Synthesis, spectroscopic and computational characterization of the tautomerism of pyrazoline derivatives from chalcones

• Synthesis of a series of new pyrazolines derived from chalcones is described.
• NMR and Raman spectroscopy study on the tautomerization equilibrium in the series of compounds.
• Energy and vibrational computational DFT calculations allowed understanding the tautomerization equilibrium.

In the present study a series of novel pyrazolines derivatives has been synthesized, and their structures assigned on the basis of FT-Raman, 1H and 13C NMR spectral data and computational DFT calculations. A joint computational study using B3LYP/6-311G(2d,2p) density functional theory and FT-Raman investigation on the tautomerism of 3-(4-substituted-phenyl)-4,5-dihydro-5-(4-substituted-phenyl)pyrazole-1-carbothioamide and 3-(4-substituted-phenyl)-4,5-dihydro-5-(4-substituted-phenyl)pyrazole-1-carboxamide are presented. The structures were characterized as a minimum in the potential energy surface using DFT. The calculated Raman and NMR spectra were of such remarkable agreement to the experimental results that the equilibrium between tautomeric forms has been discussed in detail. Our study suggests the existence of tautomers, the carboxamide/carbothioamide group may tautomerize, in the solid state or in solution. Thermodynamic data calculated suggests that the R(CS)NH2 and R(CO)NH2 species are more stable than the R(CNH)SH and R(CNH)OH species. Additionally, results found for the 1H NMR shifting, pointed out to which structure is present.

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