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TABLE 4.2
Speciic Structural Parameters for the 
Aqueous Solutions of the Cr 3+  and the 
Fe 3+  Ions
Cr 3+
Fe 3+
R Cr−O 2 .(Å)
3.91.±.0.03
R Fe−O 2 .(Å)
3.96.±.0.04
R O 2 −H 2 .(Å)
0.98.±.0.01
R O 2 −H 2 .(Å)
0.99.±.0.01
R O 1 −O 2 .(Å)
2.63.±.0.03
R O 1 −O 2 .(Å)
2.64.±.0.02
R HB .(Å)
1.65.±.0.04
R HB .(Å)
1.63.±.0.05
HB.denotes.the.hydrogen.bond.distance.between.the.irst.and.
second. shell. water. molecules. in. the. vicinity. of. these. two.
transition.metal.ions.
4.3.2  S ingle  i on and  d imerization  S tudieS of  a l 3+ in  W ater
For.the.studies.of.the.Al 3+ .ion.in.water,.we.performed.several.simulations..To.model.
Al.ions.in.water.at.the.pH.of.interest.(3.<.pH.<.7),.we.used.the.octahedral.Al(H 2 O) 3+ .
structure. (Figure. 4.2A). obtained. from. our. simulations. with. 58. water. molecules,.
removing.one.H.atom.from.a.irst.shell.water.molecule.that.is.coordinated.to.the.Al 3+ .
ion.. The. resulting. aqueous. Al(H 2 O) 5 OH 2+ . structure. loses. one. water. molecule. and.
becomes. a. penta-coordinate. Al(H 2 O) 4 OH 2+ . structure. (Figure. 4.2B). within. 0.8.ps..
This.inding.is.in.excellent.agreement.with.recent.kinetics.experiments.and.CPMD.
simulations,.which.report.a.penta-coordinated.Al(OH)(H 2 O) 2+ .complex.in.aqueous.
solution..Our.studies.show.that.the.penta-coordinated.complex.converts.to.a.trigonal.
bipyramidal.structure.(Figure.4.2C).in.water.within.the.following.1.7.ps.
Additional.CPMD.simulations.involving.breaking.a.HO-H.bond.of.a.irst.shell.
water.molecule.were.performed.in.order.to.understand.the.impact.of.the.free.proton.
on. the. solution. dynamics. and. coordination. chemistry.. This. simulation. shows. that.
the. proton. moves. to. the. second. shell. and. forms. a. Zundel-like. structure.. However,.
(A)
(B)
(C)
FIGURE  4.2  Octahedral. Al(H 2 O) 3+ (A),. square. pyramidal. Al(OH)(H 2 O) 2+ . (B),. and. tri-
gonal. bipyramidal. Al(OH)(H 2 O) 2+ . (C). species. in. aqueous. solution,. obtained. from. CPMD.
simulations.
 
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