Chemists create forbidden compounds

Exotic saline variants contradict textbook rules and revolutionize chemistry

NaCl3 - a really "impossible" KOchsalz variant. The colors symbolize the electron localization. © Artem Oganov / Stony Brook University
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According to the chemistry textbook, these compounds are unlikely to exist: researchers have produced high-pressure and heat saline variants in which each sodium atom is associated with two or more chlorine atoms. But that violates the rule that every compound strives for a full outer shell of electrons. The creation of such "forbidden" compounds would usher in a revolution in chemistry, say the scientists in the journal "Science". This is only the beginning of the discovery of completely new connections.

Common salt, with the chemical name sodium chloride (NaCl), is one of the best known, most studied and stable compounds. Its chemical composition is very simple: one sodium atom (Na) and one chlorine atom (Cl) form the cubic salt lattice. This applies at least under normal pressure and temperature conditions. Other compounds of the two chemical elements are prohibited by the rules of classical chemistry.

Because the octet rule states that all chemical elements aim for the state of the noble gases. These each have eight electrons in their outer shell, which is completely filled with it. Sodium has just a single atom too much for a closed outer shell, while chlorine lacks exactly one. When the two combine, the sodium releases its surplus electron to the chlorine, so that both atoms reach a closed outer shell with eight electrons each. This creates a strong ionic bond.

Violation of the eights rule

Under extreme conditions, however, this looks different, as has now been noted by an international team of researchers. They had compressed and heated common salt at more than 200, 000-fold atmospheric pressure, either adding an extra dose of sodium or chlorine to the sample chamber. In the light of the X-ray source PETRA III at the German Electron Synchrotron DESY in Hamburg, the researchers observed the result.

"Based on theoretical predictions, we heated the samples under pressure for a while with lasers, " explains DESY researcher Zuzana Konôpková. Oganov's team had previously calculated that under these extreme conditions, exotic connections could form and then remain stable as long as the extreme conditions persist. "We predicted and created crazy compounds that violate the textbook rules: NaCl3, NaCl7, Na3Cl2, Na2Cl, and Na3Cl, " says lead author Weiwei Zhang of DESY. display

Especially the compounds Na3Cl and NaCl3 were easy to generate. "These compounds are thermodynamically stable and will remain so once generated, " stresses Zhang. "Classical chemistry forbids its existence. Classical chemistry also says that atoms try to satisfy the octet rule - the elements take or give electrons to reach the electron configuration of the next noble gas, with a fully occupied outer electron shell which makes her very stable. Well, here this rule is not fulfilled. "

This is what a conventional saline crystal looks like: NaCl - sodium and chloride always alternate. public domain

Revolution in chemistry

The experiments can expand the view of chemistry, the authors emphasize. "I think this work is the beginning of a revolution in chemistry, " Oganov is convinced. "Even at comparably low pressures that can be achieved in the laboratory, we have found perfectly stable compounds that contradict the classical rules of chemistry. Even at a relatively moderate pressure of 200, 000 atmospheres - in the center of the earth there is a pressure of 3.6 million atmospheres - much of what we know from chemistry textbooks loses his validity. "

One reason for the surprising discovery is that textbook chemistry usually applies to the so-called normal conditions. "Here on the earth's surface, these conditions may be normal, " says Kon pkov . "But when you look at the universe as a whole, they are quite special." What is "forbidden" under terrestrial normal conditions can be possible under extreme conditions. "'Impossible' actually means that the energy is high, " says Oganov. "The rules of chemistry are not like mathematical theorems that can not be broken. The rules of chemistry can be broken, because 'impossible' is impossible. You just have to find the conditions under which the energy balance changes, and then the rules no longer apply. "

New applications possible

Apart from its fundamental importance, the discovery can also lead to new applications. "Changing the theoretical foundation of chemistry is a big deal, " says Goncharov. "But it does mean that we can make new materials with exotic properties." Among the compounds that the group has created around Oganov are, for example, two-dimensional metals, in which current flows along the structural layers,

"One of these materials - Na3Cl - has a fascinating structure, " reports Oganov. "It consists of NaCl layers and layers of pure sodium. The NaCl layers act as insulators, the pure sodium layers conduct the current. Systems with two-dimensional electrical conductivity have aroused a lot of interest. "

The salt-salt experiments may be just the beginning of the discovery of completely new compounds. "If this simple system is able to turn into such a diverse set of connections under pressure, then it is likely to do so for others, " Goncharov says. "This could help to answer open questions about young planetary cores, but also to generate new materials of practical use." (Science, 2013; doi: 10.1126 / science.1244989)

(German electron synchrotron DESY, 20.12.2013 - NPO)