Structure, intramolecular hydrogen bonding, and vibrational spectra of 2,2,6,6-tetramethyl-3,5-heptanedione

Molecular structure, intramolecular hydrogen bonding (IHB) and vibrational frequencies of 2,2,6,6-tetramethyl-3,5-heptanedione (TMHD, also known as dipivaloylmethane), have been investigated by means of the density functional theory (DFT) calculations and IR and Raman spectroscopies. In addition, the geometry of the cis- and trans-enol conformers were also optimized at the MP2/6-31G∗∗ level of theory. The results are compared with those of acetylacetone (AA), benzoylacetone (BA), and dibenzoylmethane (DBM). The energy differences between two stable E1 and E2 chelated enol forms, calculated at different levels of theory, are negligible. Comparing the calculated and experimental band frequencies and intensities suggests coexisting of two stable cis-enol conformers in comparable proportions in the sample. Also the IR and Raman spectra of TMHD and its deuterated analogue were clearly assigned. According to the theoretical calculations, TMHD has a hydrogen bond strength of about 17.8–18.0 kcal/mol, calculated at the B3LYP/6-311++G∗∗ level of theory, which is about 3.0 kcal/mol stronger than that of AA. This enhancement in the IHB strength is also consistent with the experimental results of the band frequency shifts for the modes of OH/OD and O ⋯ O stretching and OH/OD out-of-plane bending frequencies. The theoretical calculations and spectroscopic results indicate that the IHB strength of TMHD is between those of AA and DBM.