Dynamic NMR spectroscopic and computational studies of 1H-pyrazoles: determination of the prototropic barriers

Abstract

Four 1H pyrazoles, two of which are 4-nitro substituted, were studied using 1H and 13C NMR spectroscopy in HMPA-d18 and in CD3OD, the two most prevalent solvents for such investigations. Variable-temperature experiments were conducted in the latter solvent to determine the energy barriers to proton transfer. A theoretical model involving two methanol molecules satisfactorily accounts for the experimental results, including the high entropy values observed during the process. Additionally, solvent effects were analyzed using a Free-Wilson matrix, incorporating data from the existing literature.