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Energy and Electron Transfer - II

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Energy and Electron Transfer - II

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Energy and Electron Transfer - II

Chapter-7 MMP+

P. H.

December 19, 2002

Marcus Theory

-DG = ls + lv

Log k

normal

Inverted

-DG

Acc. Chem. Res., 1996, 29, 522

Example

kR >> kI

J. Am. Chem. Soc., 1989, 111, 8948

l = 0.39eV

Contact and Solvent Separated Radical Ion Pairs

CRIP

SSRIP

A

D*

D+

A-

CRIP & SSRIP - An example

DG2 = 1.8 kcal/mol

k1 values depend on solvent polarity

Dicloromethane -108 ; Hexane > 1010

Also more solvent reorganization for SSRIP than CRIP

CRIP and Exciplexes - The Transition

Electron and Energy Transfer Equilibria

Can’t even happen without

Energy transfer equilibrium

Thermodynamics of the Equlibria

For rigid systems

For example..

DS - 0.04 gibbs/mol

DS - -1.8 gibbs/mol

Electron Transfer Equilibria

Cannot observe the equlibrium if…

kbet >> k–et

Chemiluminescent Ion Recombination

1

2

Marcus Effect

Role of Diffusion

Diffusion and collision

D*A becomes DA*

DA* breaks up into D and A*

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

Cage Effect

Diffusion - Distance/Time Relationship

Diffusion coefficient (D) of benzene (25°C) - 2 x 10–5 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 10–10 s.

Effect of Charged Species

Transient Effects on Quenching

With Incresing Time

Static Quenching - Perrin Model

ln ( ° / ) = V NA [ A]

R (in Å) = 6.5 [A]1/3

(with [A] in M units)