IPPP-CLOPPA Analysis of the Hydrogen Bonds of the Adenine-Thymine Base Pair. Is C-H···O the Third Hydrogen Bond?

The IPPP-CLOPPA method is applied to investigate the feasibility of the C-H···O moiety as a third hydrogen bond in the adenine-thymine base pair, and the role of this intermolecular contact in its stability. For this purpose, an analysis of the interaction energy and the potential energy of the protons of the "conventional" intermolecular hydrogen bonds and the "unconventional" C-H···O contact is performed, in order to assess how much they contribute to the intermolecular stabilization energy of the base pair. On the same grounds, this study is complemented by the analysis of the molecular electric dipolar polarizability of the hydrogen bond moieties, in order to determine the information that this property can give about the electronic mechanisms that affect the stabilization of the hydrogen bonds, the influence of the π system on each one, and the cooperativity effects among them. The results obtained seem to confirm that the C-H···O moiety contributes to the stability of the adenine-thymine pair almost as much as the "conventional" hydrogen bonds do. Besides, this stabilization effect is strengthened by cooperativity between hydrogen bonds and, particularly and mostly, with the π system.