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Bond length

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In molecular geometry, bond length or bond distance is the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of molecule.

Explanation

Bond length is 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 (all other things being equal) a stronger bond will be shorter. In a bond between two identical atoms half the bond distance is equal to the covalent radius.

Bond lengths are measured in the solid phase by means of X-ray diffraction, or approximated in the gas phase by microwave spectroscopy. 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[1] 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.

Bond distance of carbon to other elements[1]
Bonded element Bond length (pm) Group
H 106 - 112 group 1
Be 193 group 2
Mg 207 group 2
B 156 group 3
Al 224 group 3
In 216 group 3
C 120 - 154 group 4
Si 186 group 4
Sn 214 group 4
Pb 229 group 4
N 147 - 210 group 5
P 187 group 5
As 198 group 5
Sb 220 group 5
Bi 230 group 5
O 143 - 215 group 6
S 181 - 255 group 6
Cr 192 group 6
Se 198 - 271 group 6
Te 205 group 6
Mo 208 group 6
W 206 group 6
F 134 group 7
Cl 176 group 7
Br 193 group 7
I 213 group 7

Bond lengths in organic compounds

The actual bond length between two atoms in a molecule depends on such factors as the orbital hybridization 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.[2] 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°.

cyclobutabenzene with a bond length in red of 174 pm
cyclobutabenzene with a bond length in red of 174 pm

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.[3][4] This type of bonding has also been observed in dimers of neutral phenalene dimers. The bond lengths of these so-called pancake bonds[5] 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.[6] The smallest theoretical CC single bond obtained in this study is 131 pm for a hypothetical tetrahedrane derivative.[7]

The same study also estimated that stretching or squeezing the CC bond in an ethane 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.

Bond lengths in organic compounds[8][9]
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

References

  1. ^ Handbook of Chemistry & Physics (65th ed.). CRC Press. ISBN 0-8493-0465-2.
  2. ^ Fumio Toda (2000). "Naphthocyclobutenes and Benzodicyclobutadienes: Synthesis in the Solid State and Anomalies in the Bond Lengths". European Journal of Organic Chemistry. 2000 (8): 1377–1386. doi:10.1002/(SICI)1099-0690(200004)2000:8<1377::AID-EJOC1377>3.0.CO;2-I. {{cite journal}}: Unknown parameter |month= ignored (help)
  3. ^ Novoa JJ, Lafuente P, Del Sesto RE, Miller JS (2001-07-02). "Exceptionally Long (2.9 Å) C-C Bonds between [TCNE]- Ions: Two-Electron, Four-Center *-* C-C Bonding in -[TCNE]22-". Angewandte Chemie International Edition. 40 (13): 2540–2545. doi:10.1002/1521-3773(20010702)40:13<2540::AID-ANIE2540>3.0.CO;2-O.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Lü J-M, Rosokha SV, Kochi JK (2003). "Stable (Long-Bonded) Dimers via the Quantitative Self-Association of Different Cationic, Anionic, and Uncharged -Radicals: Structures, Energetics, and Optical Transitions". J. Am. Chem. Soc. 125 (40): 12161–12171. doi:10.1021/ja0364928.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Suzuki S, Morita Y, Fukui K, Sato K, Shiomi D, Takui T, Nakasuji K (2006). "Aromaticity on the Pancake-Bonded Dimer of Neutral Phenalenyl Radical as Studied by MS and NMR Spectroscopies and NICS Analysis". J. Am. Chem. Soc. 128 (8): 2530–2531. doi:10.1021/ja058387z.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. ^ Huntley DR, Markopoulos G, Donovan PM, Scott LT, Hoffmann R (2005). "Squeezing CC Bonds". Angewandte Chemie International Edition. 44 (46): 7549–7553. doi:10.1002/anie.200502721. PMID 16259033.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ Martinez-Guajardo G, Donald KJ, Wittmaack BK, Vazquez MA, Merino G (2010). "Shorter Still: Compresing C-C Single Bonds". Organic Letters, ASAP.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Fox, Marye Anne; Whitesell, James K. (1995). Organische Chemie: Grundlagen, Mechanismen, Bioorganische Anwendungen. Springer. ISBN 978-3-86025-249-9.{{cite book}}: CS1 maint: multiple names: authors list (link)
  9. ^ Prof Chao-Jun Li, Ph.D. in lecture, March 2009 (needs citation)
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