Hydrogen-Transfer Reactions Examined in the Context of a Sputtered Ion Mechanism for Low-Energy Polyatomic Ion−Hydrocarbon Surface Collisions

An associative ion/surface reaction channel has been observed in which hydrogen is attached to incident ions when low-energy polyatomic ions collide with hydrocarbon-covered surfaces. Since sputtered protons also are observed as a result of these collisions, the thermodynamics of hydrogen transfer have been examined for a series of polyatomic organic compounds and their 2H-labeled analogues in the context of a sputtered ion mechanism. In this mechanism incident ions are neutralized via charge exchange with the adsorbate, releasing sputtered ions (protons), which then react with the neutralized incident ion to form the associative reaction product. If H addition proceeds via a sputtered ion mechanism, then the overall thermodynamics of this reaction should be related to the sum of the ionization potential (IP) and the proton affinity (PA) of the incident molecule. This hypothesis has been tested using the following series of compounds: phenol, toluene, benzene, cyclohexane, tetrahydrofuran, acetone, pyrazine, pyridine, and acetonitrile. It has been found that as the overall exothermicity for hydrogen addition increases (i.e., larger sum of IP and PA), the relative abundance of protonated molecular ions and sputtered proton-containing fragments increases. The threshold nature of the extent of hydrogen-transfer reaction as a function of IP + PA is suggestive of the reaction pathway involved in the sputtering of protons from hydrocarbon surfaces during SID MS/MS.