First Principles DFT Studies of Metal-Based Biological and Biomimetic Systems - Metallic Systems: A Quantum Chemist's Perspective - page 9

This.approach.clearly.showed.that.the.strongest.solvation.effects.(with.either.ben-
zene. or. heptane. molecules). were. occurring. for. the. proton. source,. rather. than. for.
the. catalyst.. Also,. the. molybdenum. complexes. employed. in. experiments. are. even.
larger.than.the.model.used.in.this.study,.with.the.metal.center.being.less.accessible.
to.solvent.molecules,.i.e.,.the. real .effect.of.solvent.molecules.is.expected.to.be.even.
smaller.than.that.observed.in.our.calculations.
As.reported.in.Table.1.1,.once.solvation.effects.were.included.in.the.estimation.of.
reaction.energies,.the.discrepancy.between.experimental.free.energies.and.calculated.
reaction.energies.was.drastically.reduced.to.less.than.∼5.kcal.mol −1 , 126 .leading.to.rea-
sonable.agreement.between.experimental.and.calculated.reaction.energies..Moreover,.
our.calculations.suggested.that.reactions.in.heptane.are.expected.to.be.only.slightly.
exothermic,.in.line.with.experimental.measurements.carried.out.in.benzene. 126
The.reasons.behind.such.a.strong.solvent.effect.were.revealed.by.analysis.of.the.
solvation.energies.of.the.compounds.involved.in.the.reactions.and.by.the.atoms-in-
molecules.approach..These.clearly.showed.that.LutH + .undergoes.a.larger.stabiliza-
tion.than.Lut.in.the.presence.of.explicit.solvent..In.particular,.Figure.1.2.displays.a.
schematic.view.of.C—H…π.interactions.between.heptane.molecules.and.both.forms.
of.proton.source.(Figure.1.2A′-B′)..Lut(heptane) 4 .has.seven.BCPs.corresponding.to.
C—H…π.interactions.with.a.total.electron.density.(ρ TOT ).equal.to.0.0497.au,.where.
LutH + (heptane) 4 .shows.nine.BCPs.with.ρ TOT = 0.0550.au..Similarly,.π…π.interactions.
bind.the.benzene.molecules.around.the.proton.source..These.interactions.are.stron-
ger.in.LutH + (benzene) 4 .(with.seven.BCPs.and.ρ TOT = 0.160.au).than.in.Lut(benzene) 4 .
(with.six.BCPs.and.ρ TOT = 0.137.au),.conirming.once.more.that.LutH + .can.interact.
much.more.strongly.with.solvent.molecules.than.Lut.

(A) (A')

(B) (B')

FIGURE 1.2 Views. of. solvation. models. with. four. heptane. molecules. of. (A). LutH + . and.
(B).Lut;.schematic.view.of.C—H…π.interactions.(black.arrows).in.A′.and.B′,.respectively..
(Reprinted.with.permission.from.Alessandra.Magistrato,.et.al.,. J. Chem. Theory Comput .,.
3(5),.1708..Copyright.2007.American.Chemical.Society.)