Cu + in Liquid Ammonia and in Water: Intermolecular Potential Function and Monte Carlo Simulation Harno D. Pranowo, ² A. H. Bambang Setiaji, ² and Bernd M. Rode* Theoretical Chemistry DiVision, Institute of General, Inorganic and Theoretical Chemistry, UniVersity of Innsbruck, A-6020 Innsbruck, Austria ReceiVed: July 15, 1999; In Final Form: October 8, 1999 The solvation structure of Cu + in water and in liquid ammonia has been investigated using the Metropolis Monte Carlo method. The systems consisting of one Cu + in 215 solvent molecules have been simulated at a temperature of 240 K for ammonia and 298 K for water, respectively. Cu + -ammonia and Cu + -water pair potentials have been newly developed based on ab initio calculations of double- quality. Structural properties were investigated by means of radial distribution functions and their running integration numbers, leading for the first solvation shell to an average coordination number 6 and Cu-N distance of 2.20 Å in ammonia, and to number 6 and Cu-O distance of 2.20 Å in water. The RDFs, coordination number distributions, and pair interaction energy distribution analyses indicate that ligand exchange reactions take place more easily in water than in liquid ammonia. 1. Introduction Binding of Cu + to small molecules has been the focus of several experimental and theoretical studies. These include complexes of Cu + with one and more molecules of water 1-7 and/or ammonia. 8-10 Holland and Castleman 11 reported on the basis of high- pressure mass spectroscopy that copper forms a [Cu(H 2 O) 4 ] + cluster in the gas phase, but no experimental data is available for Cu + in aqueous solution, probably due to the low solubility of its salts in water. Stevenson and his group 12,13 studied the triaminocopper(I) complex in aqueous ammonia using ultraviolet spectroscopy. The stability constants for the stepwise formation of [Cu(NH 3 ) 3 ] + were determined at 1 M ionic strength, resulting in an overall value of 0.05. The preferred Cu + species in aqueous ammonia solution is [Cu(NH 3 ) 2 ] + with a stability constant of 0.86 × 10 5 at 2 M ionic strength. So far, there is no experimental data referring to the microscopic structure of these complexes. The structural and energetic features of the hydration of Cu + in aqueous solution have been studied by Monte Carlo simula- tion, resulting in a coordination number of six for the first hydration shell of the Cu + , 14 corresponding to an octahedral structure in the first shell. The structure of a second hydration shell of Cu + could not be described because of the use of only 20 water molecules in this simulation. In most simulations, pair interaction potentials have been used to describe ion-ligand interactions. It is known, however, that the assumption of pairwise additivity can lead to serious errors in the description of ions in water 4,15,16 and liquid ammonia 17-19 as well, especially for doubly charged cations, as a significant part of the many-body effects is due to polarization effects. 20 The Cu 2+ -NH 3 and Cu 2+ -H 2 O ab initio pair potentials are inadequate to describe the solvation structure of Cu 2+ , leading especially to an overestimation of the coordination number (8 instead of 6) and of hydration energies. 15,18 The inclusion of three-body interactions markedly improves the agreement with experimental data. 21 The electrostatic interaction between Cu + and ligands should be much weaker than in the case of the doubly charged cation Cu 2+ , so that the neglect of 3-body effects seemed acceptable for the present work, especially after some ab initio calculations of [Cu(NH 3 ) n ] + and [Cu(H 2 O) n ] + complexes had been performed in order to estimate their order of magnitude. Pair potential based Monte Carlo simulations were performed, therefore, for systems consisting of one Cu + and 215 ammonia molecules and one Cu + and 215 water molecules, respectively. The results are reported and discussed in terms of structural properties and compared with other theoretical and experimental investigations. 2. Details of the Calculations 2.1. Estimation of Many-Body Effects. To investigate the influence of many-body terms on the interactions between Cu + and water and ammonia, ab initio calculations with energy optimization of Cu(L) n + complexes, where L is H 2 O or NH 3 , and n ) 1-6, were carried out using the double- valence (DZV) basis set of Scha ¨fer et al. 22 for copper. The double- plus polarization (DZP) basis sets of Dunning 23 corresponding to D95V* in the Gaussian 94 program 24 were used for water and ammonia. The experimental gas-phase geometries of the ammonia molecule 25 with N-H distance of 1.0124 Å and HNH angle of 106.67° and for water 26 with O-H distance of 0.9601 Å and H-O-H angle of 104.47° were taken as starting values for the optimization. The stabilization energies of the complexes, ∆E stb , were calculated as where E MLn , E M , and E Ln are the total energies of [Cu(NH 3 ) n ] + or [Cu(H 2 O) n ] + , Cu + and n NH 3 or H 2 O molecules in the same configuration as that of the [Cu(L) n ] + complexes, respectively. ² Permanent address: Austrian-Indonesian Centre for Computer Chem- istry, Gadjah Mada University, Yogyakarta, Indonesia. ∆E stb ) E ML n - E M - E L n (1) 11115 J. Phys. Chem. A 1999, 103, 11115-11120 10.1021/jp992410j CCC: $18.00 © 1999 American Chemical Society Published on Web 11/23/1999