Molecular structure and vibrational assignment of (trifluoroacetyl) acetone: A density functional study

Intramolecular hydrogen bonding in (trifluoroacetyl) acetone (TFAA) has been investigated by means of density functional theory (DFT) calculations and vibrational spectroscopy. Fourier transform infrared and Fourier transform Raman spectra of TFAA and its deuterated analogue were recorded in the regions 400–4000 and 150–4000 cm−1, respectively. Furthermore, the molecular structure and vibrational frequencies of this compound were investigated by a series of density functional theoretical, DFT, and ab initio calculations at the post-Hartree–Fock (MP2) level. Rigorous normal coordinate analysis has been performed at the B3LYP/6-31G** level of theory. The complete vibrational assignment for TFAA has been made on the basis of the calculated potential energy distribution (PED). Theoretical calculations predicted that 2-TFAA is more stable than the other structures, 5-TFAA and symmetric structure of chelated ring with C2v point group. Both theoretical and experimental methods showed that TFAA has an asymmetric double minimum potential energy surface with a low barrier (about 11.0 kJ mol−1). The geometrical equilibrium parameters, in the most stable structure (2-TFAA), at the B3LYP, B3PW91, BLYP, BPW91, G96LYP and G96PW91 levels of density functional theory were compared with the corresponding parameters of acetylacetone and we concluded that, in contrast to electron diffraction result, TFAA has an asymmetric chelated ring structure with hydrogen bond strength (in 2-TFAA) about 66 kJ mol−1, about 10 kJ mol−1 less than that of acetylacetone. This weakening of hydrogen bond is consistent with frequency shifts for OH/OD stretching, OH/OD out-of-plane bending and skeletal vibration such as asymmetric or symmetric CC–CO stretching modes upon substitution of methyl hydrogen atoms with fluorine atoms. The topological properties of the electron density contributions for intramolecular hydrogen bond in (trifluoroacetyl) acetone and acetylacetone have been analyzed in term of the Bader theory of atoms in molecules (AIM). These results also support the stronger hydrogen bond in the parent molecule with respect to the title compound.