Trimethylsilylazide, an efficient trap for gaseous carbenium ions. The “azide-clock” in the gas phase
Massimiliano Aschi, Marina Attina
and Andreina Ricci
Dipartimento di Studi di Chimica e
Tecnologia delle Sostanze Biologicamente Attive, Università
“La Sapienza” di Roma, P.le Aldo Moro, 5, 00185, Roma, Italy
Received 2 May 1994; accepted 1 August 1994. ;
Available online 14 August 2003.
Trimethylsilylazide has proven to be a highly efficient nucleophile for trapping gaseous carbenium ions, as shown by the results of low-pressure FT-ICR experiments and by atmospheric-pressure γ radiolysis of propane and isobutane. Of particular interest is the structural characterization of the carbenium ions R+ from the analysis of neutral alkylazides whose formation processes have been previously investigated by mass spectrometric techniques and ab initio calculations. The results demonstrate that RN3 is formed from the primary attack of R+ ions on TMSA, followed by (CH3)3Si+ displacement by a suitable nucleophile. The absolute yields of the products from the radiolytic experiments are as high as 70 to 100% of the primary ions, and point to the efficiency of the reagent, that can be used at relatively high concentrations, thus allowing to trap short-lived ionic species within 10−9 s from their formation. The nature and the yields of RN3 from propane, namely n-C3H7N3 and i-C3H7N3, and their dependence on the experimental conditions, provide compelling evidence for the role of protonated cyclopropane as an n-propylating reactant with a lifetime exceeding 10−9 s. Formation of i-C3H7+ from high-pressure radiolysis of isobutane is demonstrated for the first time. The rate constant for the hydride-transfer reaction i-C3H7+ + i-C4H10 → C3H8 + t-C4H9+ has been evaluated to be 2.4 × 10−10 cm3 molec−1 s−1 at 42°C. The small negative temperature dependence of the hydride-ion abstraction is consistent with the lack of an appreciable activation barrier, as previously noted at much lower pressures by mass spectrometric techniques.
Author Keywords: Carbenium ions; Gas phase; Ion/molecule reactions; Radiolysis; Trimethylsilylazide