"Conformational Study of trans-1,2 and cis-1,3 Ciclohexanedicarboxylic Acids in Water and DMSO Using NMR Spectroscopy"
The populations of diaxial and diequatorial conformers in the trans-1,2 and cis-1,3 isomers of cyclohexanedicarboxylic acid and their mono and dianions were determined by use of vicinal proton-proton NMR coupling constants and, specially developed, accurate Karplus equations. In order to measure the proton-proton couplings, the experimental 1H NMR spectra in D2O of these diacids and their ionization species was fitted by iterative simulations. Conformer populations were unambiguously determined to a high degree of accuracy from the magnitude of the couplings of the alpha-proton and by comparing the measured couplings in the CCH2CH2C fragments to the ones calculated by an accurate Karplus equation previously developed for these fragments in six-membered rings. From these results, Karplus relationships were obtained to estimate the couplings of the alpha-proton in these diacids and the conformational preferences in DMSO were determined from the couplings of the alpha-proton, measured from 13C NMR satellites and signals from this proton, by use of the obtained relations. The results show strong preference for the diequatorial conformers of both diacids in dieffrent ionization states in both, protic (water) and aprotic (DMSO) solvents.