The molecular structure of 3-(methylthio)pentane-2,4-dione (known as a-methylthio-acetylacetone, MTAA) has been investigated by means of Density Functional Theory (DFT) calculations and the results were compared with those of the unsubstituted parent, acetylacetone (AA). The harmonic vibrational frequencies of the cis–enol form were calculated at the B3LYP level using 6-31G**, 6-311G**, and 6-311++G** basis sets. We also calculated the anharmonic frequencies at the B3LYP level using 6-31G** and 6-311G** basis sets. The calculated frequencies and the IR intensities of light and partially deuterated compounds were compared with the reported experimental results. According to the theoretical calculations, the hydrogen bond strength in MTAA is about 7 kJ/mol stronger than in AA. This result is in excellent agreement with the experimental and theoretical vibrational and NMR spectroscopy data. Natural Bond Orbital (NBO) analyses indicate that both electron delocalization and steric effects are responsible for increasing the hydrogen bond strength in MTAA.

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