Solvent effects in nuclear magnetic resonance spectroscopy. Part XI. The mechanism of solvent shifts of proton resonances induced by benzene

Abstract

Evidence is presented in support of a theory of solvent shifts induced by benzene which modifies some of the ideas expressed in the literature. It is suggested that benzene solvent molecules will solvate the electron-deficient sites of local dipoles in solute molecules, probably in transient 1 : 1 associations. Hence it is believed that polyfunctional solutes (aromatic or aliphatic) will undergo 1 : 1 interactions at various sites, but that there is no phase relationship between the interactions. In general, it is thought that there is no basic difference between the mechanisms by which the solvent shifts are induced in aliphatic and aromatic solutes. The orientation of benzene solute molecules may well occur in a non-planar association, in which the benzene ring is as far away from the negative end of the local dipole (or molecular dipole) as possible.