Theoretical model of Cu(I)/Cu(II) hydration under the influence of variable ligand field.

Jaroslav V. Burda^a, Matěj Pavelka^a, Milan Šimánek^a, and Manoj Shukla^b

^aDepartment of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech Republic

^bDepartment of Chemistry, Jackson State University, 1325 J. R. Lynch Street, Jackson, Mississippi 39217-0510, USA

An ab initio second-order perturbation theory (MP2) and DFT calculation were performed on the copper monovalent and divalent cations. Different number of the water molecules was considered for the pure Cu(H₂O)_n^m+ ions where n=(1 to 6), m=1 or 2. In the next step, copper complexes with four and six ligands were regarded where the number of the ammonium molecules was varied. As a final step complexes of Cu-guanine with 5 water molecules were calculated.

Pure copper hydration clearly shows that while Cu(II) complexes are relatively good stabilized up to coordination with six water molecules, the Cu(I) cation prefers only two-water coordination when electron-correlation effects are included. The other water molecules leave to second hydration sphere. Some interesting theoretical consequences appear when only Hartree-Fock level is taken into account.

Under the ammonium surrounding, the individual stabilities are changed but all qualitative conclusions found for pure water complexes remain valid. Also, some small changes in geometries are apparent as a consequence of different ligand strength of the ammonium molecule.

A slightly stronger ligand – guanine in N₇ site also prefers two coordinated Cu(I) complex with the other water escaped from first hydration sphere of Cu cation. Interesting feature, from the structural point of view, can be regarded as the position where the other water molecules are placed in comparison with the hydration of the plain guanine molecule. On the contrary, hydrated complex Cu(II)-guanine exhibits the largest stabilization among all the studied Cu complexes.