Dealing with quasi-ring formation by two hydrogen bonds. Cooperativity analysis with delocalization indices

The quasi-ring formation between monomers structurally similar to DNA base pairs which interact with more than one hydrogen bond may show an extra stabilization linked with the delocalization of π electrons, commonly known as resonance-assisted hydrogen bonds (RAHB). Our aim is to reach a further understanding of their interaction energy and to find out if π delocalization associated with RAHB is an important contribution to it. Pairwise and n-centered electron delocalization indices have been used to study qualitatively the interaction energies. From our results, delocalization indices on hydrogen bonds are helpful to classify their strength and to assess that delocalization within the quasi-ring is not the main component in the sense of energetically assisting the formation of the second HB. In addition, this idea is reinforced by considering the π contribution of the n-centered delocalization index within the ring, which is not directly related to interaction energy. Regarding our results, ring reorganization energy (ERiR) appearing when going from one HB to the quasi-ring formation is mainly due to the electron reorganization within the individual fragments, accounted for by the use of delocalization indices of the main skeleton.