Editing Carbonic acid (section)

==Anhydrous carbonic acid==
At ambient temperatures, pure carbonic acid is a stable gas.<ref name="lo" /> There are two main methods to produce [[anhydrous]] carbonic acid: reaction of [[hydrogen chloride]] and [[potassium bicarbonate]] at [[Orders of magnitude (temperature)|100&nbsp;K]] in [[methanol]] and [[Proton beam|proton irradiation]] of pure [[solid carbon dioxide]].<ref name="sublime" /> Chemically, it behaves as a [[diprotic acid|diprotic]] [[Brønsted acid]].<ref name=peroxide /><ref name="Andersen" />

Carbonic acid [[monomers]] exhibit three [[conformational isomerism|conformational isomers]]: cis–cis, cis–trans, and trans–trans.<ref>{{cite journal |author1=Loerting, Thomas |author2=Bernard, Juergen |year=2010 |title=Aqueous Carbonic Acid (H<sub>2</sub>CO<sub>3</sub>) |journal=ChemPhysChem |issue=11 |pages=2305–9 |doi=10.1002/cphc.201000220}}</ref>

At low temperatures and [[atmospheric pressure]], solid carbonic acid is [[Amorphous solid|amorphous]] and lacks [[Bragg peak|Bragg peaks]] in [[X-ray diffraction]].<ref name="wi">{{cite journal |last1=Winkel |first1=Katrin |last2=Hage |first2=Wolfgang |last3=Loerting |first3=Thomas |last4=Price |first4=Sarah L. |last5=Mayer |first5=Erwin |date=2007 |title=Carbonic Acid: From Polyamorphism to Polymorphism |journal=Journal of the American Chemical Society |volume=129 |issue=45 |pages=13863–71 |doi=10.1021/ja073594f |pmid=17944463}}</ref> But at high pressure, carbonic acid crystallizes, and modern analytical spectroscopy can measure its geometry.

According to [[neutron diffraction]] of [[deuterium|dideuterated]] carbonic acid ({{chem|D|2|CO|3}}) in a [[hybrid clamped cell]] ([[Russian alloy]]/[[Copper beryllium alloy|copper-beryllium]]) at 1.85&nbsp;GPa, the molecules are planar and form [[Dimerization (chemistry)|dimers]] joined by pairs of [[hydrogen bond]]s. All three [[Carbon–oxygen bond|C-O bonds]] are nearly equidistant at 1.34&nbsp;[[Angstrom|Å]], intermediate between typical C-O and C=O distances (respectively 1.43 and 1.23&nbsp;Å). The unusual C-O bond lengths are attributed to delocalized [[Pi bond|π bonding]] in the molecule's center and extraordinarily strong hydrogen bonds. The same effects also induce a very short O—O separation (2.13&nbsp;Å), through the 136° O-H-O [[angle]] imposed by the doubly hydrogen-bonded 8-membered rings.<ref name="neutron" /> Longer O—O distances are observed in strong intramolecular hydrogen bonds, e.g. in [[oxalic acid]], where the distances exceed 2.4&nbsp;Å.<ref name="wi" />