Editing Carbon dioxide (section)

=== Structure, bonding and molecular vibrations ===
{{See also|Molecular orbital diagram#Carbon dioxide}}
The [[Molecular symmetry|symmetry]] of a carbon dioxide molecule is linear and [[centrosymmetric]] at its equilibrium geometry. The [[bond length|length]] of the [[carbon–oxygen bond]] in carbon dioxide is 116.3&nbsp;[[picometer|pm]], noticeably shorter than the roughly 140&nbsp;pm length of a typical single C–O bond, and shorter than most other C–O multiply bonded [[functional group]]s such as [[carbonyls]].<ref name=Green/> Since it is centrosymmetric, the molecule has no [[electric dipole moment]].

[[File:Co2 vibrations.svg|thumb|left|[[Infrared spectroscopy#Number of vibrational modes|Stretching and bending oscillations]] of the {{CO2}} molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes.]]

As a linear triatomic molecule, {{CO2}} has four [[Molecular vibration|vibrational modes]] as shown in the diagram. In the symmetric and the antisymmetric stretching modes, the atoms move along the axis of the molecule. There are two bending modes, which are [[Degenerate energy levels|degenerate]], meaning that they have the same frequency and same energy, because of the symmetry of the molecule. When a molecule touches a surface or touches another molecule, the two bending modes can differ in frequency because the interaction is different for the two modes. Some of the vibrational modes are observed in the [[Infrared spectroscopy|infrared (IR) spectrum]]: the antisymmetric stretching mode at [[wavenumber]] 2349&nbsp;cm<sup>−1</sup> (wavelength 4.25&nbsp;μm) and the degenerate pair of bending modes at 667&nbsp;cm<sup>−1</sup> (wavelength 15.0&nbsp;μm). The symmetric stretching mode does not create an electric dipole so is not observed in IR spectroscopy, but it is detected in [[Raman spectroscopy]] at 1388&nbsp;cm<sup>−1</sup> (wavelength 7.20 μm), with a [[Fermi resonance]] doublet at 1285&nbsp;cm<sup>−1</sup>.<ref>{{cite book | vauthors = Atkins P, de Paula J | title = Physical Chemistry | edition = 8th | publisher = W.H. Freeman | date = 2006 | pages = 461, 464 | isbn = 978-0-7167-8759-4}}</ref>

In the gas phase, carbon dioxide molecules undergo significant vibrational motions and do not keep a fixed structure. However, in a [[Coulomb explosion#Coulomb Explosion Imaging|Coulomb explosion imaging]] experiment, an instantaneous image of the molecular structure can be deduced. Such an experiment<ref>{{cite journal | vauthors = Siegmann B, Werner U, Lutz HO, Mann R | title = Complete Coulomb fragmentation of {{CO2}} in collisions with 5.9 MeV u<sup>−1</sup> Xe<sup>18+</sup> and Xe<sup>43+</sup> | journal = J Phys B  | volume = 35 | issue = 17 | page = 3755 | year = 2002 | doi = 10.1088/0953-4075/35/17/311 | bibcode = 2002JPhB...35.3755S | s2cid = 250782825}}</ref> has been performed for carbon dioxide. The result of this experiment, and the conclusion of theoretical calculations<ref name=Jensen2020>{{cite journal |vauthors = Jensen P, Spanner M, Bunker PR |title = The {{CO2}} molecule is never linear− |journal = J Mol Struct |volume = 1212 |page = 128087 |year = 2020 |doi = 10.1016/j.molstruc.2020.128087 |bibcode = 2020JMoSt121228087J |hdl = 2142/107329 |hdl-access = free }}</ref> based on an [[Ab initio quantum chemistry methods|ab initio]] [[potential energy surface]] of the molecule, is that none of the molecules in the gas phase are ever exactly linear. This counter-intuitive result is trivially due to the fact that the nuclear motion [[volume element]] vanishes for linear geometries.<ref name=Jensen2020/> This is so for all molecules except [[diatomic molecule]]s.