Bond length or bond distance in molecular geometry is the distance between two bonded atoms in a molecule.

Explanation

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Bond length is directly related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also directly related to bond strength and the bond dissociation energy as a stronger bond is also a shorter bond. In a bond between two identical atoms half the bond distance is equal to the covalent radius. Bond lengths are measured in molecules by means of x-ray diffraction. A set of two atoms sharing a bond is unique going from one molecule to the next. For example the carbon to hydrogen bond in methane is different from that in methyl chloride. It is however possible to make generalizations when the general structure is the same.

Bond lengths of carbon with other elements

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A table with experimental single bonds for carbon to other elements is given below. Bond lengths are given in picometers. By approximation the bond distance between two different atoms is the sum of the individual covalent radii (these are given in the chemical element articles for each element). As a general trend, bond distances decrease across the row in the periodic table and increase down a group. This trend is identical to that of the atomic radius.

H	106 - 112	group 1
Be	193	group 2
Mg	207	group 2
B	156	group 13
Al	224	group 13
In	216	group 13
C	154 - 120	group 14
Si	186	group 14
Sn	214	group 14
Pb	229	group 14
N	147 - 210	group 15
P	187	group 15
As	198	group 15
Sb	220	group 15
Bi	230	group 15
O	143 - 215	group 16
S	181 - 255	group 16
Cr	192	group 16
Se	198 - 271	group 16
Te	205	group 16
Mo	208	group 16
W	206	group 16
F	134	group 17
Cl	176	group 17
Br	193	group 17
I	213	group 17
Table 1. Bond distance of carbon to other elements in picometers

Carbon-carbon bond lengths

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The actual bond length between two atoms in a molecule depends on such factors as the orbital hybridisation and the electronic and steric nature of the substituents. The carbon carbon bond length in diamond is 154 pm which is also the largest bond length that exists for ordinary carbon covalent bonds.

Unusually long bond lengths do exists. In one tricyclobutabenzene a bond length of 160 pm is reported. The current record holder is another cyclobutabenzene with length 174 pm based on X-ray crystallography . In this type of compounds the cyclobutane ring would force 90° angles on the carbon atoms connected to the benzene ring where they ordinarily have angles of 120°.

The existence of a very long C-C bond length of up to 290 pm is claimed in a dimer of two tetracyanoethylenedianions although this concerns a 2-electron-4-center bond . This type of bonding has also been observed in dimers of neutral phenalene dimers. The bond lengths of these so-called pancake bonds are up to 305 pm.

Shorter than average carbon carbon bonds distances are also possible, alkenes and alkynes have bond lengths of respectively 133 and 120 pm due to increased s-character of the sigma bond.In benzene all bonds have the same length: 139 pm. In carbon carbon single bonds increased s-character is also notable in the central bond of diacetylene (137 pm) and that of a certain tetrahedrane dimer (144 pm).

In propionitrile the cyano group withdraws electrons also resulting in a reduced bond length (144 pm). Squeezing a CC bond is also possible by application of strain. An unusual organic compound exists called In-Methylcyclophane with a very short bond distance of 147 pm for the methyl group being squeezed between a trypticene and a phenyl group. In an in silico experiment a bond distance of 136 pm is estimated for neopentane locked up in fullerene . The smallest theoretical CC single bond obtained in this study is 132 pm for a hypothetical adamantane like molecule. In the same study it is estimated that for ethane it takes 2.8 kJ/mol to stretch the CC bond by 5 pm from its equilibrium value and only 3.5 kJ/mol to squeeze it by the same amount. on the other hand, stretching and squeezing by 15 pm requires 21.9 and 37.7 kJ/mol.

See also

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• Chemical bond
• Molecule

External links

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• Utah Chemists Find Longest Carbon-Carbon Bond Press release

References

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1. Handbook of Chemistry & Physics 65th Edition CRC Press ISBN 0849304652
2. Naphthocyclobutenes and Benzodicyclobutadienes: Synthesis in the Solid State and Anomalies in the Bond Lengths Fumio Toda European Journal of Organic Chemistry Volume 2000, Issue 8, Date: April 2000, Pages: 1377-1386 Abstract
3. Exceptionally Long (2.9 Å) C-C Bonds between Ions: Two-Electron, Four-Center *-* C-C Bonding in -[TCNE22- Angewandte Chemie International Edition Volume 40, Issue 13, Date: July 2, 2001, Pages: 2540-2545 Juan J. Novoa, Pilar Lafuente, Rico E. Del Sesto, Joel S. Miller Abstract
4. Stable (Long-Bonded) Dimers via the Quantitative Self-Association of Different Cationic, Anionic, and Uncharged -Radicals: Structures, Energetics, and Optical Transitions Jian-Ming Lü, Sergiy V. Rosokha, and Jay K. Kochi J. Am. Chem. Soc.; 2003; 125(40) pp 12161 Abstract]
5. Aromaticity on the Pancake-Bonded Dimer of Neutral Phenalenyl Radical as Studied by MS and NMR Spectroscopies and NICS Analysis Shuichi Suzuki, Yasushi Morita, Kozo Fukui, Kazunobu Sato, Daisuke Shiomi, Takeji Takui, and Kazuhiro Nakasuji J. Am. Chem. Soc.; 2006; 128(8) pp 2530 Abstract
6. Squeezing CC Bonds Deborah R. Huntley, Georgios Markopoulos, Patrick M. Donovan, Lawrence T. Scott, Roald Hoffmann Angewandte Chemie International Edition Volume 44, Issue 46 , Pages 7549 - 7553 2005

External links

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• Bond length tutorial

Chemical bond properties Molecular geometry