Neutralization of methyl cation via chemical reactions in low-energy ion-surface collisions with fluorocarbon and hydrocarbon self-assembled monolayer films

Abstract

Low-energy ion-surface collisions of methyl cation at hydrocarbon and fluorocarbon self-assembled monolayer (SAM) surfaces produce extensive neutralization of CH 3 +. These experimental observations are reported together with the results obtained for ion-surface collisions with the molecular ions of benzene, styrene, 3-fluorobenzonitrile, 1,3,5-triazine, and ammonia on the same surfaces. For comparison, low-energy gas-phase collisions of CD 3 + and 3-fluorobenzonitrile molecular ions with neutral n-butane reagent gas were conducted in a triple quadrupole (QQQ) instrument. Relevant MP2 6-31G*//MP2 6-31G* ab initio and thermochemical calculations provide further insight in the neutralization mechanisms of methyl cation. The data suggest that neutralization of methyl cation with hydrocarbon and fluorocarbon SAMs occurs by concerted chemical reactions, i.e., that neutralization of the projectile occurs not only by a direct electron transfer from the surface but also by formation of a neutral molecule. The calculations indicate that the following products can be formed by exothermic processes and without appreciable activation energy: CH 4 (formal hydride ion addition) and C 2H 6 (formal methyl anion addition) from a hydrocarbon surface and CH 3F (formal fluoride addition) from a fluorocarbon surface. The results also demonstrate that, in some cases, simple thermochemical calculations cannot be used to predict the energy profiles because relatively large activation energies can be associated with exothermic reactions, as was found for the formation of CH 3CF 3 (formal addition of trifluoromethyl anion).