Theoretical studies on keto-enol tautomerism, gas phase acidity and spectral properties of phenylbutazone

Abstract

The keto, enol and anion forms of phenylbutazone were optimized in a gas phase at B3LYP/6-31g(d) level with vibrational analysis. The transitional state between keto and enol forms was estimated and optimized in the same way. The energetic differences between these forms were evaluated in a gas phase and with SCRF in methanol and water with B3LYP, BP86, MP2 and HF methods, using 6-31g(d) and 6-311++g(d,p) basis sets. MP2 calculations were also performed with cc-pVDZ and cc-pVTZ basis sets for comparison. The keto form has the lowest energy and the difference between keto and enol is about 50 kJ/mol. The transitional barrier is about 300 kJ/mol above keto form. Anionic form is about 1400 kJ/mol higher than keto form. The presence of the solvent lowers the energetic difference. The gas phase acidity (B3LYP/6-31g(d)) is equal to 1362.89 kJ/mol. The vibrational spectra were interpreted and compared. The ATR-FTIR experimental spectrum is very close to the theoretical one of keto form, indicating that this form is mainly present in the crystals. Next, the TD-DFT study was carried out, visualizing orbitals and predicting UV spectra in a gas phase, methanol and water. The experimental UV spectrum in methanol has a shape similar to mixed theoretical spectra of keto, enol and anion. The experimental UV spectrum in NaOH solution is very close to  the anion theoretical spectrum, confirming dissociation in this solvent.