Ab initio study of intra- and intermolecular H-bond energies in π-conjugated molecular systems

Ab initio calculation with full geometry optimization has been performed to develop a new approach to separation of contributions of proper H-bonding and π resonance energy increment into the total stabilization energy of π-conjugated molecular systems with intra- and intermolecular H-bonds. In order to reach this goal the molecular geometries of all isomeric enol conformers of malonaldehyde (MA), acetylacetone (AA), 2-formylcyclopentane-1,3-dione (FCPD), 2-acetylcyclopentane-1,3-dione (ACPD) and dimers of MA composed from monomers according to the principle 'like-to-like' have been optimized within the framework of calculation method ab initio at the MP4/6-311G(d,p)//MP2/6-31G(d,p) level of theory. It has been shown that traditional method of the theoretical evaluation of intramolecular H-bond energy (EHB) in pseudo-aromatic six-membered chelate rings gives exceedingly high values (for example, EHB=−10.7 kcal/mol for chelate conformer of MA), and these values better correspond to the total stabilization energy of the molecular systems, including π-electron resonance energy increment. It has been found that intramolecular hydrogen bond energies in chelate rings of MA and AA are equal to 6.3 and 8.9 kcal/mol. Intramolecular H-bond formation in mentioned enols is accompanied by additional energy stabilization of each system due to the redistribution of π-electronic density by 4.5 and 4.6 kcal/mol, respectively. Such calculations were also carried out for dimers and cyclic tetramer of MA, and for endo- and exo-enols of FCPD and ACPD, as well. The question of aromaticity of conjugated chelate rings has been discussed.