Effect of fluorine substitution on the proton transfer barrier in malonaldehyde. A density functional theory study

The effect of fluorine substitution on the proton transfer barrier (ΔE_PT) of malonaldehyde (1) has been investigated with the aid of density functional theory (DFT) calculations carried out at the B3LYP/ cc-pVTZ level of theory. The results indicate that fluorine substitution in 1 increases the energy barrier for intramolecular proton transfer, which correlates linearly with both the HB distance and the O-H bond stretching frequency. The origin of this structure-energy correlation was investigated by performing a topological analysis of the electron density with Bader's theory of atoms in molecules (AIM). AIM theory indicates that H-by-F substitution in 1 has the effect to decrease the electron density of its intramolecular HB. This implies that conjugation effects are operative within the enolone fragment of 1.