Open-shell van der Waals complexes of the coinage metals: Cu…H 2O, Ag…H 2O, Cu…H 2S, and Ag…H 2S

Abstract

The interaction potential curves for complexes between the coinage metal atoms (Me ≡ Cu, Ag) and H 2X (X ≡ O, S) molecules are calculated for three selected configurations. Two of them represent H-bonded structures while the third corresponds to direct interactions between Me and the lone pair carrier of XH 2. Most calculations have been carried out at the level of the restricted open-shell Hartree-Fock (ROHF) formalism followed by a perturbation treatment of the electron correlation contribution (CASPT2). For some selected points the ROHF-based spin-adapted coupled cluster method (ROHF SA CCSD) has been used. The relativistic effects are accounted for through the mass-velocity and Darwin (MVD) corrections. We found remarkable differences in the size and the sign of MVD corrections to interaction energies for different configurations. For all structures bonded through the hydrogen of H 2X (H-bonded structures) the ROHF interaction potentials turn out to be essentially repulsive, the attractive force due to induction effects being negligible in comparison with the valence repulsion for all but very large intersystem distances. However, the direct Me…O interaction produces a significant minimum even at the level of the ROHF approximation. At the correlated levels of approximations all structures of all systems are attractive with binding energies of the order of 1 kcal mol −1. In the case of the water molecule complexes, lone pair interaction is favoured so that the Me…OH 2 bonding resembles the usual H-bonds with the Me atom playing the role of the electron acceptor. It is suggested that the Me-bonding in systems of the form Me…OH 2 may represent the coinage metal counterparts of the H-bonding. On the contrary, H-bonding is found to give the most stable structures for the Me…H 2S complexes, the interaction energies being slightly higher than those for the corresponding complexes with the water molecule.