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Intheoretical chemistry, anantibonding orbital is a type ofmolecular orbital that weakens thechemical bond between twoatoms and helps to raise theenergy of themolecule relative to the separated atoms. Such an orbital has one or morenodes in the bonding region between thenuclei. Thedensity of theelectrons in the orbital is concentrated outside the bonding region and acts to pull one nucleus away from the other and tends to cause mutual repulsion between the two atoms.[1][2] This is in contrast to abonding molecular orbital, which has a lower energy than that of the separate atoms, and is responsible for chemical bonds.
Antibondingmolecular orbitals (MOs) are normallyhigher in energy than bonding molecular orbitals. Bonding and antibonding orbitals form when atoms combine into molecules.[3] If twohydrogen atoms are initially far apart, they have identicalatomic orbitals. However, as the spacing between the two atoms becomes smaller, the electronwave functions begin to overlap. ThePauli exclusion principle prohibits any two electrons (e-) in a molecule from having the same set ofquantum numbers.[4] Therefore each original atomic orbital of the isolated atoms (for example, the ground state energy level, 1s) splits into two molecular orbitals belonging to the pair, one lower in energy than the original atomic level and one higher. The orbital which is in a lower energy state than the orbitals of the separate atoms is the bonding orbital, which is more stable and promotes the bonding of the two H atoms into H2. The higher-energy orbital is the antibonding orbital, which is less stable and opposes bonding if it is occupied. In a molecule such as H2, the two electrons normally occupy the lower-energy bonding orbital, so that the molecule is more stable than the separate H atoms.
A molecular orbital becomes antibonding when there is lesselectron density between the two nuclei than there would be if there were no bonding interaction at all.[5] When a molecular orbital changes sign (from positive to negative) at anodal plane between two atoms, it is said to beantibonding with respect to those atoms. Antibonding orbitals are often labelled with anasterisk (*) on molecular orbital diagrams.
Inhomonuclear diatomic molecules, σ* (sigma star) antibonding orbitals have no nodal planes passing through the two nuclei, likesigma bonds, and π* (pi star) orbitals have one nodal plane passing through the two nuclei, likepi bonds. ThePauli exclusion principle dictates that no two electrons in an interacting system may have the same quantum state. If the bonding orbitals are filled, then any additional electrons will occupy antibonding orbitals. This occurs in the He2 molecule, in which both the 1sσ and 1sσ* orbitals are filled.[6] Since theantibonding orbital is more antibonding than the bonding orbital is bonding, the molecule has a higher energy than two separated helium atoms, and it is therefore unstable.
In molecules with several atoms, some orbitals may bedelocalized over more than two atoms. A particular molecular orbital may bebonding with respect to some adjacent pairs of atoms andantibonding with respect to other pairs. If the bonding interactions outnumber the antibonding interactions, the MO is said to bebonding, whereas, if the antibonding interactions outnumber the bonding interactions, the molecular orbital is said to beantibonding.
For example,butadiene haspi orbitals which are delocalized over all four carbon atoms. There are two bonding pi orbitals which are occupied in theground state: π1 is bonding between all carbons, while π2 is bonding between C1 and C2 and between C3 and C4, and antibonding between C2 and C3. There are also antibonding pi orbitals with two and three antibonding interactions as shown in the diagram; these are vacant in theground state, but may be occupied inexcited states.
Similarlybenzene with six carbon atoms has three bonding pi orbitals and three antibonding pi orbitals. Since eachcarbon atom contributes one electron to theπ-system of benzene, there are six pi electrons which fill the three lowest-energy pi molecular orbitals (the bonding pi orbitals).
Antibonding orbitals are also important for explainingchemical reactions in terms of molecular orbital theory.Roald Hoffmann andKenichi Fukui shared the 1981Nobel Prize in Chemistry for their work and further development ofqualitative molecular orbital explanations for chemical reactions.[7]