Evaluation the origin of conformational preferences in trifluoroacetylacetaldehyde by detail analysis of the intramolecular hydrogen bond and π-electron delocalization in the ground and first excited states

In the current research, the origin of conformational preferences in trifluoro-acetylacetaldehyde (TFAAD) by detail analysis of the intramolecular hydrogen bond (IMHB) and pi-electron delocalization (pi-ED) was investigated. In this regard, all of the possible conformations of keto and enol tautomers of TFAAD at various theoretical levels were optimized. Our results reveal that the chelated enol forms (E11, E21) have extra stability with respect to the other conformations, and at all of the computational levels E21 is more stable than the E11 and identifies as the global minimum. We expected that the extra stability of E21 is probably related to the stronger intramolecular hydrogen bond (IMHB), but quantum chemical calculations confirm the stronger IMHB of E11. Since, the chelated forms are resonance assisted hydrogen bond (RAHB) systems, it seems that the pi-ED concept can probably justify this duality. The significance of pi-ED by the geometrical factor of Gilli (lambda) and the harmonic oscillator model of aromaticity (HOMA) were assessed. According to these indicators, E21 has higher pi-ED than E11, in excellent agreement with their stability order. Finally, the population analysis by the natural bond orbital method and atoms in molecules theory were carried out. The stabilization charge transfer energies of RAHB systems also indicate more pi- ED in E21. Consequently the pi-electron delocalization effect is the superior factor determining the global minimum. The IMHB of the chelated forms, in the first singlet excited state, were also studied. Surprisingly, the results show that the IMHB in E21 is stronger than in E11, in contrast to the ground state.