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)-HÁÁÁX À hydrogen bond depends on the counter-ions and increases in the order: Cl À < Br À < ClO 4 À .
In the crystal structure of 3Ax determined by X-ray diffraction, the ClO 4 À anion is disordered and the N(1)-HÁÁÁOClO 3 À 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)-HÁ Á ÁX À , type B with N(1)Á Á ÁH-X hydrogen bonds and type C with electrostatic interactions between the positively charged nitrogen atom N + (4) and the counter anion (X À = Cl À , Br À or ClO 4
À
). Correlations between the experimental 1 H and 13 C NMR chemical shifts (d exp ) and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors (r calc ) for 3Ao and 3Co, d exp = a + br 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.