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

1-H-2-oxo-pyrido[2,1-b][3,4]dihydropyrimidinium chloride has been obtained from 2-amino-pyridine and 3-chloropropionic acid. The crystals are orthorhombic, space group Pnma, with a = 7.7435(15) Å, b = 9.0838(18) Å, c = 12.676(3) Å, V = 841.4(3) Å3 and Z = 3 (at 293(2) K). Two conformers are present in the crystal structure. The N(1)···Cl hydrogen bond length are 3.112(3) and 3.150(3) Å, respectively. The νCO at 1714 cm−1 and the νNH band at 2660 cm−1 in the FTIR spectrum agree well with the bicyclic structure of the molecule investigated. The tentative assignments for the experimental solid state FTIR spectrum have been made on the basis of B3LYP calculated frequencies. 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.