OPEN-SHELL VAN-DER-WAALS COMPLEXES OF THE COINAGE METALS - CU...H20, AG...H2O, CU...H2S, AND AG...H2S

The interaction potential curves for complexes between the coinage met al atoms (Me = Cu, Ag) and H2X (X = O, S) molecules are calculated for three selected configurations. Two of them represent H-bonded structu res while the third corresponds to direct interactions between Me and the lone pair carrier of XH2. Most calculations have been carried out at the level of the restricted open-shell Hartree-Fock (ROHF) formalis m followed by a perturbation treatment of the electron correlation con tribution (CASPT2). For some selected points the ROHF-based spin-adapt ed coupled cluster method (ROHF SA CCSD) has been used. The relativist ic effects are accounted for through the mass-velocity and Darwin (MVD ) corrections. We found remarkable differences in the size and the sig n of MVD corrections to interaction energies for different configurati ons. For all structures bonded through the hydrogen of H2X (H-bonded s tructures) the ROHF interaction potentials turn out to be essentially repulsive, the attractive force due to induction effects being negligi ble in comparison with the valence repulsion for all but very large in tersystem distances. However, the direct Me...O interaction produces a significant minimum even at the level of the ROHF approximation. At t he correlated levels of approximations all structures of all systems a re attractive with binding energies of the order of 1 kcal mol-1. In t he case of the water molecule complexes, lone pair interaction is favo ured so that the Me...OH2 bonding resembles the usual H-bonds with the Me atom playing the role of the electron acceptor. It is suggested th at the Me-bonding in systems of the form Me...OH2 may represent the co inage metal counterparts of the H-bonding. On the contrary, H-bonding is found to give the most stable structures for the Me...H2S complexes , the interaction energies being slightly higher than those for the co rresponding complexes with the water molecule.