Inmolecular geometry,bond length orbond distance is defined as the average distance betweennuclei of twobondedatoms in amolecule. It is atransferable property of a bond between atoms of fixed types, relatively independent of the rest of the molecule.
Bond length is related tobond order: when moreelectrons participate in bond formation the bond is shorter. Bond length is also inversely related tobond strength and thebond dissociation energy: all other factors being equal, a stronger bond will be shorter. In a bond between two identical atoms, half the bond distance is equal to thecovalent radius.
Bond lengths are measured in the solid phase by means ofX-ray diffraction, or approximated in the gas phase bymicrowave spectroscopy. A bond between a given pair of atoms may vary between different molecules. For example, the carbon to hydrogen bonds inmethane are different from those inmethyl chloride. It is however possible to make generalizations when the general structure is the same.
A table with experimentalsingle bonds for carbon to other elements is given below. Bond lengths are given inpicometers. By approximation the bond distance between two different atoms is the sum of the individualcovalent radii (these are given in thechemical element articles for each element). As a general trend, bond distancesdecrease across therow in theperiodic table andincrease down agroup. This trend is identical to that of theatomic radius.
| Bonded element | Bond length (pm) | Group |
|---|---|---|
| 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 |
The bond length between two atoms in a molecule depends not only on the atoms but also on such factors as theorbital hybridization and the electronic andsteric nature of thesubstituents. Thecarbon–carbon (C–C) bond length in diamond is 154 pm. It is generally considered the average length for a carbon–carbon single bond, but is also the largest bond length that exists for ordinary carbon covalent bonds. Since one atomic unit of length (i.e., a Bohr radius) is 52.9177 pm, the C–C bond length is 2.91 atomic units, or approximately three Bohr radii long.
Unusually long bond lengths do exist. Current record holder for the longest C-C bond with a length of 180.6 pm is 1,8-Bis(5-hydroxydibenzo[a,d]cycloheptatrien-5-yl)naphthalene,[2] one of many molecules within a category ofhexaaryl ethanes, which are derivatives based onhexaphenylethane skeleton. Bond is located between carbons C1 and C2 as depicted in a picture below.
Another notable compound with an extraordinary C-C bond length istricyclobutabenzene, in which a bond length of 160 pm is reported. Longest C-C bond within the cyclobutabenzene category is 174 pm based onX-ray crystallography.[3] In this type of compound 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 adimer of twotetracyanoethylene dianions, although this concerns a 2-electron-4-center bond.[4][5] This type of bonding has also been observed in neutralphenalenyl dimers. The bond lengths of these so-called "pancake bonds"[6] are up to 305 pm.
Shorter than average C–C bond distances are also possible:alkenes andalkynes have bond lengths of respectively 133 and 120 pm due to increaseds-character of thesigma bond. Inbenzene all bonds have the same length: 139 pm. Carbon–carbon single bonds increased s-character is also notable in the central bond ofdiacetylene (137 pm) and that of a certaintetrahedrane dimer (144 pm).
Inpropionitrile thecyano group withdraws electrons, also resulting in a reduced bond length (144 pm). Squeezing a C–C bond is also possible by application ofstrain. An unusual organic compound exists calledIn-methylcyclophane with a very short bond distance of 147 pm for the methyl group being squeezed between atriptycene and a phenyl group. In anin silico experiment a bond distance of 136 pm was estimated forneopentane locked up infullerene.[7] The smallest theoretical C–C single bond obtained in this study is 131 pm for a hypothetical tetrahedrane derivative.[8]
The same study also estimated that stretching or squeezing the C–C bond in anethane molecule by 5 pm required 2.8 or 3.5 kJ/mol, respectively. Stretching or squeezing the same bond by 15 pm required an estimated 21.9 or 37.7 kJ/mol.
| 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 | 147 | Alkyne | 120 |
| sp3–sp | 146 | Allene | 130 | ||
| sp2–sp | 143 | ||||
| sp–sp | 137 |
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