Molecular Orbital and Infrared Spectroscopic Considerations on the Hydrogen Bond Formation and Proton-transfer in the 3-Methylpyrazole-Acetic Acid System in the Ground State

Abstract

In order to make clear (1) the existence of the monomers and dimers of 3- and 5-methylpyrazoles in CC14s olution, and (2) the formation of 3- and 5-methylpyrazoleacetic acid complexes and 3 ( 5) methylpyrazolium acetate in the 3-methylpyrazoleacetic acid system in the ground state, the ab initio molecular orbital calculation and the IR measurements were carried out.
From these results the following conclusions were drawn: (1) In the IR spectrum of 3-methylpyrazole in CCl₄ solution, the bands at 3480 and 3470 cm^-1 were assigned to the N-H stretching band of the monomers of 3- and 5-methylpyrazoles, respectively, and the strong and broad bands at 3200 and 2980cm^-1 were assigned to the N-H stretching vibrations (B_U) of the dimers of 3- and 5-methylpyrazo les, respectively. (2) In the ground state the calculated stabilization energies for the models of the 3-methylpyrazoleformic acid and 5-methylpyrazoleformic acid complexes are 80.20 and 80.62 kJ mol^-1, respectively. The magnitudes of these values suggest the possibility of the formation of the hydrogen bond complexes (abbrev.: H-bond complex) between 3- and 5-methylpyrazoles and acetic acid. (3)The calculated stabilization energy of the 3- or 5-methylpyrazolium formate is 51.44 kJ mol^-1. Although the magnitude of the stabilization energy is smaller than those of the H-bond complexes, it suggests the possibility of the formation of 3- or 5-methylpyrazolium acetate (abbrev.: cation complex)through the proton-transfer process. (4) Experimentally, the characteristic bands ascribed to the Hbond complexes and the cation complex were observed in the region from 3500 to 1300 cm^-1 of the IR spectrum of the 3-methylpyrazoleacetic acid system. The bands at 3280 and 2540 cm^-1 were assigned to the N-H and O-H stretching vibrations of the H-bond complexes, respectively. The bands at 1900 and 1360 cm^-1 were assigned to the N-H stretching vibration and the inplane bending vibration of the N-H bonds of the cation complex, respectively. (5) The good linear relationship exists between the N-H stretching vibrational frequencies (ν^c)o f the optimized models of 3-methylpyrazole and its relat ed compounds used for the present calculation and their N-H bond distances.