Prototropic equilibrium between 1-H-2-oxo-pyrido[2,1-b][3,4]dihydropyrimidinium chloride and 3-(2-aminopyridinium)propionate hydrochloride studied by X-ray, FTIR, Raman, NMR and ab initio methods

Two of the most stable conformers have been analyzed by the B3LYP and MP2 calculations. The energy barrier about the C(3)-C(4) bond between them is 1.7 kcal mol−1. Correlations between the experimental 1H and 13C NMR chemical shifts (δexp) and the GIAO/B3LYP/6-311++G(d,p) calculated magnetic isotropic shielding tensors (σcal) for both conformers, δexp = a + b σcal, are reported. A good linear relationship between the experimental and the calculated data was obtained only for carbon atoms. In a DMSO solution containing traces of water, 1-H-2-oxo-pyrido[2,1-b][3,4]dihydropyrimidinium chloride exists in an equilibrium with 3-(2-amino-pyridinium)propionate hydrochloride.