Hydrogen bond strength and vibrational assignment of the enol form of 3-(phenylthio)pentane-2,4-dione

The molecular structure of 3-(phenylthio)pentane-2,4-dione (PTPD) has been investigated by means of Density Functional Theory (DFT) calculations and the results were compared with those of the unsubstituted parent, pentane-2,4-dione (known as acetylacetone, AA). The harmonic vibrational frequencies of the cis-enol form were calculated at the B3LYP level using 6-31G∗∗ and 6-311G∗∗ basis sets. The calculated frequencies and the Raman and IR intensities were compared with the experimental results. The infrared and Raman spectra of PTPD and its deuterated analogue are recorded in the 4000–100 cm−1 range. The observed vibrational wave numbers were analyzed in light of the computed vibrational spectra. ing to the theoretical calculations, the hydrogen bond strength for PTPD is 6.1–6.8 kJ/mol stronger than in AA. This result is in agreement with the OH/OD stretching, OH/OD in-plane bending, O···O stretching frequencies, and NMR chemical shift data. l Bond Orbital (NBO) analyses indicate that both steric and resonance effects are responsible for increasing the hydrogen bond strength in PTPD.