Bronsted-acid behavior of C-6(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, Bronsted 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 degrees C for an elementary D+ transfer step within the complex. An estimated rate of ca. 10(7) s(-1) at 40 degrees 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 degrees C, of radiolytic experiments. All gathered evidence points to a sigma-complex structure for protonated benzene.