Substituent R-Effects on Intermolecular Resonance-Assisted Hydrogen (H) Bonds: Theoretical Analysis of Double H-Bonded Dimers of Carboxylic Acids

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<jats:title>Abstract</jats:title> <jats:p>Substituent dependence of geometries and binding energies calculated for a number of carboxylic acid dimers with R-substituents using 2nd-order Møller–Plesset perturbation theory and Gaussian 4 level quantum mechanical method, is analyzed to characterize intrinsic substituent effects. Charge distribution and charge-transfer interaction of reaction centers are examined to evaluate π-electron delocalization and individual H-bond structures using natural bond orbital analysis. For (RCOOH)2 homodimers, parametric substituent-treatment by Taft and Topsom revealed that π-donor substitution increases the π-electron delocalization of conjugated C=O/C–O frames of carboxylic acids and cooperatively stabilizes the equivalent H-bonds depending on the potentiality |σR+| of π-resonance effect, i.e., substituent-enhanced RAHBs (resonance-assisted H-bonds). In contrast, π-accepting substituents are found actually inactive in these electron deficient C=O/C–O fields yielding the minimum available RAHBs similar to the reference (HCOOH)2. In (R1–COOH)(R2–COOH) heterodimers, the π-accepting substituent can only be activated under the presence of π-donor potentiality from the counter molecule, generating a cooperative substituent push–pull interaction through the corresponding H-bond. Out of two H-bonds generated, one is significantly strengthened while the other being quenched due to the substituent effects in the opposite direction. The intermolecular electron push–pull interaction is quantitatively interpreted in terms of the charge-transfer of each H-bond influenced by the π-resonance substituent effects.</jats:p>

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