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15-December-2009

Zeinab Mokhtari. 15-December-2009. 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.

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15-December-2009

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  1. Zeinab Mokhtari 15-December-2009

  2. 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 Definitively can be measured, because it is large. The electron affinity (EA) is the energy difference between the anion and the neutral. R + e → R- H = -EA It has a small value. So difficult to be measured.

  3. E Vacuum level =0 eV EA LUMO IP HOMO

  4. Oxidation and Reduction reduction oxidation Vacuum level e EA IP LUMO LUMO e HOMO HOMO Eox Ered

  5. 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.

  6. THE METHOD OF CALCULATING EA’S FROM HALF WAVE REDUCTION POTENTIALS 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.

  7. 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:

  8. Bergman Dewar set E1/2 MINDO/3 CURES EC Semiempirical methods

  9. Gas phase EA Some Calculative Methods

  10. Slope = 1 Different intercepts for different groups

  11. σ(EA) = EA(Experimental) – EA(calculated)

  12. σ(EA) = EA(Experimental) – EA(calculated) More precise results

  13. ‘‘Is the electronegativity constant?’’ Constant Slope = 1

  14. A series of aromatic hydrocarbons Electronegativity EN is almost constant vs. size and close to the workfunction of graphite (4.3 eV).

  15. Thanks

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