squeezing by

Bond length

In molecular geometry, bond length or bond distance is the average distance between nuclei of two bonded atoms in a molecule.


Bond length is inversely related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely 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

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 individuales 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 120 - 154 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 6
Se 198 - 271 group 16
Te 205 group 16
Mo 208 group 6
W 206 group 6
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

Bond lengths in organic compounds

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 exist. 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.

Bond lengths in organic compounds
C-H length (pm) C-C length (pm) Multiple bonds length (pm)
sp3-H 110 sp3-sp3 154 Benzene 140
sp2-H 109 sp3-sp2 150 Alkene 134
sp-H 108 sp2-sp2 135 Alkyne 120
sp3-sp 146 Allene 130
sp2-sp 143
sp-sp 120

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