Lone pair interactions with coinage metal atoms: Weak van der Waals complexes of the coinage metal atoms with water and ammonia

Interaction energies between the coinage metal atoms (Cu, Ag, and Au) and lone-pair donating molecules (H2O and NH3) are calculated by the spin adapted restricted open-shell Hartree-Fock coupled cluster method with the scalar relativistic effects accounted for by the Douglas-Kroll approximation. All ammonia complexes CuNH3, AgNH3, and AuNH3 are found to be of C-3v symmetry with the counterpoise corrected interaction energies equal to -16.68, -6.87, and -14.64 mH for Cu, Ag, and Au, respectively. In the case of the water molecule the complexes are much weaker with the counterpoise corrected interaction energies equal to -3.78, -1.81, and -1.77 mH, for the three metal atoms, respectively. Moreover, all complexes with the water molecule are nonplanar. For both lone-pair donating molecules the structure and energetics of their complexes with the coinage metal atoms is mostly due to electron correlation effects. The relativistic effects are found to increase the bonding energies in the series of the ammonia complexes, whereas they reduce the bonding energy in the AgOH2 complex and are essentially negligible for CuOH2 and AuOH2. The calculated complex geometries and interaction energies are discussed in terms of different models. The pattern of interaction energies is discussed in terms of the balance between long-range induction and dispersion contributions and short-range forces. Also the possibility of some charge transfer from the lone-pair donor to the metal atom is considered and supported by analysis of the ionization potential and electron affinity data. The relativistic reduction of the size of the coinage metal atoms is found to be of importance as well. The calculated structural data are used to interpret the experimental observation concerning the existence of well resolved resonantly enhanced multiphoton ionization (REMPI) spectra of the ammonia-silver complexes and the absence of the corresponding spectra of the water-silver complex. This experimental difference between the ammonia and water complexes is explained in terms of the very flat interaction energy function for the wagging motion of the water molecule in AgOH2. This large amplitude vibration makes the structure of the complex undefined and is responsible for the nonexistence of the vibrationally resolved REMPI spectrum of the complex.(C) 2003 American Institute of Physics.