Adelia J. A. Aquino

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MODELING OF THE SURFACE OF THE MINERAL GOETHITE
Adelia J. A. Aquino Institute for Theoretical
Chemistry und Structural Biology, University of
Vienna and Institute of Soil Research, University
of Natural Resources and Applied Life Sciences
Vienna - Vienna, Austria
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OUTLINE
A. GOETHITE COMPLEXES B. 2,4-DICHLOROPHENOXYACET
IC ACID HERBICIDE COMPLEXES
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BACKGROUND

• Goethite (a-FeOOH) is a common component of
  soils.
• It belongs to the group of ferric oxyhydroxides,
  which are able to sorb large amounts of heavy
  metal cations, anions and oxyanions and also
  organic pollutants (e.g. polycyclic aromatic
  hydrocarbons).

• Even though the bulk structure of goethite is
  relatively simple the surface structure is
  complicated due to the existence of several types
  of adsorption surface sites.

Goethite- Hydrated iron oxide Size 6X8
cm Origin Brazil

• The surfaces of ferric oxyhydroxides are
  predominantly formed from hydroxyl groups.

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STUDIED SYSTEMS
I- Isolated clusters Fe4, Fe6 and Fe8
II - Complexes formed of each isolated cluster
and water, acetic acid, acetate,
2,4D-diclhorophenoxiacetic acid,
2,4D-diclhorophenoxiacetate
III Fe6C6H6
GOAL
The main aim of the present work is the study of
adsorption complexes on goethite. We show the
structural manifold of the hydroxyl groups of a
goethite surface in their interaction with a set
of adsorbents occurring in soil environments. For
this purpose we have selected a series of
molecular species containing small model
molecules like water and acetic acid and acetate
representing typical polar interactions in soils.
Beyond that the interaction of the herbicide
2,4-dichloro-phenoxyacetic acid (2,4-D) and of
benzene with the goethite surface has been
studied. The latter choice resulted from the
absorption capability of goethite concerning
aromatic compounds.
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STRUCTURAL AND COMPUTATIONAL DETAILS
-The goethite structure consists of a network of
distorted octahedra with Fe(III) cations in their
centers which are connected via
µ-oxo-bridges. -Cluster models used in the
calculations were constructed from the (110) slab
surface.The surface of this model contains three
different OH types. -All calculations were
performed at DFT/B3LYP level of theory with the
TURBOMOLE program. -SCF calculations for
isolated clusters and the water complexes were
carried out at low and high-spin as well as at
closed shell levels. -Basis Set SVP, SVPsp
Only the O-H groups highlighted in the cluster
model picture were optimized. All other
geometric parameters were kept frozen.
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RESULTS
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Geometrical parameters (in Å) of isolated iron
clusters at low spin (LSPIN), high spin (HSPIN)
and closed shell (CSHELL) using B3LYP/SVP approach
System Method R O1-H RO2-H RO3-H RO4-H RO5-H RO6-H RO7-H RO8-H RO9-H
Fe4 LSPIN 0.969 0.971 0.968 0.970
HSPIN 0.969 0.971 0.970 0.965
CSHELL 0.973 (0.973) 0.980 (0.977) 0.969 (0.966) 0.966 (0.969)
Fe6 LSPIN 0.984 0.971 0.968 0.967 0.969 0.968
HSPIN 0.988 0.972 0.966 0.966 0.967 0.969
CSHELL 1.001 (0.999) 0.972 (0.971) 0.969 (0.968) 0.977 (0.973) 0.969 (0.968) 0.979 (0.977)
Fe8 LSPIN 0.969 0.969 0.970 0.987 0.977 0.972
HSPIN 0.967 0.967 0.968 0.987 0.977 0.971
CSHELL 0.981 (0.982) 0.977 (0.973) 0.969 (0.967) 0.969 (0.972) 0.977 (0.968) 0.969 (0.967) 0.988 (0.983) 0.975 (0.974) 1.003 (1.006)
a values in parentheses are results obtained with
the SVPsp basis set
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Hydrogen bond distances (Å) between goethite
clusters and the water molecule using the
B3LYP/SVP approach.
a values in parentheses are results obtained with
the SVPsp basis set
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Interaction energies, ?E, of the water molecule
adsorbed on four different goethite clusters
using the B3LYP/SVP approach. Energies are given
in kcal/mol.
Fe4-H2O (Fig. 1a) Fe4-H2O (Fig. 1a) Fe4-H2O (Fig. 1a) Fe6-H2O (Fig. 1c) Fe6-H2O (Fig. 1c)
Low-Spin -16.4 -16.4 Low-Spin -21.3
High-Spin -20.9 -20.9 High-Spin -24.6
Closed Shella -19.2(-16.5) -19.2(-16.5) Closed Shell -20.1(-16.5)
Fe6-H2O (Fig. 1b) Fe6-H2O (Fig. 1b) Fe6-H2O (Fig. 1b) Fe8-H2O (Fig. 1d) Fe8-H2O (Fig. 1d)
Low-Spin Low-Spin -18.3 Low-Spin -16.8
High-Spin High-Spin -21.7 High-Spin -17.8
Closed Shella Closed Shella -17.5(-13.2) Closed Shell -15.2(-13.1)
a values in parentheses are results
obtained with the SVPsp basis set
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1
.
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Interaction energies, ?E, of acetic acid,
acetate, 2,4-D, 2,4-D and benzene adsorbed on
two goethite clusters using the closed shell
B3LYP approach and two basis sets. Energies are
given in kcal/mol.
System Figure ?E(kcal/mol) (SVP basis) ?E(kcal/mol) (SVPsp basis)
Fe4-HAc 2a -22.7 -25.3
Fe6-HAc 2b -23.7 -25.0
Fe4-Aca 2c -55.4 -43.4
Fe6- Ac 2d -58.3 -50.6
Fe4-2,4-D 3a -20.9 -21.1
Fe6-2,4-D 3b -23.9 -25.9
Fe4-2,4-D 3c -38.2 -32.1
Fe6-2,4-D 3d -37.4 -31.3
Fe6- C6H6 4 -2.6(-13.1)b -4.4
a proton transfer from the goethite surface to
the Ac anion b in parentheses single point
MP2/SVP result
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CONCLUSIONS

