Evidence for Sevenfold Coordination in the First Solvation Shell of Hg(II) Aqua Ion Giovanni Chillemi, ² Giordano Mancini, ²,‡ Nico Sanna, ² Vincenzo Barone,* ,§ Stefano Della Longa, | Maurizio Benfatto, ⊥ Nicolae V. Pavel, ‡ and Paola D’Angelo* ,‡ Contribution from the CASPUR, Consortium for Supercomputing. Applications,Via dei Tizii 6b, 00185 Rome, Italy, Department of Chemistry, UniVersity of Rome “La Sapienza”, P.le Aldo Moro 5, 00185 Rome, Italy, Department of Chemistry, UniVersity of Naples Federico II, Via Cintia, 80126 Naples, Italy, Department of Experimental Medicine, UniVersity of L’Aquila, 67100 L’Aquila, Italy, and INFN, Frascati National Laboratories, 00044 Frascati, Italy Received October 2, 2006; E-mail: baronev@unina.it; p.dangelo@caspur.it Abstract: A quite unexpected sevenfold coordination of the hydrated Hg(II) complex in aqueous solution is revealed by an extensive study combining X-ray absorption spectroscopy (XAS) and quantum mechanics/ molecular dynamics (QM/MD) calculations. As a matter of fact, the generally accepted octahedral solvation of Hg(II) ion cannot be reconciled with XAS results. Next, refined QM computations point out the remarkable stability of a heptacoordinated structure with C2 symmetry, and long-time MD simulations by new interaction potentials including many-body effects reveal that the hydrated complex has a quite flexible structure, corresponding for most of the time to heptacoordinated species. This picture is fully consistent with X-ray absorption near-edge structure experimental data which unambiguously show the preference for a sevenfold instead of a sixfold coordination. Introduction Mercury is a toxic and an environmentally hazardous element able to replace biological Zn(II) in enzymes, proteins, and nucleic acids, altering the normal activity of these species. 1 Once released to the environment by a variety of sources (e.g., waste combustors or coal-fired power plants), metallic mercury is oxidized to Hg(II) by water and ozone, and when it falls on the ground of acidic soil (around pH ) 4), it transforms to the neurotoxin methyl mercury that causes severe neurological damages (e.g., Minamata disease). 2,3 Despite the calamitous effects on human health arising from mercury pollution of streams, lakes, and oceans, 4 the solution structure of aqua Hg- (II) ion is still poorly defined due to the lack of experimental techniques able to provide reliable information about the coordination structure of this ion. 5 Hg(II) is normally described as being hexacoordinated by water, 5,6 but due to the occurrence of hydrolysis and of different coordination geometries that mercury adopts in complexes, a conclusive description of the structural properties of this ion is still lacking. 5 Moreover, like Zn(II) and Cd(II), the d 10 Hg(II) ion has no electronic spectroscopic handle, and the few attempts made to study its hydration structure by X-ray diffraction are in support of retention of the same octahedral structure as present in the solid- state X-ray structure of Hg(ClO 4 ) 2 ‚6H 2 O. 6 However, the radial distribution function obtained from X-ray diffraction of Hg(II) in aqueous solution showed a broad peak corresponding to an unexpectedly large variation in the Hg-O bond lengths. 7 The wide bond distance distribution has been explained by a pseudo Jahn-Teller effect in the hexahydrated mercury(II) complexes leading to four equatorial Hg-O bonds about 0.05 Å shorter than the axial ones. 7 In addition, the residence time of water molecules in the first hydration shell of Hg(II) is quite short (of the order of nanoseconds) 8 as compared with divalent first row transition ions which form octahedral hydration complexes in aqueous solution. 5 We have undertaken a combined experimental and theoretical investigation to unveil the detailed structure and dynamics of the hydrated Hg(II) ion complex in aqueous solution. We used a combined extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) analysis to explore the Hg(II) hydration structure. Quantum mechanical ab initio calculations support the experimentally determined struc- tural results, and molecular dynamics simulations reveal the dynamic behavior of the system. ² CASPUR. ‡ Department of Chemistry, University of Rome “La Sapienza”. § Department of Chemistry, University of Naples Federico II. | Department of Experimental Medicine, University of L’Aquila. ⊥ Frascati National Laboratories. (1) Jennette, K. W. EnViron. Health Perspect. 1981, 40, 233-252. (2) Benoit, J. M.; Fitzgerald, W. F.; Damman, A. W. H. Mercury Pollution: Integration and Synthesis; Watras, C. J., Huckabee, J. W., Eds.; Lewis: Boca Raton, FL, 1994; pp 187-202. (3) Goyer, R. A. Toxicological Effects of Methylmercury; National Academy Press: Washington, DC, 2000. (4) Seller, P.; Kelly, C. A.; Rudd, J. W. M.; MacHutchon, A. R. Nature 1996, 380, 694-697. (5) Richens, D. T. The Chemistry of Aqua Ions; Wiley: Chichester, 1997. (6) Johansson, G.; Sandstro ¨m, M. Acta Chem. Scand., Ser. A 1978, 32, 109- 113. (7) Sandstro ¨m, M.; Persson, I.; Ahrland, S. Acta Chem. Scand., Ser. A 1978, 32, 627-41. (8) Eigen, M. Pure Appl. Chem. 1963, 6, 97-115. Published on Web 04/06/2007 5430 9 J. AM. CHEM. SOC. 2007, 129, 5430-5436 10.1021/ja066943z CCC: $37.00 © 2007 American Chemical Society