Cleavage of C−C and C−F Bonds by Xe+• and I+ Ions in Reactions at a Fluorinated Self-Assembled Monolayer Surface: Collision Energy Dependence and Mechanisms

Abstract

Collisions of Xe+• with a fluorinated self-assembled monolayer surface cause C−C and C−F bond cleavage as evidenced by the reactively scattered ions XeCF2+•, XeCF+, and XeF+. The projectile ion extracts difluorocarbene from the fluorocarbon to form XeCF2+• in a low-energy reaction, while simple fluorine abstraction also occurs and yields XeF+ in a higher energy but entropically favored process. The intact trifluoromethyl iodide radical cation, ICF3+•, resulting from simple C−C cleavage, is observed as a scattered product when I+ is chosen as projectile, as are analogous ions IF+•, ICF+•, and ICF2+, resulting from C−F and C−C bond cleavage with concomitant I−F and I−C bond formation. Multiple F-atom abstraction occurs in a single collision evidenced by the product, IF2+. Density functional theory calculations confirm that the reactions that lead to XeF+ and XeCF+ are more endothermic than XeCF2+• formation. The experimental observations and enthalpy calculations suggest that two reaction pathways contribute to XeF+ formation: oxidative insertion at low collision energy and formation of a fluoronium ion (−F+−) at high collision energy. The generation of XeCF2+• products is also accounted for through an oxidative insertion mechanism. Although chemical sputtering, i.e., charge exchange with liberation of fluorocarbon cations from the surface, also occurs even at very low collision energy, it does not appear to contribute to the formation of ion/surface reaction products discussed.