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

Abstract 2-Aminopyridine betaine (1-carboxymethyl-2-aminopyridinium inner salt) forms crystalline complexes with HCl, HBr and HClO 4 . 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 − − 4 − . In the crystal structure of 3Ax determined by X-ray diffraction, the ClO 4 − anion is disordered and the N(1)–H···OClO 3 − 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 ClO 4 − ). Correlations between the experimental 1 H and 13 C NMR chemical shifts ( δ exp ) and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors ( σ calc ) for 3Ao and 3Co, δ exp  =  a  +  bσ calc , 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.