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


Department of Materials Science and Engineering, Faculty of Engineering, Tokyo Denki University; Kanda, Chiyoda-ku, Tokyo 101-0054 Japan

In order to make clear (1) the existence of the monomers and dimers of 3- and 5-methylpyrazoles in CCl4 solution, and (2) the formation of 3- and 5-methylpyrazole--acetic acid complexes and 3(5)-methylpyrazolium acetate in the 3-methylpyrazole--acetic 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 2980 cm−1 were assigned to the N-H stretching vibrations (BU) of the dimers of 3- and 5-methylpyrazoles, respectively. (2) In the ground state the calculated stabilization energies for the models of the 3-methylpyrazole--formic acid and 5-methylpyrazole--formic 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 H-bond complexes and the cation complex were observed in the region from 3500 to 1300 cm−1 of the IR spectrum of the 3-methylpyrazole--acetic 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 (vc) of the optimized models of 3-methylpyrazole and its related compounds used for the present calculation and their N-H bond distances.

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