Energy and Electron Transfer II

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Energy and Electron Transfer - II
Chapter-7 MMP
P. H.
December 19, 2002
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Marcus Theory
-DG ls lv
Log k
normal
Inverted
-DG
Acc. Chem. Res., 1996, 29, 522
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Example
kR gtgt kI
J. Am. Chem. Soc., 1989, 111, 8948
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l 0.39eV
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Contact and Solvent Separated Radical Ion Pairs
CRIP
SSRIP
A
D
D
A-
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CRIP SSRIP - An example
DG2 1.8 kcal/mol
k1 values depend on solvent polarity
Dicloromethane -108 Hexane gt 1010
Also more solvent reorganization for SSRIP than
CRIP
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CRIP and Exciplexes - The Transition
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Electron and Energy Transfer Equilibria
Cant even happen without
Energy transfer equilibrium
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Thermodynamics of the Equlibria
For rigid systems
For example..
DS - 0.04 gibbs/mol
DS - -1.8 gibbs/mol
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Electron Transfer Equilibria
Cannot observe the equlibrium if
kbet gtgt ket
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Chemiluminescent Ion Recombination
1
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Marcus Effect
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Role of Diffusion

• Diffusion and collision
• DA becomes DA
• DA breaks up into D and A

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Rate Constants

• kOBS, is close to calculated kDIFF.
• kOBS is a function of T/?.
• kOBS is essentially invariant for quenchers of
  widely varying structure.
• kOBS reach a limiting value which corresponds to
  the fastest bimolecular rate constant measured
  for that solvent.

Obscured the Marcus Inverted Region
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Cage Effect
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Diffusion - Distance/Time Relationship
Diffusion coefficient (D) of benzene (25C) - 2
x 105 cm2/s if we assume the encounter to be
over when one of the molecules has traveled a
distance equivalent to the size of several
solvent molecules (e.g. x 10 Å, equivalent to
about 2 benzene molecules), then we obtain a
rough estimate of the time required by applying
above equation of about 2.5 x 1010 s.
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Effect of Charged Species
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Transient Effects on Quenching
With Incresing Time
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Static Quenching - Perrin Model
ln (? /? ) V NA A
R (in Å) 6.5 A1/3
(with A in M units)