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15-December-2009
Zeinab Mokhtari
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Two important fundamental molecular properties :- The ionization potential (IP)- The electron affinity (EA)
The ionization potential (IP) is the energy required to remove an electron from a molecule.
R → R+ + e H = IP
The electron affinity (EA) is the energy difference between the anion and the neutral.
R + e → R- H = -EA
Definitively can be measured, because it is large.
It has a small value. So difficult to be measured.
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Vacuum level =0 eV
E
HOMO
LUMO
IP
EA
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Oxidation and Reduction
HOMO
e
LUMO
HOMO
e
reduction oxidation
EA
IP
Ered Eox
LUMO
Vacuum level
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The major purpose :To obtain EA of aromatic hydrocarbons from half wave reduction potentials in aprotic solvents
- Fullerenes- Organic molecules containing O, N, and halogen atoms- Nucleic acids and other biologically significant molecules
A second objective: To calculate the electronegativities, (EN)=(IP+EA)/2 , of the aromatic hydrocarbons by using ionization potentials in the literature.
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EA evaluationmethods
Gas phasemeasurment
EA= 2EN-IP EA = -E1/2Semiempirical
methods
ECD or NIMS PES TCT
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The first experimental method for obtaining relative electron affinities.
-G is the solution energy difference for the reaction in the gas phase and in solution.
mddG(-G) is a function of charge density.
THE METHOD OF CALCULATING EA’S FROM HALF WAVE REDUCTION POTENTIALS
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Before the 1960’s:it was assumed that the mddG were the same for different molecules.
By gas phase electron affinities measurement, the mddG values range from 1.7 eV for the large fullerenes to 2.7 eV for small anions.
They make some different groups with different mddG:
9Semiempirical methods
Bergman Dewar set
E1/2CURES EC MINDO/3
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Gas phase EA SomeCalculativeMethods
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Slope = 1
Different intercepts for different groups
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σ(EA) = EA(Experimental) – EA(calculated)
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σ(EA) = EA(Experimental) – EA(calculated)
More precise results
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Slope = 1
Constant
‘‘Is the electronegativity constant?’’
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A series of aromatic hydrocarbons
Electronegativity EN is almost constant vs. size and close to the workfunction of graphite (4.3 eV).
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