Assignment of configuration at a tertiary centre in an organic molecule is often difficult. There are no vicinal proton-proton couplings and often the most useful type of information, particularly in cyclic compounds, is based on 'XI chemical shift data'.'or-i./c,, coupling”. The interaction of sodium ions with certain polyoxygenated compounds may induce4 changes in the chemical shifts of the resonances of particular protons, and the signals which have the ...