• Our investigations showed that the (110)
  goethite surface formed by three types of the
  hydroxyl groups offers a variety of possibilities
  for hydrogen bond formation with appropriate
  polar adsorbents.
• Two OH types, hydroxo- and µ-hydroxo, have
  sufficient flexibility for bending allowing them
  to act as proton acceptors while the third type,
  µ3-hydroxo, acts only as proton donor due to its
  more pronounced rigidity.
• Calculated interaction energies on different
  sites are ca. -20 kcal/mol for the water
  molecule, a number which is in line with the
  number and type of hydrogen bonds formed.
  Slightly larger interaction energies were
  observed for neutral acetic acid and 2,4-D in
  comparison to the goethite/water complexes.
• The aromatic ring actively participates in the
  interaction with the goethite surface groups.
  Interactions with the nonpolar, aromatic benzene
  molecule are much weaker. However, the estimated
  interaction energy range of -5 to -8 kcal/mol is
  still significant. This result rationalizes why
  goethite plays an important role for the
  retention of polyaromatic hydrocarbons in soils.
•  
•  

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INTERACTION OF THE 2,4-DICHLOROPHENOXYACETIC ACID
HERBICIDE WITH SOIL ORGANIC MATTER
The term soil organic matter (SOM) is generally
used to represent the organic constituents in the
soil Humic substances (HS) are one of the major
constituents of the terrestrial (SOM) and aquatic
(dissolved SOM) carbon pool Humic acids - the
fraction of HS that is not soluble in water under
acidic conditions (pH lt 2) but is soluble at
higher pH values Fulvic acids - the fraction of
HS that is soluble in water under all pH
conditions Humin - the fraction of HS that is
not soluble in water at any pH value
BACKGROUND
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STUDIED SYSTEMS

