Bronsted 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 10-8−103 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 109 s-1 is obtained at 40 °C for an elementary D+ transfer step within the complex. An estimated rate of ca. 107 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 C6(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.