Gas-phase H/D exchange between arenium ions and selected bases. The site of protonation of simple aromatics

H/D exchange between arenium ions bearing various substituents (Y = Et, Me(2)CH, CF(3), CF(3)CO, NO(2), Me(3)Si) on a perdeuterated ring and bases having at least one H atom at the active site are described. A radiolytic technique operating at atmospheric pressure and allowing the recovery of neutral end products was used. The exchange depends on features of both the base (structure, proton affinity) and the arenium ion. The elementary step for back proton transfer from the protonated base to the neutral arene has been studied by NMR analysis of the H distribution within YC(6)D(4)H. The protonation was found to involve the most basic ring positions: ortho/para for alkyl substituents (Y = Et, Me(2)CH) and ortho/meta for electron-withdrawing substituents (Y = CF(3), CF(3)CO, NO(2)).