GOAL
Humic acids contain several relevant functional
groups, mainly carboxyl, carbonyl, alcoholic and
phenolic units, which play a major role in
binding of polar molecules from a polar solvent
environment. The aim of this work was to study
the interactions of molecular and anionic forms
of 2,4-D herbicide with these functional groups.
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COMPUTATIONAL DETAILS
All calculations were performed at DFT level of
theory with the TURBOMOLE and GAUSSIAN03
programs Density functional B3LYP Basis Set
SVP, SVPsp The polarizable continuum model, PCM
and the conductor-like screening model, COSMO
were used to computer the calculations in
solution Two models were used to perform the
calculation in solution the microsolvation (g)
and the global solvation (gs) and the combination
of them (gsm) All results are BSSE corrected
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RESULTS
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Interaction energies, enthalpies and Gibbs free
energies for complexes of 2,4-D and selected MS
and water molecules. All calculations were
performed at the B3LYP/SVPsp level of theory.
Energies are BSSE corrected a and given in
kcal/mol.
Complex formation a ?Eg ?Hg ?Gg ?Egs ?Hgs ?Ggs
Me-CHO 2,4-D ? Me-CHO2,4-D -11.4 -8.6 1.2 -2.6 0.2 10.0
Me-OH 2,4-D ? Me-OH2,4-D -12.5 -9.4 0.2 -4.2 -1.1 4.1
Me-NH2 2,4-D ? Me-NH22,4-D -13.9 -11.1 -2.0 -7.3 -4.5 4.6
Me-COOH 2,4-D ? Me-COOH2,4-D -18.0 -15.1 -4.0 -1.6 1.3 12.4
(H2O)2 2,4-D ? 2H2O2,4-D -18.8 -15.2 -2.5 -4.0 -0.4 12.3
Me-NH3 2,4-D ? Me-NH32,4-D -33.2 -29.6 -18.9 -4.9 -1.3 9.4
Subscript g denotes the gas phase calculations.
Subscript gs denotes the results obtained with
the global solvation approach (PCM calculations)
?Hgs ?Hg - ?Eg ?Egs ?Ggs ?Gg - ?Eg
?Egs
MS 2,4D ? MS2,4D MS 2,4D ?
MS2,4D
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Energies, enthalpies and Gibbs free energies of
reactions between the 2,4D2H2O complex and
MS2H2O complexes for the microsolvation and
combined micro- and global solvation approaches.
All calculations were performed at the
B3LYP/SVPsp level of theory. Energies are given
in kcal/mol.
microsolvation microsolvation microsolvation global solvation microsolvation global solvation microsolvation global solvation microsolvation
Model reactiona ?Eg ?Hg ?Gg ?Egsm ?Hgsm ?Ggsm
Me-CHO2H2O 2,4-D2H2O ? Me-CHO2,4-D (H2O)4 -2.2 -1.8 -1.5 -4.1 -3.7 -3.4
Me-OH2H2O 2,4-D2H2O ? Me-OH2,4-D (H2O)4 -3.9 -3.6 -4.3 -8.9 -8.6 -9.3
Me-NH2 2H2O 2,4-D 2H2O ? Me-NH22,4-D (H2O)4 -3.1 -3.1 -3.8 -4.7 -4.7 -5.4
Me-NH3 2H2O 2,4-D 2H2O ? Me-NH32,4-D (H2O)4 -2.8 -2.2 0.9 -0.6 -0.1 3.0
Me-COOH 2H2O 2,4-D2H2? Me-COOH2,4-D (H2O)4 -0.8 -0.8 -1.5 -0.2 -0.2 -0.9
Subscript gsm denotes the results obtained with
combined approach.
a Me -CH3
MS2H2O 2,4-D2H2O ? MS2,4-D (H2O)4

