NMR is at present one of the basic direct methods for the study and structure determination of carbonium ions. Quite a number of carbocations have been obtained in superacid media (SbF,-S02, FS03H-SbF,-S02FC1 etc.) under the so-called 'long lifetime' conditions, thus allowing their study by NMR.' Novel important information about the well-known problem of the classical or non-classical structure of some carbocations was yielded in this manner.2 In a number of cases it is quite difficult, on the basis of chemical data, to prefer the non-classical model of carbonium ions as intermediates in chemical reactions, rather than the oscillation model corresponding to a fast rearrangement of two or more equivalent classical structures. In fact, the problem is, which of the structures-classical or non-classical-is energetically more advantageous.In principle, this can be solved by theoretical methods. However, ab initio calculations of small molecules show that the results are essentially dependent on the approximations used.3 For example, for the (C2H,)+ cation with a 4-31G basis set, the Hartree-Fock energy of the classical structure is 7.3 kcal mol-' lower than that of the non-classical ion4 and with a 6-31G basis, taking into account the polarization d functions, it is 0.1 kcal mol-' lower? whereas by consideration of the polarization p functions it is the non-classical structure which is 0.4 kcal mol-' lower in en erg^.^ Calculations allowing for the configuration interaction lead to a stability as high as 9 kcal mol-' for the non-classical structure.6 For the (C,H3)+ cation the Hartree-Fock approximation shows the classical structure to be the more stable by 19.2-5.7 kcal mol-', again depending on the while the non-classical structure is more stable by 7 kcal mol-' by consideration of the configuration interactiom6 More detailed calculations' show, in fact, de-* Author to whom correspondence should be addressed. generation of these structures. Thus, the level of energy calculations achieved does not allow unambiguous conclusions even about the structures of simple carbocations, particularly since, in real systems, interactions with the solvent (neglected in computations) may be of great importance.The NMR technique allows the unambiguous determination of the cation structure, provided its spectrum corresponds to an asymmetrical classical structure. Some cations, however, have symmetrical spectra down to the lowest temperatures used which, as shown above, can be attributed to either a non-classical structure, or rearrangements of classical structures with a frequency exceeding the frequency difference of the averaged NMR lines. Olah8 has proposed a criterion according to which an ion has a non-classical structure if its spectrum differs essentially from the average spectrum of classical structures. This criterion is based on two suppositions: first, that from empirical relationships one can, with sufficient precision, predict the NMR spectra of classical structures which perhaps do not exist; and, second, that the spec...