Brønsted-Acid Behavior of C6(H,D)7+ Benzenium Ions. A Combined Approach by Radiolytic, FA-SIFT, and FT-ICR Methodologies

Benzenium ions of varying deuterium content have been formed by H + /D + transfer and exchange processes with neutral benzene in the gas phase at 10 -8 −10 3 Torr. Brønsted acid reactivity with benzene itself is characterized by comparable efficiencies in the wide pressure range examined and by significant primary kinetic isotope effects in the competitive H + vs D + transfer. The reactivity of benzenium ions with selected bases is highly dependent on structural features of the ensuing collision complex. When the benzenium−benzene complex is allowed to reach thermal equilibrium with the environment at the upper pressure limit allowed by the radiolytic technique, a rate constant of 10 9 s -1 is obtained at 40 °C for an elementary D + transfer step within the complex. An estimated rate of ca. 10 7 s -1 at 40 °C is derived for the D + shift within the aromatic ring from a temperature dependence radiolytic study. Accordingly, complete H/D scrambling in C 6 (H,D) 7 + ions is observed in the long-lived (10 -3 −1 s) ions sampled by FA-SIFT or FT-ICR mass spectrometry and is verified only at the highest temperature, 120 °C, of radiolytic experiments. All gathered evidence points to a σ-complex structure for protonated benzene.