A NMR investigation of proline and its derivatives. II—Conformational implications of the ¹H NMR spectrum of L‐proline at different pH

The 250 MHz 1 H NMR spectrum of L ‐proline is comprehensively analysed by computer simulation for different pH values. A fast endo – exo interconversion has already been proposed from chemical shift and coupling constant data. A critical comparison of the vicinal coupling constants leads to the conclusion that in basic solution the equilibrium is shifted towards a more endo conformation. The position of the carboxylic plane in the ring space is given by the relative intramolecular chemical shift of the geminal protons and by the typical patterns of the H α multiplet. These results are applied to the analysis of the spectra of ProNH 2 and AcProNH 2 .