The malonaldehyde equilibrium geometry: A major structural shift due to the effects of electron correlation

Complete theoretical optimizations of the equilibrium geometry of malonaldehyde have been carried out within the framework of the self-consistent-field (SCF) approximation. Both Huzinaga–Dunning double zeta plus polarization (DZ+P) and Pople 6–31G** basis sets have been used, resulting in very similar results. The predicted O ⋅ ⋅ ⋅ H hydrogen bond distance is 1.88 Å, in poor agreement with the value 1.68 Å deduced from experiment. It appears that the Hartree–Fock approximation is incapable of describing the equilibrium geometry of malonaldehyde in a qualitatively correct manner. However, second-order perturbation theory yields a structure (O ⋅ ⋅ ⋅ H distance 1.69 Å) in good agreement with experiment. The structures of the keto tautomer and the transition state for symmetric intramolecular hydrogen transfer have also been determined, as have harmonic vibrational frequencies for all stationary points.