Ions R3C in Solution

Silylium ions cannot be formed under superacid conditions because of the high affinity of Si to O, F, and Cl atoms normally contained in these media. [78] 

All available experimental results [87] suggest that carbocations are just weakly coordinated to counterion or solvent. For trialkyl and triaryl substituted carbenium ions generated under superacid conditions, this has been demonstrated in a series of X-ray diffraction experiments in particular carried out by Laube and co-workers. [98-102] For example, for the 3,5,7-trimethyl-l-adamantyl cation and its counterion Sb2Fn, the closest C-F distance was measured to be 2.88 A suggesting that interactions between cation and anion are rather weak. [98] 

In an interesting study on the rotational reorientation of the 1-adamantyl cation in different superacid solutions, Kelly and Leslie confirmed that interactions between carbocation and the surrounding medium are also weak. 

It was shown that the major difference between carbenium and silylium ions in solution arises from the fact that internal stabilization of R3O ions can be much larger than for I 3Si+ ions. [6] For example, three methyl groups stabilize a carbenium ion by 72 kcal/mol (9, Table 3) because of hyperconjugative interactions. In this way, the LUMO is raised in energy and, accordingly, it is less available for a nucleophilic interaction partner. Hence, increasing internal stabilization of a carbenium ion makes specific external solvation more difficult. 

For carbenium ions with none or just one alkyl substituent attached to C+, internal stabilization is lowered and strong coordination of solvent molecules can take place. For example, when CH3F is dissolved in SbFs/S02, the solvent is methylated so that instead of CH3+ the complex H3COSO+ is formed. [104,105] This occurs also for ethyl and iso-propyl fluoride while use of SO2F2 instead of SO2 prevents dissociation of CH3F or other monoalkyl fluorides. [105] Obviously, there seems to be a competition between solvent and anion to coordinate to the carbocation. 

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