A further look at π-delocalization and hydrogen bonding in 2-arylmalondialdehydes

This paper examines in detail the tautomerism of 2-aryl-substituted malondialdehydes, mainly focused on enolal structures, by means of gas-phase and solvent-based DFT calculations performed at the M06-2X/6-311++G(d,p) level. Open and pseudo-cyclic structures can be envisaged, the latter being decorated by an intramolecular hydrogen bond acting in symbiosis with π-electron delocalization. We have checked the relationship between the computed hydrogen bond energies and a variety of descriptors for electron delocalization. Application of the natural bond orbital (NBO) method to interrogate the nature of donor-acceptor interactions unravels that stabilization energies mainly arise from lone-pair and σ-antibonding interactions. Transition structures involved in proton transfer along the reaction pathway could be identified and TS exhibit a nearly complete electron delocalization characteristic of aromatic systems. All data gathered herein provide a description of quasi-aromaticity more consistent with hydrogen-bonding-assisted resonance (HBAR) than the widely held RAHB (resonance-assisted hydrogen bonding) formalism. Overall, the paper does not invoke a new concept, but rather the fact that the effects of H-bonding on delocalization and the effects of the latter on H-bonding can accurately be captured by computation, yet representing two sides of the same coin.