Normal coordinate analysis, hydrogen bonding, and conformation analysis of heptane-3,5-dione

• The theoretical and experimental results confirm that HPD's IHB is the same as AA.
• The IHB strength of HPD has been estimated by using DFT, AIM, NMR, IR, and Raman.
• We study vibrational spectra and NMR chemical shifts theoretically and experimentally.
• Existence of some cis-enol forms could be confirmed by IR, Raman and DFT results.

Fourier transform Raman and infrared spectral measurements have been made for the heptane-3,5-dione (HPD) and simultaneously compared with those of acetylacetone (AA) to give a clear understanding of substitution effect of ethyl groups (in β-positions) on the structure, electron delocalization, and intramolecular hydrogen bonding (IHB). Molecular structure, conformational stabilities, and intramolecular hydrogen bonding of different oxo-enol forms of HPD, have been investigated by MP2, BLYP, B2PLYP, TPSSh, and B3LYP methods, using various basis sets, and experimental results. The energy differences between four stable E1-E4 chelated forms are relatively negligible. The theoretical and experimental results obtained for stable oxo-enol forms of HPD have been compared with each other and also with those of AA. According to the theoretical calculations, HPD has a hydrogen bond strength of about 15.9 kcal/mol, calculated at the B3LYP/6-311++G** level, which is the same as AA, 15.9 kcal/mol. This similarity in the IHB strength is also consistent with the experimental results of the band frequency shifts for the OH/OD and O···O stretching and OH/OD out-of-plane bending frequencies and chemical shift of the O–H group. The molecular stability and the hydrogen bond strength also were investigated by applying the topological analysis, geometry calculations, and spectroscopic results. Potential energy distribution (PED) and normal coordinate analysis have also been performed. A complete assignment of the observed band frequencies has been suggested the presence of four HPD forms at comparable amounts in the sample.

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