This is an author version of the contribution published on:Questa

A comparative study of the metal−glycine bonding for the biologically relevant Cu+ and Cu2+ pair is presented. The structure and vibrational frequencies for several coordination modes of Cu+ and Cu2+ to glycine have been determined using the hybrid three-parameter B3LYP density functional approach. Single-point calculations have also been carried out at the modified coupled pair functional (MCPF) and single- and double- (triple) excitation coupled cluster (CCSD(T)) levels of theory and using larger basis sets. Calculations have shown that the metal−glycine bonding and the energy ordering of the different conformers are very different in Cu+-glycine than in Cu2+-glycine. Whereas for Cu+-glycine, the ground state structure is found to have a bidentated η2-N,O coordination in which Cu+ interacts with the nitrogen of the amino group and the carbonyl oxygen, the ground state structure of Cu2+-glycine is the η 2 -O,O (CO2 -) one, derived from the interaction of the metal cation with the CO2 - terminus of the zwitterionic glycine. In this case, the results seem to indicate that glycine acquires an important radical character that changes the relative metal affinities of the different basic sites, which favors the interaction of the metal cation with the CO2 group compared with other coordinations.

Correlated calculations show the proton-transferred OH−H3O+ isomer to be the ground-state structure of the (H2O)2 + dimer ion, with the C 2 h hemibond structure being ca. 8 kcal/mol less stable. Modern density functionals however favor the hemibond structure, overestimating the strength of the three-electron bond by ca. 17 kcal/mol. The wrong prediction of the relative stability of the two isomers is attributed to overestimation by the exchange functionals of the self-interaction part of the exchange energy in the hemibond ion due to its delocalized electron hole. It is cautioned that this erroneous behavior of the density functionals for exchange, if unrecognized, may lead to wrong predictions for ground-state structures of systems with a three-electron bond.

The structure and binding energies are determined for many of the M(H2O)+n and M(H2O)2+n species, for n=1–3 and M=Mg, Ca, or Sr. The trends are explained in terms of metal sp or sdσ hybridization and core polarization. The M(NH3)+n systems, with M=Mg or Sr, are also studied. For the positive ions, the low-lying excited states are also studied and compared with experiment. The calculations suggest an alternative interpretation of the SrNH+3 spectrum.