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 al so are observed as a result of these collisions, the thermodynamics of hydrogen transfer have been examined for a series of polyatomic organ ic compounds and their H-2-labeled analogues in the context of a sputt ered ion mechanism. In this mechanism incident ions are neutralized vi a charge exchange with the adsorbate, releasing sputtered ions (proton s), which then react with the neutralized incident ion to form the ass ociative 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 aff inity (PA) of the incident molecule. This hypothesis has been tested u sing the following series of compounds: phenol, toluene, benzene, cycl ohexane, tetrahydrofuran, acetone, pyrazine, pyridine, and acetonitril e. It has been found that as the overall exothermicity for hydrogen ad dition increases (i.e., larger sum of TP and PA), the relative abundan ce of protonated molecular ions and sputtered proton-containing fragme nts increases. The threshold nature of the extent of hydrogen-transfer reaction as a function of IP + PA is suggestive of the reaction pathw ay involved in the sputtering of protons from hydrocarbon surfaces dur ing SID MS/MS.