Computational Evidence for a Variable First Shell Coordination of the Cadmium(II) Ion in Aqueous Solution Giovanni Chillemi,* Vincenzo Barone, † Paola D’Angelo, ‡ Giordano Mancini, ‡ Ingmar Persson, ⊥ and Nico Sanna § CASPUR, Consortium for Supercomputing in Research, Via dei Tizii 6b, 00185 ROMA, Italy, and Dipartimento di Chimica, UniVersita ` di Roma “La Sapienza”, P.le Aldo Moro 5, 00185 ROMA, Italy, and Dipartimento di Chimica, UniVersita ` di Napoli “Federico II”, Complesso UniVersitario di Monte S. Angelo, Via Cintia, 80126 NAPOLI, Italy, and Department of Chemistry, Swedish UniVersity of Agricultural Sciences, P.O. Box 7015, SE-750 07 Uppsala, Sweden ReceiVed: January 26, 2005; In Final Form: March 3, 2005 In this paper, we present a state-of-the-art 100 ns molecular dynamics simulation of a cadmium(II) aqueous solution that highlights a very flexible ion first coordination shell which transits between hexa- and heptahydrated complexes. From this investigation, a dynamical picture of the water exchange process emerges that takes place through an associative mechanism for the solvent substitution reaction. Our procedure starts from the generation of an effective two-body potential from quantum mechanical ab initio calculations in which the many-body ion-water terms are accounted for by the polarizable continuum method (PCM). This approach is computationally very efficient and has allowed us to carry out extremely long molecular dynamics simulations, indispensable to reproduce the dynamic properties of the cadmium(II) aqueous solution. Quantum mechanical ab initio calculations of the hexa- and heptahydrated complexes extracted from MD configurations have revealed stable minima for both clusters with the water molecules arranged in T h and C 2 symmetries in the hexa- and heptahydrated complexes, respectively, with a slight energetic preference for the heptahydrated one. Finally, a comparison of the calculated hexa- and heptahydrated cluster IR and Raman spectra with the experimental data in the literature, has demonstrated that the IR spectroscopy is not able to distinguish between the two species, whereas the Raman spectrum of the Cd 2+ -(H 2 O) 7 cluster provides a better agreement with the experimental data. 1. Introduction A detailed understanding of aqueous electrolyte solutions is of fundamental importance to the chemical physics of solvation. Consequently, a substantial amount of research work has been aimed to the description of the physical properties of solutions, especially with the purpose of elucidating the coordination structure of hydrated ions. 1 Experimental techniques such as X-ray diffraction, neutron diffraction, Raman, X-ray absorption spectroscopy, and NMR have been employed to perform structural investigations of hydrated ions. However, the char- acterization of the dynamic properties of ions and water molecules in the bulk and in the hydration spheres is more elusive and very difficult to be obtained from experimental techniques only. On the contrary, the combined use of experi- mental and theoretical methods has expanded our knowledge on the water exchange mechanism of electrolyte solutions, 2 and it has been recently successfully applied, for example, to highlight the existence of a flexible coordination shell of water molecules around the calcium(II) ion. 3 Computer simulation techniques, such as molecular dynamics (MD), are powerful tools in the analysis of both static and dynamic properties of solvated ions in solution and have been extensively used, in the last two decades, for the study of aqueous electrolyte solutions. 1 Historically, the first studies were carried out using two-body ion-water interaction potentials with a limited number of parameters, such as Coulombic and Lennard-Jones terms. Classical MD simulations are still carried out using additive potential models, which are usually built using data obtained from experiments (i.e., solvation enthalpy or free energy) or from ab initio quantum mechanical calculations. However, it has been demonstrated that this class of models is not suf- ficiently accurate for a reliable description of the ion-solvent interaction, and the inclusion of many-body effects is crucial. Recently, ab initio quantum mechanics (QM) or mixed QM/ molecular mechanics (MM) MD simulations have been used to circumvent the need of accurate analytical potentials 4 even if this approach is limited both in the dimension of the systems (hundreds of atoms) and in the length of the simulation (tens of picoseconds), owing to its computational cost. Although this very limited statistic has enabled some insight into the fast exchange mechanism in the first solvation shell of the Au(I) ion in aqueous solution 5 and of the Cu(II) ion in ammonia solution to be gained, 6 the study of dynamic mechanisms in the nanosecond time scale is well beyond the capability of this methodology. Recently, we demonstrated that improvements and optimiza- tions of the polarizable continuum model (PCM) allow for a very efficient inclusion of the averaged many body ion-water effects in a two-body classical potential (effective two-body potential). 7 In particular, we produced, for the first time, an * To whom correspondence should be addressed. E-mail: g.chillemi@ caspur.it. † Universita ` di Napoli “Federico II”. ‡ Universita ` di Roma “La Sapienza”. § CASPUR. ⊥ Swedish University of Agricultural Sciences. 9186 J. Phys. Chem. B 2005, 109, 9186-9193 10.1021/jp0504625 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/30/2005