Structure of pure carbonic acid discovered

Hello dear friends.

What we call carbonic acid, is an acid that belongs to the category of oxacids and is derived from the reaction of carbon dioxide with water, it is also known by the names of dihydrogen trioxycarbonate or trioxocarbon acid. And although there is a lot of information about this acid, the structure of its pure form has been elusive for chemists due to its instability, since the slightest presence of water catalyzes its decomposition, so it is always found in its ionic forms (bicarbonate anion and carbonate anion), however theoretically it is believed that in the total absence of water it could exist in its pure form, but so far it has not been possible to synthesize the crystalline form of this acid.

Molecular structure of carbonic acid. Source: @emiliomoron, image elaborated in Powerpoin.

The molecular formula of carbonic acid is H₂CO₃, and this molecule is formed when anhydrous carbon dioxide (CO₂) dissolves in water according to the following reaction:

In this equilibrium the hydrated CO₂ species predominates, since the dissociation constant of this acid is very small, Ka = 0.00000045, which makes the equilibrium lean towards the reactants, that is why it is said that in solution there is only dissolved CO₂, since it is estimated that in solution there would be barely 0.003% as carbonic acid.

Specifically, since carbonic acid is a diprotic acid with two acidity constants, KA1 = 1.3 x10^-4 and KA2 = 4.84 x10^-11, in solution it can lose one or more protons, decomposing easily into the anions bicarbonate (if it loses one proton) and carbonate (if it loses two protons).

This is why the preparation of pure carbonic acid has so far been theoretically real, but in practice impossible to detect. Despite a pioneering synthesis that was carried out in dimethyl ether and other research from which two different structures of carbonic acid (α and β) are believed to exist, but due to spectroscopic ambiguities in frequency and phase assignment, the literature has been controversial, so its actual structure is still unknown.

However, information about the structure of this acid could soon be rewritten in chemistry textbooks, as a team of scientists from the Technical University of Munich RWTH and the Hoffmann Institute for Advanced Materials (HIAM) in Shenzhen, China, working at the FRM II Research Neutron Source, has succeeded in obtaining pure carbonic acid and has been able to analyze its crystal structure for the first time.

To solve the enigma surrounding this acid, this team of researchers conducted a neutron diffraction study on deuterated carbonic acid, D₂CO₃, although this option would require a relatively large sample volume and GPa conditions, their previous studies suggested a stability below 2 GPa, so they had to manufacture a capsule that could withstand such conditions, designing a cell with walls of a special alloy and a diamond window that can be subjected to pressure with an anvil. In this cell they tested equimolar mixtures of frozen water and dry ice at the temperature of liquid nitrogen and pressurized with a load of 4 tons, and when returning to room temperature, even more pressure was applied, to reach a final value of 1.85 GPa.

And to better determine the composition of the crystals formed, the team performed a characterization by neutron powder diffraction, because while X-ray diffraction can have an interaction with the electrons of the atoms, the neutron beam interacts with the nuclei, being able to characterize even very light atoms, such as hydrogen, which is essential for the experiment because it allowed to locate more precisely where the two hydrogen atoms of the molecule are located.

Using this approach, the group of scientists was able to determine the structure of the crystals formed inside the cell, confirming that they are indeed carbonic acid molecules linked by hydrogen bonds that form a monoclinic crystalline structure, with four molecular units in total, very similar to the typical structure of inorganic salts.

Experimentally determined crystal structure for carbonic acid. Source: image elaborated in Powerpoint with research data, the acid structure was taken from Wikipedia.org.

And although this type of research is quite fundamental and the structure of carbonic acid turned out to be quite common, knowing the crystalline structure of pure substances is important for chemists, it is something that must be known because other knowledge can be derived from it.

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Thanks for coming by to read friends, I hope you liked the information. See you next time.

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