Determination of isotope effects on acid–base equilibria by 13C NMR spectroscopy

Abstract

New results from isotope effect measurements on acid–base equilibria by ¹³C NMR spectroscopy show that in fatty acids deuterium isotope effects extend at least up to seven bonds and have unexpectedly small attenuation. In deuterium-substituted benzoic acids the isotope effect on pK_a is practically independent of the site of deuterium substitution. ²H isotope effects on carboxy ionization in glycine and alanine are smaller than in the corresponding carboxylic acids. Different protonation shifts in unlabelled and labelled compounds are recorded. ¹⁸O and ¹³C isotope effects in acetic acid and ¹⁵N effects in glycine isotopomers were measured. The application of auxiliary isotope substitution is demonstrated for the measurement of very small differences in pK_a values.