An integrated approach to the study of intramolecular hydrogen bonds in malonaldehyde enol derivatives and naphthazarin: trend in energetic versus geometrical consequences

Abstract

AbstractDensity functional and atoms‐in‐molecules (AIM) and natural bond orbital (NBO) approaches have been applied in the study of energetic (E), geometrical (G) and electronic (AIM and NBO) consequences of H bonding in malonaldehyde (MAE) derivatives and naphthazarin (NZ). AIM parameters and other measures of HB strength were used: (a) for the verification of (i) the reliability of the O···O distance (G consequence) as an indicator of IMHB strength; (ii) the capacity of the classically computed energetic parameters (ΔESEs) to serve as acceptable measures of IMHB strength; and (b) for the separation of the ΔESEs into (i) stabilization (HB) energies (EHSEs) that serve as apparent IMHB energies (EHB,As), and (ii) stabilization (isomerization) energies (ENSEs) that do not (owing to intractable contributions that are not germane to the solitary HB donor(D)‐acceptor(A) interactions). Some of the sources of the anomalies have been rationalized. AIM topological properties were used to study the nature of the IMHB interactions. An exponential parametric model for the correlation of EHSE with the O···O distance, which has asymptotic characteristics at long O···O distances, was obtained. The model (a) has predictive ability, that is, can be used to estimate, in an empirical manner, EHB,As that are otherwise grossly underestimated, and (b) can treat both the MAE derivatives and the NZ systems even though they possess very different resonant spacers connecting the HB D‐A segments. MAE and NZ are also demonstrated to have essentially the same IMHB strength. By contrast, a quadratic model for EHSE‐HB distance correlation was found to be unphysical. Use of electronic consequences of H bonding was shown to be essential for study of IMHBs with intractable interactions. Thus, AIM energy density and NBO second‐order interaction energy parameters were used for the verification of predictions of IMHB strengths made on the bases of energetic and geometrical consequences. Copyright © 2006 John Wiley & Sons, Ltd.