Effet de solvant en résonance magnétique nucléaire de 13C. I. Cas d'un soluté apolaire: n-pentane

Abstract

The quantitative importance (several ppm) of the effect of solute–solvent van der Waals' interactions on the 13C chemical shift is shown for a non polar solute, n-pentane, in solution in 17 solvents and in the gas phase. The van der Waals screening constant term is discussed in respect to several theoretical functions of the solvent refractive index. The extension to the case of 13C of the solvent parameter scale which characterises the van der Waals interactions in 19F nmr explains quantitatively this phenomenon and allows the separation of the anisotropic term in the case of C6H6. This parameter scale is extended to several solvents. The linear variation of this van der Waals effect on the chemical shift in terms of the volume fraction is interpreted leading to a relationship which allows calculation of this parameter for a solvent mixture. The variation of the sensitivity to this effect of the different magnetic sites depending on their position in the carbon chain is discussed. The theoretical expression of the site factor leads only to a qualitative explanation of this variation.