Intramolecular hydrogen bonding and vibrational assignment of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione

Intramolecular hydrogen bonding (IHB) and vibrational frequencies of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione (TFDMHD) have been investigated by means of density functional theory (DFT) calculations, Atoms in Molecules (AIM) analysis, natural bond orbital (NBO) theory, and IR and Raman spectroscopies. The results are compared with those of 1,1,1-trifluoro-2,4-pentanedione (TFAA) and 5,5-dimethyl hexane-2,4-dione (DMHD). The hydrogen bonding energies (EHB) of the most stable chelated enol forms, obtained by AIM theory are in the range of 17.1–20.0 kcal/mol. The IR and Raman spectra of TFDMHD and its deuterated analog were clearly assigned. Comparing the calculated and experimental band frequencies and intensities suggests coexisting of three stable cis-enol forms in comparable proportions in the sample. The theoretical calculations and spectroscopic results indicate that the IHB strength of TFDMHD is between those of TFAA and DMHD.