MEDIUM EFFECTS ON PROTON CHEMICAL SHIFTS OF NON-POLAR MOLECULES IN LIQUIDS

The displacements of the proton resonance signal for a number of non-polar molecules on going from the gas phase to infinite dilution in a series of solvent molecules have been measured. It is concluded that the displacement due to the van der Waals forces is related to several factors the main one of which is the degree of ‘exposure’ of the solute proton. Furthermore, the van der Waals shift is not apparently related to any simple property of the solvent molecule such as the molecular polarizability. Contrary to generally held belief, intermolecular magnetic fields are not always identically zero for isotropic solvent molecules since only by invoking such fields can one explain the surprisingly small displacements observed for solutes dissolved in tetranitromethane. The present work underlines the sensitivity of proton chemical shifts to very weak specific interactions and evidence is presented for the formation of very weak hydrogen bonds in liquid tetramethoxysilane and between the tetramethoxysilane and benzene molecules. Some comments are also made concerning the use of tetramethylsilane as a standard for internal referencing in chemical shift measurements. © 1969 Taylor and Francis Group, LLC.