Solvation of Silylium Ions

A solvated molecule is rather difficult to describe since there can be specific interactions with one or more solvent molecules apart from nonspecific interactions with one or more solvent shells. Although the investigation of specific solvent complexes provides a description of major solvent effects it is far from giving an exact picture of a solvated molecule. This can only be obtained by excessive solvent modelling using Monte Carlo methods which becomes too costly when carried out for a larger number of silylium cations in different solvents. 

Alternatively, one can create a situation, in which primary solvation of the soluted molecule occurs in a well-defined way. For this purpose, the idea of internal solvation was developed, [44] which is based on the following If the solute and solvent molecules are part of the same molecular system, then the molecular link between them will exactly define magnitude and type of solute-solvent interactions. 

Realization of the idea of internal solvation in the case of silylium ions can be done with the help of a template to which both the group R2Si+ and a nucleophilic solvent -CH2-Z are bonded. For example, the bidentated molecular templates used by Corriu and co-workers to study intramolecular coordination, [121-124] are suitable to investigate a well-defined internal solvation situation. The general structure of these templates, originally 

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In Sections 5, 6 and 7, three different approaches to the problem of silylium ions in solution are described. First, the typical gas phase versus solution phase ab initio (DFT) description of silyl compounds and silylium ions is given (Section 5). In Section 6, the NMR/ab initio/IGLO and NMR/DFT/IGLO methods are used to investigate solvation of silylium ions in different solvents. This work demonstrates how complex the solvation process of a silylium ion can be and, therefore, there is a need to generate silylium ions under well-defined situations in solution which simplify investigations. Out of this necessity, the idea of intramolecular solvation of silylium ions was born, which is discussed in Section 7. 

When Z in 41 represents a group with little coordination ability, intermolecular solvation of silylium ions in a weakly nucleophilic solvent can be modelled. [44] For Z = CH3, cations 46 and 47 shown in Figure 11 are obtained, which should... 

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