DETERMINATION OF ISOTOPE EFFECTS ON ACID-BASE EQUILIBRIA BY C-13 NMR-SPECTROSCOPY

New results from isotope effect measurements on add-base equilibria by C-13 NMR spectroscopy show that in fatty acids deuterium isotope effe cts extend at least up to seven bonds and have unexpectedly small atte nuation. In deuterium-substituted benzoic acids the isotope effect on pK(a) is practically independent of the site of deuterium substitution . H-2 isotope effects on carboxy ionization in glycine and alanine are smaller than in the corresponding carboxylic acids. Different protona tion shifts in unlabelled and labelled compounds are recorded. O-18 an d C-13 isotope effects in acetic acid and N-15 effects in glycine isot opomers were measured. The application of auxiliary isotope substituti on is demonstrated for the measurement of very small differences in pK (a) values.