Acidity and Structure of Isosaccharinate in Aqueous Solution: A Nuclear Magnetic Resonance Study

Dilute aqueous solutions of the calcium and sodium salts of -d-isosaccharinate (ISA) have been analyzed by ¹³C and ¹H NMR spectroscopy. The positions of the six ¹³C ISA NMR lines show a pH dependence that can be used to infer that the most acidic proton is the one associated with the carboxylate group, with an aver- age log k⁰ of –3.27 for the reaction H(ISA) H⁺ + ISA^– in solutions made with the sodium salt. In acidic solutions (pH < 4),=" nmr=" signals=" were=" found=" that=" could=" be=" assigned=" to=">-d-isosaccharinate-1,4-lactone (ISL), formed from the dehydration of H(ISA). The pH dependence of the H(ISA) ¹³C line positions, furthermore, reveals that the deprotonated H(ISA) at neutral to high pH assumes a conformation with the carboxylate anion hydrogen-bonded to the secondary alcohol and the cation (Na⁺ or Ca²⁺) interacting with the tertiary alcohol.