) kcal mol À1 and 81.0/60.4 (X ¼ none), 71.2/43.6 (X ¼ CO 2 ), and 63.3/36.4 (X ¼ SO 2 ) kcal mol À1 , respectively. Furthermore, deprotonation of the [Cs 8 Nb 6 O 19 /X] H + -core and protonation of the phosphonic acids i-MPA and MPFA is expected to be very complex and may proceed via several pathways depending on the reaction conditions. One of these may involve any surrounding water molecules, in real experimental conditions, via a concerted protonationdeprotonation mechanism involving the hydrogen-bonded water-based network. However, this process depends on multiple factors (including, but not limited to, the concentration of water in the system and the reaction temperature) and was not studied in this paper. We also failed to locate a transition state associated with the direct deprotonation of [Cs 8 Nb 6 O 19 /X]H + and protonation of the phosphonic acids because of the high stability of the corresponding pre-reaction complexes Cs 8 Nb 6 O 19 /X-(i-MPA) and Cs 8 Nb 6 O 19 /X-MPFA. The solution to this issue requires special comprehensive experimental and computational studies, which are in progress.

Conclusions
This paper, for the rst time, addresses the impact of environmentally-signicant ambient gas molecules, NO 2 , CO 2 and SO 2 , on the structure, stability and decontamination activity of a basic polyoxometalate species. Specically, Cs 8 Nb 6 O 19 in the presence of these gases has been studied in depth by complementary computational and experimental approaches. It was found that: (1) Cs 8 Nb 6 O 19 absorbs ambient gas molecules of X ¼ CO 2 , NO 2 and SO 2 more strongly than it absorbs water or Sarin (GB) molecules. The calculated Cs 8 Nb 6 O 19 -X binding energy follows the trend for DG (X ¼ CO 2 ) < DG (NO 2 ) < DG (SO 2 ).
(2) The impacts of the diamagnetic CO 2 and SO 2 molecules on polyoxoniobate Cs 8 Nb 6 O 19 are fundamentally different than that of the NO 2 radical. At ambient temperatures, weak coordination of the rst NO 2 radical to Cs 8 Nb 6 O 19 confers partial radical character on the polyoxoniobate and promotes a stronger coordination of the second NO 2 radical to form a stable diamagnetic Cs 8 Nb 6 O 19 /(NO 2 ) 2 species; meanwhile, at low temperatures, NO 2 radicals form weakly stable dinitrogen tetraoxide (N 2 O 4 ), which interacts weakly with Cs 8 Nb 6 O 19 .
(3) Similar to the case without ambient gas molecules, reported previously, 24 in the presence of X, GB hydrolysis by Cs 8 Nb 6 O 19 /X proceeds via general base hydrolysis involving: (a) adsorption of water and the nerve agent on the Cs 8 Nb 6 O 19 /X catalyst, (b) concerted hydrolysis of the adsorbed water molecule on a basic oxygen atom of the polyoxoniobate and nucleophilic addition of the nascent OH group to the phosphorus center of the nerve agent, (c) rapid reorganization of the resulting pentacoordinated-phosphorus intermediate followed by dissociation of either HF or isopropanol with formation of POM-bound isopropyl methyl phosphonic acid (i-MPA) or methyl phosphonouoridic acid (MPFA), respectively.
(4) Cs 8 Nb 6 O 19 adsorbs ambient gas molecules X at its basic O t (or O m ) reactive centers, which shields them from involvement in the base hydrolysis. As a result, one of the O centers of the coordinated ambient gas molecule becomes an active hydrolysis center. This increases the energies of the stationary points relative to the asymptote of the reactants and increases the hydrolysis barrier. These changes are closely correlated with the Cs 8 Nb 6 O 19 -X complexation energy; the stronger the Cs 8 Nb 6 O 19 -X bond, the higher the barrier for Sarin hydrolysis.
(5) The most energetically stable products of the GB hydrolysis and decontamination reaction are Cs 8 Nb 6 O 19 /X-MPFA-(i-POH) and Cs 8 Nb 6 O 19 /X-(i-MPA)-HF both in the absence and presence of ambient gas molecules. The high stability of these intermediates is due in part to the strong hydrogen bonds between the adsorbates and the protonated [Cs 8 Nb 6 O 19 /X/H] +core and to interactions between the Cs counterions and the electronegative atoms of the adsorbates.
(6) Desorption of HF or/and (i-POH) and regeneration of the catalyst requires deprotonation of the [Cs 8 Nb 6 O 19 /X/H] + -core with protonation of the phosphonic acids i-MPA and MPFA. Regeneration of the catalyst is a highly endergonic process and is the rate-limiting step for GB hydrolytic decontamination, both in the absence and presence of ambient gas molecules.

Notes
A. Computational and experimental procedures A1. Computational methodology. A major computational challenge in the present work is to properly describe the noncovalent interactions involving the various ions in the systems studied: the Cs + counter-cations, [Nb 6 O 19 ] 8À with its large negative charge, and the H + and OH À ions resulting from heterolytic water dissociation. These interactions are expected to be well described by the M06-L density functional, 27 a pure density functional designed for transition metal bonding and non-covalent interactions. Therefore, all presented calculations have been carried out with the M06-L density functional, as implemented in the Gaussian09 code. 28 In these calculations, we used the 6-31++G(d,p) basis set for the elements S, P, F, O, C, H and the Lanl2dz basis set with corresponding Hay-Wadt effective core potentials for Nb and Cs, as implemented in Gaussian09. The sets of diffuse functions (++) were added specically to obtain proper descriptions of the diffuse charge densities and long-range interactions. All reported stationary points were conrmed to have either all real frequencies (minima) or one imaginary frequency (transition states). The latter were further veried to connect the corresponding minima by IRC calculations. All reported enthalpy and Gibbs free energies were computed at a temperature of 298.15 K and 1 atm pressure.
A2. Experimental methodology. The synthesis of Cs 8 Nb 6 -O 19 $14H 2 O followed a known literature procedure. A solution of cesium hydroxide (14.6 g, 50% by weight) was heated to 90 C in an Erlenmeyer ask. 5 g of hydrous, amorphous niobium oxide was added in small portions, with full dissolution of each portion before addition of the subsequent portion. Evaporative crystallization yielded giant hexagonal crystals.
Infrared spectroscopic experiments were performed in a stainless-steel high-vacuum chamber with a base pressure of $1 Â 10 À8 Torr. The Cs 8 Nb 6 O 19 sample was pressed, as a 7 mm diameter disk, into a tungsten grid, which was then clamped onto a sample mount coupled to a precision manipulator. An empty region of the grid was used to monitor the gas phase species in the chamber and was also employed as a background for surface adsorption and desorption studies. The grid was resistively heated, and the temperature was monitored via a K-type thermocouple spot-welded adjacent to the sample. A PID controller mediated the sample temperature to within AE1 K. Details of the vacuum chamber and sample mount can be found in a previous publication. 29 An FTIR spectrometer (Thermo, Nicolet, Nexus 470 FTIR) with an external liquid-N 2 -cooled MCT-A detector and a spectral resolution of 2 cm À1 was used for collection of the infrared spectra.