Solute-solvent interactions probed by intermolecular NOEs

Nuclear Overhauser effects arising from the interactions of spins of solvent molecules with spins of a solute should reveal the exposure of solute spins to collisions with solvent. Such intermolecular NOEs could, therefore, provide information regarding conformation or structure of the solute. Determinations of solute-solvent NOES of 1,3-di-tert-butylbenzene in solvents composed of perfluoro-tert-butyl alcohol, tetramethylsilane, and carbon tetrachloride have been carried out. A crude, but apparently reliable, method for prediction of intermolecular solvent-solute NOES based on hard (noninteracting) spheres was developed. Comparison of experimental to predicted NOES indicates that tetramethylsilane interacts with the solute according to the model. By contrast, intermolecular NOE data indicate attractive interactions between the solute and perfluoro-tert-butyl alcohol. All NOE results and the corresponding predictions confirm that proton H2 of the solute is protected by the flanking tert-butyl groups from interactions with solvent molecules.