Highest occupied molecular organic radical ions

The concept of molecular orbitals (MOs) helps to explain the electron structure of ion-radicals. When one electron abandons the highest occupied molecular orbital (HOMO), a cation radical is formed. HOMO is a bonding orbital. If one electron is introduced externally, it takes the lowest unoccupied molecular orbital (LUMO), and the molecule becomes an anion-radical. LUMO is an antibonding orbital. Depending on the HOMO or LUMO involved in the redox reaction, organic donors appear as n, a, or n species, whereas organic acceptors can be tt or a species. Sometimes, a combination of these functions takes place. 

Isotope-containing organic compounds as ion radical precursors have important significance for ion radical organic chemistry. The link between the isotopic substitution at the reaction center and the change in the kinetic properties is not so obvious for one-electron transfer. In that case, the highest occupied molecular orbit (MO) of a donor loses one electron. This electron is then shuttled to the lowest unfilled MO of an acceptor. 

In aprotic nonaqueous media, the organic electrochemistry of anodic and cathodic reactions is concerned predominantly with radical-ion chemistry in many cases involving aromatic substances, the radicals are of sufficient stability for them to be characterized spectroscopically by conventional absorption spectrophotometry and by esr spectroscopy. Linear relations are found between the cathodic and anodic half-wave potentials and the ionization potentials or electron affinities determined in the gas phase. The oxidation and reduction potentials can also be related to the theoretically calculated energies of the highest occupied (anodic process) or lowest vacant (cathodic process) molecular orbitals. 

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