Chemical Ionization of TNT and RDX with Trimethylsilyl Cation

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Fourier transform ion cyclotron resonance mass spectrometry has been used to examine the reactions of Si(CH3)3+ with nitrobenzene, TNT, and RDX. With nitrobenzene, the only reaction observed is adduct formation which generates the C6H5NO2Si(CH3)3+ ion. The bimolecular rate constant for the reaction of Si(CH3)3+ with nitrobenzene is measured to be 1.8 × 10-9 cm3 s-1 molecule-1. With TNT, fragmentation and adduct formation were observed. The bimolecular rate constant for the reaction of Si(CH3)3+ with TNT is measured to be 0.85 × 10-9 cm3 s-1 molecule-1. With RDX, the dominant reaction observed is adduct formation, but some fragmentation is seen as a minor reaction pathway. The bimolecular rate constant for the reaction of Si(CH3)3+ with RDX is estimated to be similar to that observed with TNT (∼0.7 × 10-9 cm3 s-1 molecule-1). Collision-induced dissociation experiments performed on both the TNT−Si(CH3)3+ and the RDX−Si(CH3)3+ adducts using off-resonance collisional activation show the same fragmentation pattern that is observed during adduct formation. This fragmentation pattern appears to be a “fingerprint” for both adducts. These reactions appear to be driven by the high affinity of Si for oxygen and the attraction of the Si(CH3)3+ ion to the formal negative charge of oxygen in a nitro group. A reaction coordinate diagram for reactions of RDX with Si(CH3)3+ is derived (from known thermochemistry and ab initio calculations on the reactive intermediates) and its implications are discussed. Reactions of this type could be useful as a detection scheme for common explosives.