Halogen bonds in adenine-5-bromouracil complexes

Ab initio and density functional calculations were employed to investigate the bonding patterns in the adenine-5-bromouracil (AT⁺) complexes. It is shown that the Br atom in 5-bromouracil (T⁺) is involved in bonding both with the hydrogen atom of the amino group of adenine (A) and with N7(A) (or N1(A)). With this motif, the Br atom interacts with a nucleophile (H) in a "head-on" fashion and an electrophile(N) in a "side-on" fashion, forming both hydrogen and halogen bonds. Electrostatic attraction between the Br atom in T⁺ and N7 (or N1) of adenine was found via the electrostatic potential analysis. The existence of the Br· · ·N interactions in the pairs was further confirmed by means of Bader's atoms in molecules theory. A bond critical point is identified for the halogen bonds and the topological parameters at the bond critical point indicate the typical closed-shell interactions in the pairs. Natural bond orbital analysis suggests that the charge transfer from the lone pair of the nitrogen atom of adenine is mainly directed to the C-Br antibonding orbital. Finally, halogen bonds in the T⁺AT⁺A tetrads were also explored.