Organic solvents effect donor-acceptor interaction

On the basis of a study of the solvent dependence of the chemical shift of Taft el al, [Ta 63] suggested that the solvent effect can be attributed to donor-acceptor chemical interactions. The first really convincing confirmation of this concept was provided by the Na NMR studies by Erlich, Popov et al [Er 70, Er 71, Gr 73, Po 79]. In this work a linear correlation was demonstrated between the Gutmann donicities of the solvents and the chemical shift values for solutions of sodium perchlorate and sodium tetraphenylborate in various organic solvents. This clearly showed that the variation of the chemical shift of the Na in the systems investigated was caused predominantly by the donor-acceptor interaction between the donor solvent and the sodium ion. 

The mobile phase in RP chromatography contains water and one or more organic solvents, most frequently acetonitrile, methanol, tetrahyrofuran, or propanol. By the choice of the organic solvent, selective polar interactions (dipole-dipole, proton-donor, or proton-acceptor) with analytes can be either enhanced or suppressed, and the selectivity of separation can be adjusted. Binary mobile phases are usually well suited for the separation of a variety of samples, but ternary or, less often, quaternary mobile phases may offer improved selectivity for some difficult separations. The retention times are controlled by the concentration of the organic solvent in the aqueous-organic mobile phase. Equation 1 is widely used to describe the effect of the volume fraction of methanol or acetonitrile

[Pg.1440]

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