LIFETIMES OF GASEOUS ION-NEUTRAL COMPLEXES - THE RATE OF ISOTOPIC SCRAMBLING WITHIN ETHYL IONS AS AN INTERNAL CLOCK

The lifetimes of typical ion-neutral complexes (INCs), the charged int ermediates of electrophilic aromatic alkylation, were investigated by a kinetic approach that utilizes as an internal clock the rate of H/D scrambling within labeled carbenium ions bound to a nucleophile as an INC. The specific process investigated was the aromatic ethylation pro moted by addition of a CF3C6X6+ (X = D or H) arenium ion to ethylene, C2Y4 (Y = H or D). This process belongs to a class of reactions, the s o-called Crafts-Friedel alkylation, that was previously demonstrated t o occur within gaseous INCs and represents an alternative to electroph ilic aromatic substitution. Following preliminary study by Fourier-tra nsform ion cyclotron resonance (FT-ICR) and triple-quadrupole (TQ) mas s spectrometry, the reaction was investigated by the radiolytic techni que in CF4 at 720 Torr in the temperature range from 298 to 393 K, by measuring the D(H) incorporation into the alpha position of the side c hain of the ethylated products CF3C6X4C2Y4X. Kinetic analysis of the r esults led to lifetimes of 2.0 x 10(-10) s (298 K), 1.2 x 10(-10) s (3 15 K), 4.4 x 10(-11) s (353 K) and 1.9 x 10(-11) s (393 K) based on th e H/D scrambling within C2H4D+, whereas the lifetime deduced from HID scrambling within C2D4H+ is 9.0 x 10(-12) s at 393 K. The results are discussed and compared with those of an independent approach based on a chemical activation technique.