Conformation, molecular structure, and intramolecular hydrogen bonding of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione

Conformations, molecular structure, keto-enol content, and intramolecular hydrogen bonding of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione (also named as (Z)-6,6,6-trifluoro-5-hydroxy-2,2-dimethylhex-4-en-3-one), known as pivaloyltrifluoroacetone, have been investigated by means of the density functional theory calculations and experimental NMR, IR and Raman spectroscopies. The geometries and the electronic energies of different cis-enol conformers of this molecule have been obtained at BLYP, B3LYP, BPW91, B3PW91, G96LYP, G96PW91 and also MP2 levels of theory, using various basis sets. The results of conformational stabilities, molecular structures, NBO atomic charges, estimated energies of intramolecular hydrogen bonding, barrier height for proton transfer, and some spectroscopic properties related to the intramolecular hydrogen bonding for stable chelated enol forms of the target molecule have been compared with each other and also with those of some similar compounds. This is suggested by the theoretical studies that this β-diketone is completely in the enol form, in the best agreement with the experimental analyses done. Also, the theoretical calculations besides the 1H NMR and vibrational spectroscopic results indicate that the intramolecular hydrogen bond strength of this molecule is considerably weaker than that of 5,5-dimethyl-2,4-hexanedione, but stronger than that of 1,1,1-trifluoro-2,4-pentanedione.

► All possible conformations of TFDMHD are theoretically studied.
► The intramolecular hydrogen bond of TFDMHD is theoretically investigated.
► The NMR and vibrational spectroscopies are used to check the theoretical studies.
► The NMR and vibrational bands related to the IHB are assigned.
► This is confirmed that the TFDMHD’s IHB strength is between those of TFAA and DMHD.