An ab initio study of the rotamers and rotations of propane-1,3-dial by DFT and SCF calculations

Eight planar rotamers of the enol form of malonaldehyde were considered at the HF (Hartree-Fock) and DFT (density functional theory) levels with 6-311G** and D95** (Dunning/Huzinaga full double-zeta) basis sets with the aim to establish the most stable of them and to find the energy barriers of their conversions. The results show that the rotamer with an intramolecular hydrogen bond is the most stable one. High energy barriers were ascertained for the conversions including rotations around a CC double bond. Most of the reactions connected with breaking of the hydrogen bond display strongly asymmetric energy barriers. Their transition states were determined as first-order saddle points because of one imaginary frequency in the IR spectrum related with a negative energy gradient.