Earlier, we said that most reactions can be called either Lewis acid/base or redox. LUMO is Lowest Unoccupied MO, the lowest-energy MO that doesn't have any electrons in it.įrontier MOs are very important for reactivity. HOMO is Highest Occupied MO, the highest-energy MO that has electrons in it. The frontier MOs are called the HOMO and the LUMO. In this case, we are interested in the MOs at the border between occupied and empty. Distinguish and describe the significance of frontier MOsįrontier means a border area, between two things (often, between 2 countries).Hence if a molecule has a very small HOMO-LUMO gap then the molecule will have maximum stability.\) If the mobility of these pi-electrons increases, then the conjugated system will be large, due to which the energy distribution will be even throughout the molecule and will make it stable. Hence the pi-electrons will now become mobile as it will be easy for an electron to jump at a high energy level. The HOMO-LUMO gap will be small when the aromatic system is large. This is mostly observed in transitional metal complexes and also in large aromatic systems. If the HOMO-LUMO gap converges, then the excitations will become very easy. Due to this factor, this HOMO-LUMO gap is of high importance. AsĮnergies of these orbitals are close to each other, hence all the excitations will take place at this HOMO-LUMO gap. Hence out of all the orbitals inside a molecule, these orbitals will be closest in terms of energy. LUMO will be low in energy for orbitals in which there are no electrons, but HOMO will be high in energy for orbitals which have electrons. In terms of energy, the higher molecular orbital while drawing will always be LUMO, while HOMO will always be the lower one. They are called as SLUMO (second-lowest unoccupied molecular orbital) and NHOMO (next-to-highest occupied molecular orbital). The molecular orbitals which exist oneĮnergy level above LUMO and one energy level below HOMO also have a very crucial role in the frontier molecular orbital theory. SOMO is also called semi- occupied molecular orbital. A half-filled HOMO level for a radical is similar to a singly occupied molecular orbital which is also termed as SOMO. Similarly, the LUMO level is used for describing a conduction band minimum. The HOMO level in an organic semiconductor is generally used for describing the maximum valence bond which is similar to the quantum dots and inorganic semiconductors. It can also be used for predicting the colors which they will produce inside a solution. The stability and strength of a transitional metal complexĬan be predicted by using the energy difference which exists between two frontier orbitals. FMOT), HOMO and LUMO are often described as frontier orbitals. In frontier molecular orbital theory (i.e. The HOMO-LUMO gap is a concept used for describing the difference in energy between LUMO and HOMO. The types of molecular orbitals are called LUMO and HOMO which are used as acronyms for describing lowest unoccupied molecular orbital and highest occupied molecular orbital. Hence, by using these observations, the FMO theory helps in understanding and simplifying the prediction related to the reactivity for interactions between the HOMO-LUMO pairs of two reactants which are matched energetically. The first two observations help in understanding the Coulombic interaction and filled-filled interaction, while the third observation helps in understanding the HOMO-LUMO interactions. between LUMO and HOMO) due to which they both will get attracted to each other.īy using the Klopman-Salem equation, the total energy change for the reactants can be described when they approach a transitional state. This gave three major observations which are as follows:Ī) There will always be repulsion between the occupied orbitals of two different molecules.ī) There will always be an attraction between negative charges of one molecule and positive charges of another molecule.Ĭ) There will be an interaction between unoccupied orbital of one molecule and occupied orbital of another molecule (i.e. This fact was determined by using the MOT for interaction between two molecules. It was clearly understood that by using the frontier orbitals of HOMO and LUMO, a goodĪpproximation of reactivity can be achieved. In chemistry, an application of molecular orbital theory (MOT) is called frontier molecular orbital theory (FMOT) which is generally used for describing interactions between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
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