TRIMETHYLSILYLAZIDE, AN EFFICIENT TRAP FOR GASEOUS CARBENIUM IONS - THE AZIDE-CLOCK IN THE GAS-PHASE

Trimethylsilylazide has proven to be a highly efficient nucleophile fo r trapping gaseous carbenium ions, as shown by the results of low-pres sure FT-ICR experiments and by atmospheric-pressure gamma radiolysis o f propane and isobutane. Of particular interest is the structural char acterization of the carbenium ions R+ from the analysis of neutral alk ylazides whose formation processes have been previously investigated b y mass spectrometric techniques and ab initio calculations. The result s demonstrate that RN3 is formed from the primary attack of R+ ions on TMSA, followed by (CH3)3Si+ displacement by a suitable nucleophile. T he absolute yields of the products from the radiolytic experiments are as high as 70 to 100% of the primary ions, and point to the efficienc y of the reagent, that can be used at relatively high concentrations, thus allowing to trap short-lived ionic species within 10(-9) s from t heir formation. The nature and the yields of RN3 from propane, namely n-C3H7N3 and i-C3H7N3, and their dependence on the experimental condit ions, provide compelling evidence for the role of protonated cycloprop ane as an n-propylating reactant with a lifetime exceeding 10(-9) s. F ormation of i-C3H7+ from high-pressure radiolysis of isobutane is demo nstrated for the first time. The rate constant for the hydride-transfe r reaction i-C3H7+ + i-C4H10 --> C3H8 + t-C4H9+ has been evaluated to be 2.4 x 10(-10) cm3 molec-1 s-1 at 42-degrees-C. The small negative t emperature dependence of the hydride-ion abstraction is consistent wit h the lack of an appreciable activation barrier, as previously noted a t much lower pressures by mass spectrometric techniques.