Water Exchange from the Second Coordination Shell

4 Water Exchange from the Second Coordination Shell I 155 

Information on the second coordination shell of water molecules around cations is much poorer than that regarding the first shell [4]. Properties of solvent molecules in this coordination shell are often very similar to those of the bulk, making their investigation extremely difficult. The analysis of radial distribution functions, g(r), 

From MD simulations it can be concluded that for trivalent ions (Cr, lanthanides) in general, first shell water molecules form two hydrogen bonds to second shell water oxygens. In that way the number of second shell water molecules (CNu) is roughly twice that of the first shell coordination number (CNi). For [Cr(H20)6] a mean second shell number of 12.9 H2O molecule was found and for lanthanides, CNii-values of 17.61 (for [Nd(H20)9] ) and 16.74 (for [Yb(H20)g] +) were obtained (Fig. 4.7). 

Munoz-Paez et al. [114] performed extended X-ray absorption fine structure studies on aqueous solutions of Cr and Zn +. They detected second coordination shells in both cases with coordination numbers of 13.3 + 1 (Cr +) and 11.6 + 1.5 (Zn ). The same group performed Monte Carlo [115] and molecular dynamic [116, 117] simulations of [Cr(H20)6] in dilute aqueous solutions using an ab initio Cr + hydrate-water interaction potential. They found second shell coordination numbers of 14 from both simulations. Furthermore, from simulations and EXAFS measurements they concluded that chloride ions are situated beyond the second hydration shell. 

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