Kinetics of Ion−Molecule Reactions with 2-Chloroethyl Ethyl Sulfide at 298 K: A Search for CIMS Schemes for Mustard Gas

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The rate constants and product ion branching ratios have been measured in a selected ion flow tube (SIFT) at 298 K for a variety of positive and negative ions reacting with 2-chloroethyl ethyl sulfide (2-CEES), a surrogate for mustard gas (HD). This series of experiments is designed to elucidate ion-molecule reactions that have large rate constants and produce unique product ions to guide the development of chemical ionization mass spectrometry (CIMS) detection methods for the chemical weapon agent using the surrogate instead. The negative ions typically used in CIMS instruments are essentially unreactive with 2-CEES, that is, SF 6 (-), SF 4 (-), CF 3O (-), and CO 3 (-). A few negative ions such as NO 2 (-) and NO 3 (-) undergo three-body association to give a unique product ion, but the bimolecular rate constants are small in the SIFT. Positive ions typically react at the collisional limit, primarily by charge and proton transfer, some of which is dissociative. For ions with high proton binding energies, association with 2-CEES has also been observed. Many of these reactions produced ions with the 2-CEES intact, including the parent cation, the protonated cation, and clusters. G3(MP2) calculations of the thermochemical properties for 2-CEES and mustard have been performed, along with calculations of the structures for the observed product cations. Reacting a series of protonated neutral molecules with 2-CEES brackets the proton affinity (PA) to between 812 ((CH 3) 2CO) and 854 (NH 3) kJ mol (-1). G3(MP2) calculations give a PA for 2-CEES of 823 kJ mol (-1) and a PA for mustard of 796 kJ mol (-1), indicating that the present results for 2-CEES should be directly transferable to mustard to design a CIMS detection scheme.