Molecular orbital and infrared spectroscopic considerations on the proton-transfer and amino-imino tautomerization in the 2-aminopyridine-acetic acid system in the ground state

In order to make clear the formation of 8-aminopyridinium acetate complex in the 2-aminopyridine-acetic acid system in the ground state, the ab initio molecular orbital calculation at the 6-31G level with a geometry-optimization was carried out for the 2-aminopyridine-formic acid system used as a model system for the 2-aminopyridine-acetic acid system, and the IR spectrum of the 2-aminopyridine-acetic acid system was measured to ascertain the formation of the 2-aminopyridinium acetate complex. From these results the following conclusions were drawn: (1) In the ground state the 2-aminopyridine-formic acid complex (amino complex) is most stable, and its stabilization energy is 75.69 kJ mol(-1) (2) The next stable complex is the 2-aminopyridinium formate complex (cation complex), and its stabilization energy is 67.63 kJ mol(-1). Its stabilization energy is comparable in magnitude to the corresponding one of the former complex model. (3) The calculated potential barrier height of the proton-transfer process from the amino complex to the cation complex is about 13 kJ mol(-1) and its potential energy curve has a stable potential minimum which corresponds to the energy of cation complex. These results suggest that the proton-transfer may occur easily. (4) The third stable complex is the 2 (1H)-pyridinimine-formic acid complex and its stabilization energy is 28.67 kJ mol(-1). The calculated potential energy surface suggests that the amino-imino tautomerization may proceed through the cation complex from the amino complex. (5) From similarity of formic acid to acetic acid in chemical property the above conclusion may be applicable to the 2-aminopyridine-acetic acid system. (6) Experimentally, the characteristic bands ascribed to the amino and cation complexes were observed in the region from 4000 to 1800 cm(-1) of the IR spectrum of the 2-aminopyridine-acetic acid system. Especially, the bands at 3000 and 2500 cm(-1) were attributed to the 2-aminopyridine-acetic acid complex and the bands at 3200 and 1925 cm(-1) to the 2-amino pyridinium acetate complex. These band assignments were done by referring to the results of normal coordinate calculation of the amino and cation complexes and IR spectra of the 2-methylaminopyridine- and 2-amino-3-nitropyridine-acetic acid systems.