Rotational isomerism—XXI: The conformation of 2-amino-3-fluoropropanoic acid (2-afp) and 2-fluoro-3-aminopropanoic acid (3-afp) as the zwitterion, cation and anion, an nmr and mo study

Abstract The NMR spectra of 2-AFP And 3-AFP in neutral, acid and alkaline solutions have been obtained and completely analysed to give the 1 H, 19 F and the 13 C chemical shifts and the HH, HF and CF coupling constants. In the analysis the use of FT spectra with inversion recovery sequences enabled an unambiguous assignment of the complex proton region to be made in certain cases due to the different relaxation times of the protons. The rotamer populations and hence relative energies have been obtained from the 3 J HH couplings, using calculated rotamer couplings which explicitly include the orientation dependence of electronegative substituents. A MO investigation of the rotamer energies of the zwitterions, anions and cations is given. The inclusion of the counterion into the CNDO wave function gives calculated rotamer energies in complete agreement with those observed for 2-AFP. In 3-AFP, the counter-ion method cannot fully compensate for the larger interactions between the charged groups. Calculations on related molecules including β-alanine show this is a general effect, possibly due to the solvated water molecules.