Proton transfer to the fluorine atom in fluorobenzene. Temperature and pressure dependence

Abstract

Proton transfer from CH₅⁺ to the fluorine atom substituent of fluorobenzene has been studied as a function of temperature and pressure using a high-pressure ion source and collision-induced dissociation (CID) mass spectrometry. The behaviour at low temperatures examined (<350 K) is similar to that expected of a “locking” ion–dipole mechanism. However, the third-order pressure and overall T^–1.46 temperature dependences are consistent with a mechanism involving the formation of an intermediate complex. This complex has been isolated from a benzene/fluorobenzene mixture in methane at ca. 200 K and was identified as having the C₆H₅F–H⁺–CH₄ structure, in a potential-energy well depth of 10–11 kJ mol^–1 according to theory. Although decomposition directly from the complex is very likely, under the conditions investigated thermal decomposition from the F-protonated isomer does not appear to be a significant factor in determining its population relative to the ring-protonated species.