Structure of 1-methyl-2-oxo-pyrido[2,1b][3,4]dihydropyrimidinium bromide studied by X-ray diffraction, FTIR and NMR spectroscopy and DFT calculations

1-Methyl-2-oxo-pyrido[2,1b][3,4]dihydropyrimidinium bromide (1) has been obtained from 2-methylamino-pyridine and 3-bromopropionic acid. The crystals are orthorhombic, space group Pnma, with a = 12.3146(8), b = 6.7869(4), c = 11.5270(9) Å, V = 963.40(11) Å3, and Z = 4 (at 100 K). Two conformers are present in the crystal structure as a result of a disorder. The structure of 1 was confirmed by 1H and 13C NMR spectra in DMSO-d6 and D2O solutions using 2D NMR techniques (COSY, NOESY, HSQC, and HMBC). the most stable conformers of 1 have been analyzed by the B3LYP calculations. The energy barrier is 4.9 kcal/mol according to PM3 calculation. The νCO band at 1714 cm−1 and the lack of a νNH band above 2000 cm−1 in the FTIR spectrum agree well with the bicyclic structure of the title molecule. Correlations between the experimental 1H and 13C NMR chemical shifts (δexp.) have been found and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors (σcalc.) for both conformers, δexp. = a + b·σcalc., are reported. A good linear relationship between the experimental and calculated data has been obtained only for carbon atoms.