Carbohydrate and Trivalent Iron Ion Interactions in the Gas Phase and in Aqueous Solution - Metallic Systems: A Quantum Chemist's Perspective - page 91

3.4 RESULTS AND DISCUSSION

3.4.1 c oordination c hemiStry S tudieS of m and f e 3+ in the g aS P haSe
The.chemical.reactivity.calculations.using.the.condensed.FFs.indicate.that.the.O2.
and.O4.atoms.(see.Figure.3.1).with. f HOMO .values.of.0.38.and.0.32,.respectively,.pos-
sess.strong.nucleophilic.characters.(see.Table.3.1)..In.other.words,.the.O2.and.O4.
atoms.of.M.are.the.hard.bases.that.react.with.the.hard.acid.(Fe 3+ ).based.on.the.acid-
base. concept. of. Pearson. [54].. Furthermore,. the. calculated. natural. partial. charges.
show.that.the.hydroxyl.oxygen.atoms.of.M.have.larger.partial.negative.charges.com-
pared.to.the.ring.oxygen.(O5).and.methoxy.oxygen.(O1).atoms..These.indings.indi-
cate.a.better.Lewis.base.reactivity.for.the.hydroxyl.group.oxygen.atoms.
CPMD.simulations.of.M.and.a.single.Fe 3+ .ion.in.the.gas.phase.show.that.the.metal.
ion.coordinates.to.the.O2,.O3,.and.O4.atoms.of.M.(Figure.3.2).within.25.ps.and.stays.
coordinated. for. the. rest. of. the. simulation. (45.ps).. These. simulation. results. support.

TABLE 3.1
Calculated Chemical Reactivity Indices f HOMO in
the Gas Phase and in Water Based on
Condensed FF

M M-Fe 3+

Atom Gas Phase In Water Gas Phase In Water

O2 0.38106 0.18068 0.50533 0.01602

O3 0.01906 0.00107 0.01135 0.00125

O6 0.00153 0.00004 0.00080 0.00022

O4 0.01999 0.00412 0.07802 0.00309

O5 0.32149 0.00036 0.07753 0.00092

O1 0.01602 0.00165 0.18454 0.00002

H4

Fe A

O6

O4

H3

O3

H2

O5

O2

O1

C1

C7

ψ

FIGURE 3.2 Methyl-α-d-mannopyranoside. and. its. complex. with. one. Fe 3+ . ion. in. the. gas.
phase,.obtained.from.CPMD.simulations.