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Electrochemistry of Stimulation Electrodes: Part I: Page 4

Electron Transfer in a Hypothetical System Containing a Single Molecule.

Quantum theory tells us that electrons in atomic orbitals occupy discrete levels of energy that may be determined from the wave function (). The probability of electron being in an elementary volume is a function of ||^2. Theoretical and experimental value for the lowest energy level of the 1s orbital of the Hydrogen atom has been found to be –13.6 eV (electron-volt, 1 eV = 1.6 x 10^-19 J). For spherically symmetric orbitals, the discrete energy levels are given by

E(n) = -13.6/n^2 ( for n = 1,2,3 ……).

Orbital electrons are characterized by the quantum numbers, n, l and m, which are eigenvalues of the general solution of the wave equation. Following the Pauli Exclusion Principle, no discrete level may be occupied by electrons with the same quantum number. Each atomic orbital can have a maximum of two electrons with opposite spin quantum number. In the ground state (minimum energy) the orbitals are filled from the lowest levels progressively.


In the hypothetical case of a single molecule shown on the Left Panel, the electron states are filled up to a certain energy level (Highest occupied molecular orbital, HOMO) and the next possible energy level is unoccupied (Lowest unoccupied molecular orbital, LUMO).
If this molecule were sufficiently close to a metal electrode and the Fermi Level of the electrons in the electrode was elevated to the same level as the unoccupied energy state (LUMO) of the molecule, an electron could move from the electrode to the unoccupied state of the molecule (Right Panel). In this manner, current applied to an electrode can alter the ionic state of the molecules in the vicinity of the electrode. It is also worth noting that electron transfer does not occur for lower energy levels because the allowable states in the metal and that of the molecule are full at the matched energy level.


Similarly, if the Fermi Level of the electrons in the electrode were lowered to the level of the highest occupied states, by the application of anodic current (positive potential), an electron can transfer from the molecule to the electrode when the unoccupied energy states are matched in the metal with the occupied state of the molecule. This electron transfer would alter the ionization state of a molecule.
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