B. Chiavarino et al., BRONSTED-ACID BEHAVIOR OF C-6(H,D)(7)(- A COMBINED APPROACH BY RADIOLYTIC, FA-SIFT, AND FT-ICR METHODOLOGIES() BENZENIUM IONS ), Journal of physical chemistry, 100(40), 1996, pp. 16201-16208
Benzenium ions of varying deuterium content have been formed by H+/Dtransfer 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 ex
amined and by significant primary kinetic isotope effects in the compe
titive H+ vs D+ transfer. The reactivity of benzenium ions with select
ed bases is highly dependent on structural features of the ensuing col
lision 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 th
e complex. An estimated rate of ca. 10(7) s(-1) at 40 degrees C is der
ived for the D+ shift within the aromatic ring from a temperature depe
ndence 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 hi
ghest temperature, 120 degrees C, of radiolytic experiments. All gathe
red evidence points to a sigma-complex structure for protonated benzen
e.