Paper SS1+AS+TF-ThM3
Ab-Initio Computational Approaches to the Adsorption of Chemical Warfare Agents

Thursday, November 12, 2009, 8:40 am, Room M

Computational chemistry can be used to great advantage in dealing with chemical warfare agents (CWAs). Computation can be applied to reagents which are much too dangerous for routine experimentation as well as to non-traditional species which have not yet been (but could be) synthesized. Examples will be given from recent work¹ on the interaction of the simulant dimethyl methyphosphonate (DMMP) and the real CWAs Sarin and VX with γ‑Al₂O₃ surfaces. The γ‑Al₂O₃ surface acts as a Lewis acid (electron acceptor) when OH-free and a Brønsted acid (proton donor) when hydroxylated. Thus γ‑Al₂O₃ is an excellent prototype for a wide range of other metal oxides. DMMP, Sarin and VX all adsorb in the same way and with a similar adsorption energy (ΔE_ads) which depends strongly on surface acidity. A coordinative­ly-unsaturated tetrahedral Al(T_d) site, which is a strong Lewis acid, forms an Al‑‑‑O=P dative bond to the phosphonyl group. This is always the most favorable site, and an agent adsorbed by hydrogen bonding at an OH site will migrate to a bare Al(T_d) site if one is available. The γ-Al₂O₃ lowest unoccupied orbital is a surface state associated with the Al(T_d) site which lies just below the con­duction band minimum. The energy of this state relative to vacuum is a quantitative measure of Lewis acidity and is directly reflected in ΔE_ads. The highest occupied orbital of the molecule is a non-bonding orbital on the phosphonyl O atom. The one-electron energy of this orbital also directly affects ΔE_ads. The hydroxylated surface forms Type I, II and III OH sites with O bonded to one, two or three Al’s respectively. Brønsted acidity, and ΔE_ads for OH‑‑‑O=P bond formation, increase in the order I<II<III with the Type I interaction being almost negligible. The amine group in VX is found to be a strong base, capable of deprotonating an acidic Type III OH site to form an ammonium ion even in the absence of a polar solvent. The electronic ex­citations of DMMP, Sarin and VX have also been investigated with the goal of identifying possible photochem­ical effects due to terrestrial solar radiation (TSR). This issue has been largely ov­er­look­ed in previous agent-fate studies. DMMP and Sarin are insensitive at energies be­low ~7 eV; howev­er, vapor-phase VX can be excited near 4.3 eV which is at the upper end of the TSR. The excita­tion threshold changes slightly for adsorbed VX, shifting to the red or to the blue depending on the nature of the surface. These results provide a predictive framework for understanding the effects of surface condition on the adsorption of VX and G-series CWAs on oxide surfaces. 

(1) Supported by the Defense Threat Reduction Agency (DTRA)