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Interaction energies, enthalpies and Gibbs free
energies for complexes of 2,4-D anion and
selected MS and water molecules. All calculations
were performed at the B3LYP/SVPsp level of
theory. Energies are BSSE correcteda and given in
kcal/mol.
Complex formation a ?Eg ?Hg ?Gg ?Egs ?Hgs ?Ggs
Me-CHO 2,4-D ? Me-CHO2,4-D -11.5 -9.4 -1.3 1.4 3.5 11.6
Me-OH 2,4-D ? Me-OH2,4-D -15.3 -13.0 -4.4 -1.6 0.7 9.3
Me-NH2 2,4-D ? Me-NH22,4-D -8.2 -6.1 1.6 2.1 4.2 11.9
Me-COOH 2,4-D ? Me-COOH2,4-D -21.2 -19.6 -8.6 -2.4 -0.8 10.2
2H2O 2,4-D ? 2H2O2,4-D -26.7 -24.0 -12.4 -3.8 -1.1 10.5
Me-NH3 2,4-D ? Me-NH32,4-D -116.0 -115.8 -106.6 -0.6 -0.5 8.8
a Me -CH3
Subscript g denotes the gas phase
calculations. Subscript gs denotes the results
obtained with the global solvation approach (PCM
calculations).
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Energies, enthalpies, enthalpies and Gibbs free
energies of reactions between the 2,4-D?2H2O
complex and MS2H2O complexes for the
microsolvation and combined micro- and global
solvation approaches. All calculations were
performed at the B3LYP/SVPsp level of theory.
microsolvation microsolvation microsolvation global solvation microsolvation global solvation microsolvation global solvation microsolvation
Model reactiona ?Eg ?Hg ?Gg ?Egsm ?Hgsm ?Ggsm
Me-CHO2H2O 2,4-D2H2O ? Me-CHO2,4-D? (H2O)4 5.7 5.3 5.1 -1.1 -1.5 -1.7
Me-OH2H2O 2,4-D2H2O ? Me-OH2,4-D (H2O)4 1.2 0.8 0.1 -7.4 -7.8 -8.5
Me-NH22H2O 2,4-D2H2O ? Me-NH22,4-D (H2O)4 10.5 10.2 9.2 4.1 3.8 2.8
Me-NH3 2H2O 2,4-D2H2O ? Me-NH32,4-D? (H2O)4 -88.3 -89.2 -86.5 3.1 2.2 4.9
Me-COOH 2H2O 2,4-D2H2O ? Me-COOH2,4-D (H2O)4 3.9 2.5 2.8 -2.3 -3.7 -3.4
Ca2(H2O)6 2,4-D2H2O Ac? 2H2O ? -220.9
-221.0 -216.3 -11.5 -11.6
-6.6 2,4-D?Ca2(H2O)2Ac?2(H2O)4
a Me -CH3 Subscript gsm denotes the results
oobtained with combined approach. Energies are
given in kcal/mol.
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It has been shown that the consideration of this
combined solvation model is crucial for the
evaluation of chemical reaction energies The
application of the exchange reaction showed that
the neutral 2,4-D molecule is able to form stable
complexes in a polar solvent environment with a
large variety of functional groups On the other
hand, the anionic form of 2,4-D is found to form
stable complexes in a polar solvent like the soil
solution only with hydroxyl and carboxyl
functional groups In general, the interactions
of solvated ionic species are very stable in the
gas phase and in the microsolvation
model Continuum solvation has a destabilizing
effect due to a preferred solvation of the
individual charged reactants as compared to the
neutral or charged complexes The cation
bridge, which is by far the most important
interaction mechanism in soil, has been found to
be very stable with a final ?G value of -6.6
kcal/mol taking Ca2 as example.
CONCLUSIONS
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Acknowledgments