Theoretical Study on the Gas-Phase Acidity of Multiple Sites of Cu+−Adenine and Cu2+−Adenine Complexes

The acidities of multiple sites in Cu+−adenine and Cu2+−adenine complexes have been investigated theoretically. To compare, the acidities of adenine (A) and adenine radical cation (A•+) have also been included. The results clearly indicate that the acidities of C−H and N−H groups in Cu+/2+−adenine are significantly enhanced relative to the neutral adenine. The acidic order for a given site on adenine and adenine derivatives is as follows: Cu2+−adenine > A•+ > Cu+−adenine > A. For Cu+−adenine and Cu2+−adenine, N3-coordination exhibits N9−H acid, and N1- and N7-coordination exhibits N6−Ha and N6−Hb acid, respectively. Additionally, it is found that C2−H group is surprisingly acidic in the coordination complexes. Calculations in aqueous solution reveal that our results can be extrapolated to aqueous solution. Analyses of the electronic properties interpret the highest acidity of Cu2+−adenine among the adenine derivatives studied. Also, Electrostatic potential calculations of [A(−H+)]− and [A(−H+)]• indicate that the removal of Ha or Hb from the amino group favors the bidentate coordination, which provides a dative bond from the deprotonated N and the original coordination ligand to copper ion besides the electrostatic interaction between them and thereby stabilizes the [A(−H+)]−/[A(−H+)]•. NBO analysis confirms the electrostatic potential result.