2ND COORDINATION SHELL WATER EXCHANGE-RATE AND MECHANISM - EXPERIMENTS AND MODELING ON HEXAAQUACHROMIUM(III)

From our combined experimental and computer modeling study we found a structurally and kinetically well-defined second coordination shell ar ound chromium(III) ions in aqueous solution. Strong hydrogen binding d ue to polarization of first coordination sphere water molecules leads to a mean coordination number of 12.94 water molecules in the second s hell and to short first shell hydrogen-second shell oxygen distances o f about 1.4 Angstrom. The experimentally measured exchange rate consta nt of k(ex)(298) = (7.8 +/- 0.2) x 10(9) s(-1) (Delta H double dagger = 21.3 +/- 1.1 kJ mol(-1), Delta S double dagger = +16.2 +/- 3.7 J K-1 mol(-1)) corresponds to a lifetime of 128 ps for one water molecule i n the second coordination shell and compares very well with a lifetime of 144 ps as observed from molecular dynamics simulation of a [Cr-(H2 O)(6)](3+) complex in aqueous solution. The geometry and the partial a tomic charges of [Cr(H2O)(6)](3+) were determined by density functiona l theory (DFT) calculations. Water exchange from the second coordinati on shell to the bulk of the solution proceeds between a H2O sitting in the second shell and an adjacent one which just entered this shell fr om the bulk. By a small rotation of the first coordination shell water molecule, one of its two hydrogen bonds jumps to the entered water mo lecule and the one which lost its hydrogen bond leaves the second shel l of the [Cr(H2O)(6)](3+). This associative reaction mode is a model f or water exchange between water molecules which are bound by strong hy drogen bonds, as in the case for strongly polarizing 3+ ions such as A l3+ or Rh3+. Furthermore, the exchange phenomenon between second spher e and bulk water involving only two adjacent water molecules is strong ly localized and independent of other water molecules of the second sh ell. In this respect it may be considered as a starting point for a st udy of water exchange on a protonated metal oxide surface.