Conformational stability, molecular structure, intramolecular hydrogen bonding, and vibrational spectra of 5,5-dimethylhexane-2,4-dione

Complete conformational analyses of all possible keto and enol forms, molecular structure, intramolecular hydrogen bonding (IHB), and vibrational frequencies of 5,5-dimethyl hexane-2,4-dione (DMHD, also known as acetylpinacolin and pivalylacetone), were investigated by means of ab initio calculations and IR and Raman spectroscopies. The results are compared with those of acetylacetone (AA) and 2,2,6,6-tetramethyl-3,5-heptanedione (TMHD). The energy differences between three stable E1, E2, and E3 chelated enol forms are negligible. Comparing the calculated and experimental band frequencies and intensities suggests the coexisting of these three conformers in comparable proportions in the sample. The vibrational frequencies of DMHD and its deuterated analogue were also clearly assigned. According to the theoretical calculations, the stable cis-enol conformers of DMHD have an average hydrogen bond strength of 16.6 kcal/mol, calculated at the B3LYP/6-311++G** level, which is about 0.7 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 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 DMHD is between those of AA and TMHD.

► Substitution of bulky groups (such as t-butyl) in the β-positions of acetylacetone increases the enol content in the sample.
► Both theoretical and experimental data indicate that the hydrogen bond in DMHD is stronger than that in acetylacetone.
► More than one enol conformer exists in the sample.