Molecular Orbital Considerations on Interaction of 2-Aminopyridine with Formic Acid in the Ground State

Abstract

Abstract In order to ascertain the formation processes of the tautomer and proton-transferred complexes in the 2-aminopyridine–acetic acid system in the ground state, a molecular orbital calculation was carried out for the 2-aminopyridine–formic acid system by the ab initio STO-3G method with full-geometry optimization of the complex. The following conclusions were drawn from the present calculation: (1) The amino–imino tautomerization in the 2-aminopyridine–formic acid system proceeds through a double proton transfer process between 2-aminopyridine and formic acid. (2) The formation of a proton-transferred complex in the 2-aminopyridine–formic acid system is difficult in the ground state. This conclusion may be applicable to the 2-aminopyridine–acetic acid system. (3) In the case of the ab initio STO-3G calculation a full-geometry optimization of the complex plays an important role in elucidating the stabilization energies and geometries of the tautomer and proton-transferred complexes. (4) A full-geometry optimization of the complex model is not so important for evaluating the hydrogen bond energy of the stable hydrogen-bonded complex, but is necessary for elucidating the effect of hydrogen bond formation on the molecular geometry of the complex. (5) The bisectal angle of the NH2 group, calculated by the ab initio STO-3G method, becomes much larger than the experimental value. Further, the energy difference between the planar and pyramidal 2-aminopyridine with respect to the NH2 group is much larger than the experimental value.