Structure of 1H-2-oxo-2,3-dihydroimidazo[1,2-a]pyridinium perchlorate studied by X-ray diffraction, DFT calculations and by FTIR and NMR spectroscopy

2-Aminopyridine betaine (1-carboxymethyl-2-aminopyridinium inner salt) forms crystalline complexes with HCl, HBr and HClO4. These complexes on heating in ethanol cyclize to 1H-2-oxo-2,3-dihydroimidazo[1,2-a]pyridinium chloride (1Ax), bromide (2Ax) and perchlorate (3Ax), respectively. Infrared spectra of the 1H-2-oxo-2,3-dihydroimidazo[1,2-a]pyridinium complexes indicate that the length of the N(1)-H···X− hydrogen bond depends on the counter-ions and increases in the order: Cl− < Br− < ClO4−. In the crystal structure of 3Ax determined by X-ray diffraction, the ClO4− anion is disordered and the N(1)-H···OClO3− distance cannot be determined accurately. Three types of cyclic molecules were optimized by the B3LYP/6-31G(d,p) level of theory: type A with N(1)-H⋯X−, type B with N(1)⋯H-X hydrogen bonds and type C with electrostatic interactions between the positively charged nitrogen atom N+(4) and the counter anion (X− = Cl−, Br− or ClO4−). Correlations between the experimental 1H and 13C NMR chemical shifts (δexp) and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors (σcalc) for 3Ao and 3Co, δexp = a + bσcalc, are reported. Tentative assignments of the experimental anharmonic solid state vibrational frequencies of the perchlorate complex, 3Ax, based on the B3LYP/6-31G(d,p) calculated harmonic frequencies, are presented.