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

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 a-framework (Delta rho(sigma)) and the electronic pi-framework (47,). However the stabilization due to Delta rho(sigma) is four times as important as the stabilization (RAHB) due to Delta rho(pi). Stabilization due to the electrostatic interaction (Delta/E-elstat) between the two monomers that are brought together to form the hydrogen bonds is also important However Delta E-el cannot alone account for the strength of the hydrogen bonds as it is more than compensated for by the repulsive Pauli repulsion (Delta E-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 (-NH2) 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