From our combined experimental and computer modeling study we
found a structurally and kinetically
well-defined second coordination shell around chromium(III) ions
in aqueous solution. Strong hydrogen binding
due to polarization of first coordination sphere water molecules leads
to a mean coordination number of 12.94 water
molecules in the second shell and to short first shell
hydrogen−second shell oxygen distances of about 1.4 Å.
The
experimentally measured exchange rate constant of
= (7.8 ± 0.2) × 109 s-1
(ΔH
⧧ = 21.3 ± 1.1 kJ
mol-1,
ΔS
⧧
= +16.2 ± 3.7 J K-1
mol-1) corresponds to a lifetime of 128 ps
for one water molecule in the second coordination
shell and compares very well with a lifetime of 144 ps as observed from
molecular dynamics simulation of a [Cr(H2O)6]3+ complex in aqueous
solution. The geometry and the partial atomic charges of
[Cr(H2O)6]3+ were
determined
by density functional theory (DFT) calculations. Water exchange
from the second coordination 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
from the bulk. By a small rotation of the first coordination shell
water molecule, one of its two hydrogen bonds
jumps to the entered water molecule and the one which lost its hydrogen
bond leaves the second shell of the [Cr(H2O)6]3+. This associative
reaction mode is a model for water exchange between water molecules
which are bound
by strong hydrogen bonds, as in the case for strongly polarizing 3+
ions such as Al3+ or Rh3+.
Furthermore, the
exchange phenomenon between second sphere and bulk water involving only
two adjacent water molecules is strongly
localized and independent of other water molecules of the second shell.
In this respect it may be considered as a
starting point for a study of water exchange on a protonated metal
oxide surface.
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