Short Intramolecular Hydrogen Bonds: Derivatives of Malonaldehyde with Symmetrical Substituents

A systematic study of various derivatives of malonaldehyde has been carried out to explore very short hydrogen bonds (r OO < 2.450 Å). Various electron-withdrawing groups, including CN, NO2, and BH2, have been attached to the central carbon atom, C2. To C1 and C3, strong electron donors and/or sterically hindered substituents were used to strengthen the intramolecular hydrogen bond, including but not limited to NH2, N(CH3)2, and C(CH3)3. Seven molecules (Figure ) were found to have extremely short intramolecular hydrogen bonds. The chemical systems investigated are intriguing due to their low energetic barriers for the intramolecular hydrogen atom transfers. Classical barriers were predicted using correlated methods including second-order perturbation theory and coupled cluster theory in conjunction with the Dunning hierarchy of correlation consistent basis sets, cc-pVXZ (X = D, T, Q, 5). Focal point analyses allowed for the barriers to be evaluated at the CBS limit including core correlation and zero-point vibrational energy corrections. B3LYP energies are benchmarked against highly accurate correlated energies for intramolecular hydrogen bonded systems. The focal point extrapolated method, including coupled cluster full triple excitation contributions, gives a hydrogen transfer barrier for malonaldehyde of ∼4 kcal mol−1. We describe two compounds with extremely low classical barriers, nitromalonamide (0.43 kcal mol−1) and 2-borylmalonamide (0.60 kcal mol−1). An empirical relationship was drawn between the B3LYP energetic barriers and the predicted coupled cluster barriers at the CBS limit. By relating these two quantities, barrier heights may be estimated for systems too large to presently use highly correlated electronic structure methods.