Banana binding
A bent bond ( engl. Banana-bond ) is a special type of covalent bond atom . It occurs mainly in ring-shaped molecules and is precisely described by molecular orbital models.
In normal single bonds, two electron clouds ( atomic orbitals ) overlap , so that the resulting molecular orbital lies on the bond axis. This guarantees maximum overlap and thus the greatest energy gain. Because the smallest possible angle between two atomic orbitals is 90 °, this arrangement can be impossible due to geometric constraints. In such a case, the bonding electron pair can span the bonding axis. The shape of the bond is then reminiscent of a banana. Because the bond electron density is not on the axis between the two connected atoms, the orbital overlap is not optimal and the bond is therefore weaker. This makes compounds with banana bonds more unstable and much more reactive. This is also expressed in a greater heat of combustion compared to isomeric compounds .
Examples of banana ties
Cyclopropane
The simplest example of a banana bond can be found with cyclopropane C 3 H 6 . The molecule is a ring made up of three atoms. This would result in a bond angle of 60 °. However, since the electron clouds are arranged in an sp³- hybridized carbon atom at an angle of approximately 109 °, an elongated arrangement is not possible during the bond. The sp³ hybrid orbitals are each rotated about 25 ° from the bond axis. The Walsh model , which starts from sp² hybridized carbons, provides a better approximation .
Ethene
Two models are discussed for C = C double bonds. Linus Pauling describes double bonds as consisting of two equivalent bonds, the banana bonds presented above. Erich Hückel , on the other hand, suggests that it be composed of a sigma and a pi bond. Which explanation is better has not yet been conclusively clarified, although some theoretical chemists consider both representations to be equivalent.
Diborane
With the compound diborane (B 2 H 6 ) a special case of a banana binding can be found. It is a 2-electron, 3-center bond . The two electrons are in a molecular orbital that is delocalized over three atoms.
Web links
Individual evidence
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^ Linus Pauling: The nature of the chemical bond. Application of results obtained from the quantum mechanics and from a theory of paramagnetic susceptibility to the structure of molecules . J. Am. Chem. Soc. 1931 , 53 1367-1400.
F. Weinhold, Clark R. Landis: Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective , Cambridge University Press, 2005, ISBN 0-52183128-8 . - ^ E. Hückel: Z. Phys 1930 , 60 , 423 .; Penney, WG Proc. R. Soc. London 1934 , A144 , 166; A146 , 223.