How resonance assists hydrogen bonding interactions: An energy decomposition analysis

Abstract

Block-localized wave function (BLW) method, which is a variant of the ab initio valence bond (VB) theory, was employed to explore the nature of resonance-assisted hydrogen bonds (RAHBs) and to investigate the mechanism of synergistic interplay between delocalization and hydrogen-bonding interactions. We examined the dimers of formic acid, formamide, 4-pyrimidinone, 2-pyridinone, 2-hydroxpyridine, and 2-hydroxycyclopenta-2,4-dien-1-one. In addition, we studied the interactions in -diketone enols with a simplified model, namely the hydrogen bonds of 3-hydroxypropenal with both ethenol and formaldehyde. The intermolecular interaction energies, either with or without the involvement of resonance, were decomposed into the Hitler-London energy (DE HL ), polarization energy (DE pol ), charge transfer energy (DE CT ), and electron correlation energy (DE cor ) terms. This allows for the examination of the character of hydrogen bonds and the impact of conjugation on hydrogen bonding interactions. Although it has been proposed that resonance-assisted hydrogen bonds are accompanied with an increasing of covalency character, our analyses showed that the enhanced interactions mostly originate from the classical dipoledipole (i.e., electrostatic) attraction, as resonance redistributes the electron density and increases the dipole moments in monomers. The covalency of hydrogen bonds, however, changes very little. This disputes the belief that RAHB is primarily covalent in nature. Accordingly, we recommend the term ''resonance-assisted binding (RAB)'' instead of ''resonance-assisted hydrogen bonding (RHAB)'' to highlight the electrostatic, which is a long-range effect, rather than the electron transfer nature of the enhanced stabilization in RAHBs.

Theoretical Analysis of the Resonance Assisted Hydrogen Bond Based on the Combined Extended Transition State Method and Natural Orbitals for Chemical Valence Scheme†

“…We note that our estimate of ∆E int ) -18.9 kcal/mol for 3 is in good agreement with the MP2 value of -17.2 kcal/mol obtained by Beck and Mo. 63 These authors made use of decomposition scheme similar to that of Góra 62 et al They were only indirectly able to estimate a resonance energy ∆E π RAHB with a value of -12.3 kcal/mol, which seems very large in absolute terms.…”

Theoretical description of hydrogen bonding in oxalic acid dimer and trimer based on the combined extended-transition-state energy decomposition analysis and natural orbitals for chemical valence (ETS-NOCV)

“…However, a few theoretical studies were reported recently for familiar molecules, i.e . for pyrrole-2-carboxylic acid and formic acid dimers, containing as well the two equivalent O---HO interactions, supported further by RAHB contribution [57, 58]. In these studies the electronic enthalpies (per one O---HO hydrogen bond) are−5.8 kcal mol −1 and−8.5 kcal mol −1 for formic and pyrrole-2-carboxylic dimers, respectively.…”

The nature of resonance-assisted hydrogen bonds: a quantum chemical topology perspective

“…Such a reduction is of course accompanied by a concomitant increase in the total number of localised electrons, and becomes a powerful argument against the traditional role assigned to resonance in RAHBs, in agreement with previous experimental and theoretical studies which found no indication of an increased electron delocalisation in these types of H-bonds. [29][30][31][32][33][36][37][38][39][40] Since, as stated above, the interacting quantum atom partition provides a unique combination of electron and energy descriptors, we may now explore the energy features of the formation of the HB associated with the previously discussed changes in R(r) and R 2 (r 1 ,r 2 ). Following the previous line of reasoning, we first examine the energetics of resonance and bond order equalization, which are indicated in terms of the covalent, i.e.…”