Antibonding molecular orbital

 ( Chemical Bonding ) 

In theoretical chemistry, an antibonding orbital is a type of molecular orbital that weakens the chemical bond between two and helps to raise the Energy level of the molecule relative to the separated atoms. Such an orbital has one or more nodes in the bonding region between the Atomic nucleus. The Electron density of the 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.Atkins P. and de Paula J. Atkins Physical Chemistry. 8th ed. (W.H. Freeman 2006), p.371 Miessler G.L. and Tarr D.A., Inorganic Chemistry 2nd ed. (Prentice-Hall 1999), p.111 This is in contrast to a bonding molecular orbital, which has a lower energy than that of the separate atoms, and is responsible for chemical bonds.

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Diatomic molecules Antibonding molecular orbitals (MOs) are normally higher in energy than bonding molecular orbitals. Bonding and antibonding orbitals form when atoms combine into molecules. If two hydrogen atoms are initially far apart, they have identical . However, as the spacing between the two atoms becomes smaller, the electron begin to overlap. The Pauli exclusion principle prohibits any two electrons (e-) in a molecule from having the same set of . Therefore each original atomic orbital of the isolated atoms (for example, the ground state energy level, 1 s) 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 less electron density between the two nuclei than there would be if there were no bonding interaction at all. When a molecular orbital changes sign (from positive to negative) at a nodal plane between two atoms, it is said to be antibonding with respect to those atoms. Antibonding orbitals are often labelled with an asterisk (*) on molecular orbital diagrams.

In homonuclear diatomic molecules, σ* ( sigma star) antibonding orbitals have no nodal planes passing through the two nuclei, like sigma bonds, and π* ( pi star) orbitals have one nodal plane passing through the two nuclei, like pi bonds. The Pauli 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. Since the antibonding 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.

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Polyatomic molecules In molecules with several atoms, some orbitals may be delocalized over more than two atoms. A particular molecular orbital may be bonding with respect to some adjacent pairs of atoms and antibonding with respect to other pairs. If the bonding interactions outnumber the antibonding interactions, the MO is said to be bonding, whereas, if the antibonding interactions outnumber the bonding interactions, the molecular orbital is said to be antibonding.

For example, butadiene has which are delocalized over all four carbon atoms. There are two bonding pi orbitals which are occupied in the ground 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 the ground state, but may be occupied in .

Similarly benzene with six carbon atoms has three bonding pi orbitals and three antibonding pi orbitals. Since each carbon atom contributes one electron to the pi bond 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 explaining chemical reactions in terms of molecular orbital theory. Roald Hoffmann and Kenichi Fukui shared the 1981 Nobel Prize in Chemistry for their work and further development of qualitative data molecular orbital explanations for chemical reactions.

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See also

• Bonding molecular orbital
• Valence and conduction bands
• Valence bond theory
• Molecular orbital theory
• Conjugated system

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Further reading

• Orchin, M. Jaffe, H.H. (1967) The Importance of Antibonding Orbitals. Houghton Mifflin. ISBN B0006BPT5O

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