THE O-18 ISOTOPE SHIFT IN C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY .15. STUDIES OF [O-18,1,4-C-13(2)]SUCCINIC ACID AND [O-18]DIMETHYLMALEIC ANHYDRIDE

The complex C-13 NMR spectra arising from the various isotopomers in solutions of O-18-labeled [1,4-C-13(2))-succinic acid and dimethylmaleic anhydride are investigated. These studies reveal a way to measure the coupling constant between the carboxyl carbons of succinic acid by utilizing isotopic substitution to remove the chemical shift degeneracy. The pH dependence of this coupling constant and the isotope shift give rise to AB systems of varying strengths that dramatically influence spectral appearance. The eight signals observed in the carbonyl and olefinic regions of 53% O-18-labeled dimethylmaleic anhydride are interpreted in terms of isotope shifts over one, two, and even three bonds.