Part 6: Redox Potential
This is a basic introduction to the redox potential in electrochemistry.
The topics are listed below:
Redox Potential I: About HOMO and LUMO
Professor Noriyuki Watanabe
One of the purposes of the electrochemical measurement is to understand the redox potential of the subject. Oxidation-reduction potential is a characteristic feature of molecules that can be used for the identification and characterization of the substances. That is how to determine the redox potential. If you know this, you can use it as a guide to designing a substance with a desired redox potential.
The outer layer electrons of the substance (considering molecular species here) occupy the electronic energy level (also called orbital) in the turn from the low level to high level. The highest occupied energy level is defined as the highest occupied level (HOMO; Highest Occupied Molecular Orbital), the unoccupied energy level above it is called the lowest unoccupied molecular orbital (LUMO：Lowest Unoccupied Molecular Orbital), these electronic energy levels are involved in the reaction of molecular species.
For this reason, HOMO and LUMO are also called frontier levels. Regarding electrochemical reactions, oxidation is to remove electrons from HOMO, and reduction is to add electrons to LUMO.
Because the electronic level is the inherent feature of the substance, the redox potential is also a unique value of the substance. Formally, it is called standard oxidation reduction potential or formula amount oxidation reduction potential, but here it is expressed as redox potential (redox is a combination of reduction and oxidation).
As the redox potential of a molecular species depends on the locations of the HOMO - LUMO, it is possible to obtain it by theoretically quantum mechanical calculation. However, it may also be significance to understand which factors affect redox potential in quality. For this reason, it will be described in several times later.
HOMO, LUMO may determine the redox potential and cause the redox potential fluctuation. The fluctuation factor for the electronic energy level is mainly the electron density in the redox center (the site where electron transfer takes place), the electric charge around it (positive, negative, its size), and the influence of the medium is added as other factor too. When an organometallic compound is used as a material, its characterization can be considered.
Firstly, let’s consider the increase or decrease of the electron density of the redox center (site). For example, when the electron density of the redox center increases due to the influence of a substituent (electron-donating group), the electron level rises (becomes unstable), and the redox potential easily shifts to the negative direction, which makes it easy to be oxidized. On the other hand, if the electron density is lowered by a substituent (electron-withdrawing group), the opposite situation will occur.
For an example, let’s compare the acetyl group and the methyl group as the substituents of the ferrocene. The former acetyl group is electron-withdrawing group and reduces the electron density around iron which is a redox site, and methyl group is an electron donating group. Their effectiveness is almost directly proportional to the number of the substituents. The redox potential of ferrocene is shifted to the positive in 0.25 V when one acetyl substituent and 0.47 V positive shift when diacetyl group substituent. On the other hand, there is negative shift of 500mV when deca-methyl substituted ferrocene (when all hydrogen atoms are replaced by methyl groups). A negative shift of 50 mV is caused by per methyl group.