Survey of ion energetics properties of chemical weapon agent (CWA) breakdown products using G3(MP2) theory

Abstract

The ion energetics properties of the major breakdown products of chemical weapons agents (CWAs) have been calculated using G3(MP2) theory to gain insight into which reactant ions in chemical ionization mass spectrometry (CIMS) would be optimal for detecting the CWAs. These results would also offer insight into which ions are formed in ion mobility spectrometry (IMS) detectors for CWA that use atmospheric pressure ionization sources. The ionization energies (IEs), proton affinities (PAs), electron affinities (EAs), and fluoride affinities (FAs) have been calculated for the major degradation products of sulfur mustard, the G-series nerve agents, and VX based on optimized structures using G3(MP2) theory. Electron attachment is found to yield an unstable parent anion or leads to an ionic structure with negative EA. The sulfur and nitrogen containing degradation products all have IEs<9eV, implying that charge exchange with NO+ should be a favorable detection route. The strictly alkylphosphorus compounds all have IEs around 10eV, implying that they would likely form a unique association product with NO+. All of the compounds with a PO moiety most favorably add an H+ on the O atom with PAs of ∼900kJmol−1. 2-(Diisopropylamino) ethane thiol (DESH) and ethyl-N,N-dimethyl phosphoramidate (EDPA) both have amine nitrogens that can readily accept a proton, with PAs>900kJmol−1. The least favorable protonation site is the oxygen that connects the phosphorus center to the different alkyl groups. The PAs indicate that non-dissociative proton transfer with protonated acetone or ammonia should provide good selectivity, likely with a large rate constant as seen in previous kinetics studies with CWA simulants. All of the breakdown products except 1,4-dithiane can attach a F− ion in at least one stable location. However, the corresponding FAs are too low to favor this process with the normal CIMS fluoride transfer anions.