tically determining formation of an early complex [Eq. (c)], a low isodurene/toluene intermolecular selectivity should be contrasted by the retention of intramolecular selectivity in the nitration of 2. The latter has been studied, as in the case of the other substrates, by chemical ionization (CI) mass spectrometry (in CH,, 0.5 torr, 160°C) and by a radiolytic technique[" (in CH4, 720 torr, 37.5 "C). The nitrating reactant 1 (R=Me, CF3CH2) has been obtained in both cases by protonation of the corresponding alkyl nitrate with the C,H: ( n = 1,2) ions from the ionization of CH,. The CI spectra reveal formation of the expected [Arene . . . NO,]@ adducts, of unknown structure. The radiolytic results provide clear-cut evidence for the rate-limiting formation of an early adduct [Eq. (c)] since the k,,odurcne :ktoluene ratio is as low as 1.5, while the intramolecular selectivity in the nitration of 2 is much higher, the activated ring reacting at least 20 times faster than the unsubstituted one, despite the unfavorable (2 : 5 ) statistical factor. Nor can such a result be traced to isomerization of primary arenium ions, i. e., to the energetically favored intramolecular migration of NO: to the more activated ring. Convincing evidence against such a process is provided by control experiments based on metastable-ion kinetic energy (MIKE) and collisionally induced dissociation (CID) spectrometry of suitable model ions."]In conclusion, the results show that in the nitration of sufficiently activated substrates by gaseous [RN03Hle ions the rate-determining and the product-controlling steps are kinetically distinct, according to the scheme outlined in Equation (c).Whenever k2 exceeds k -, (i. e., the activation barrier to
