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

Abstract

We have analyzed hydrogen bonding in a number of species, containing from two to four hydrogen bonds. The examples were chosen in such a way that they would enable us to examine three different hydrogen bonds involving OH-O, NH-O, and NH-N. A common feature of the investigated systems is that they all are expected to exhibit resonance assisted hydrogen bonding (RAHB) in the electronic pi-framework. Our analysis was based on a recently developed method that combines the extended transition state scheme with the theory of natural orbitals for chemical valence (ETS-NOCV). We find that hydrogen bonding is associated with charge rearrangement in both the electronic sigma-framework (Deltarho(sigma)) and the electronic pi-framework (Deltarho(pi)). However the stabilization due to Deltarho(sigma) is four times as important as the stabilization (RAHB) due to Deltarho(pi). Stabilization due to the electrostatic interaction (DeltaE(elstat)) between the two monomers that are brought together to form the hydrogen bonds is also important. However DeltaE(el) cannot alone account for the strength of the hydrogen bonds as it is more than compensated for by the repulsive Pauli repulsion (DeltaE(Pauli)). When N' is part of an aromatic ring, N'H-O and N'H-N bonds are similar in strength to OH-O links involving carboxylic groups. However, NH-O bonds involving amide groups (-NH(2)) are considerably weaker than the OH-O links mentioned above. In systems with different hydrogen bonds, their relative strength is determined collectively in such a way as to optimize the total interaction. This can result in that one of the bonds (OH-O, NH-O, and NH-N) becomes particularly strong or exceptionally weak. Even within the same dimer two X'-HX bonds of the same type can show quite different strength.

The Role of Aromaticity, Hybridization, Electrostatics, and Covalency in Resonance-Assisted Hydrogen Bonds of Adenine-Thymine (AT) Base Pairs and Their Mimics

“…In previous work, we established theoretically that, for the hydrogen bonds in DNA base pairs, the electrostatic interactions and orbital interactions are of equal importance, and that, indeed, the π electrons provide an additional stabilizing component. This finding was reconfirmed by others . However, our work also showed computationally that the synergetic interplay between the delocalization in the π‐electron system and the donor–acceptor interactions in the σ‐electron system was small, that is, the simultaneous occurrence of the π and σ interactions is only slightly stronger than the sum of each of these interactions occurring individually.…”

ETS-NOCV description of σ-hole bonding

“…This picture is further enriched by providing the energetic contributions (Δ E i ) to the bond energy within ETS-NOCV scheme [39]. It has been shown that NOCV’s lead to a compact description of not only metal–ligand or covalent connections [36, 37, 39], but also of hydrogen bonding [40]. We have verified recently the applicability of ETS-NOCV scheme in a description of halogen bonding, showing in particular, that the dominating contribution to the deformation density exhibits the negative-value area that corresponds to σ-hole [41].…”

Evidences for Cooperative Resonance-Assisted Hydrogen Bonds in Protein Secondary Structure Analogs

“…For example, the electron transfer chain between two heme rings in cytochrome b 561 is clarified as serials of RAHBs16. However, some other works attribute the cooperativity to long-range electrostatic interactions17, van der Waals interaction18, σ- orbital interaction19, electronic σ- framework rearrangement20 or charge interaction21. Unfortunately, there has been no experimental evidences reported on RAHB in proteins and protein analogs, to the best of our knowledge.…”