The mystery of the “magical” quantum stability of hydrocarbons is solved!

We are going to celebrate during this decade the centenary of the discovery of the principles and equations quantum mechanics, essentially via contributions from Heisenberg, Born, Schrödinger and Dirac. But we could also cite Louis de Broglie, Niels Bohr, Wolfgang Pauli and of course Lev Landau and Enrico Fermi.

Very quickly, thanks to the famous equation discovered by Schrödinger, the quantum revolution will go from physical to chemistry with the work of several researchers, first of all Walter Heitler and Fritz London who will lay the foundations of a quantum theory of the chemical bond in the molecule ofhydrogen in 1927. This work will be extended in the years that will follow, in particular by the Nobel Prize in Chemistry Linus Pauling and other researchers – we can cite Friedrich Hund, Robert Mulliken and John C. Slater. We will thus see appearing the theory of the binding of valencethat of molecular orbitals but also notions of quantum chemistry such ashybridization atomic orbitals and also the mesomery.

Pauling will then be able to explain the existence and properties of aromatic hydrocarbonsin particular from benzene. Simple explanations on this subject can be found in the famous quantum mechanics course of another Nobel Prize, Richard Feynman. For the brave, already well informed, it is enriching to read the works of Pauling, both his introduction to general chemistry that his treatise on the chemical bond (even his treatise on quantum chemistry), the latter being a great reference in the chemistry of the XX^e century.

Uspex and hydrocarbons

An exciting new breakthrough in the field of quantum chemistry has just been published via an article in The Journal of Physical Chemistry Letters. It concerns the hydrocarbons in general and it follows again from work of the Russian physicist, chemist and crystallographer Artem Oganov.

Futura regularly talks about its contributions with colleagues, particularly in the field of physico-chemistry of materials at high pressure. We explained that his scientific trajectory had taken him to several renowned global research centers, from Moscow State University to Stony Brook University, including University College London and ETH in Zürich. Member for a time of the mythical Moscow Institute of Physics and Technology (MIPT for Moscow Institute of Physics and Technology, Московский Физико-Технический институт, in Russian), dubbed PhysTech (Физтех), as we talk about l’X in France or MIT (Massachusetts Institute of Technology) in the United States, he continued his research while stationed at Skolkovo Institute of Science and Technology (Skoltech) which can be considered the Russian equivalent of MIT in the United States.

The breakthrough obtained today in the field of hydrocarbons was made possible once again thanks to the algorithm named Uspex (Universal Structure Predictor: Evolutionary Xtallography) that we owe initially to Artem Oganov, but which was developed by his research group. In Russian uspekh means “success” and we can already convince ourselves of its accomplishments when we know that it is used by more than 7,000 researchers worldwide. It allows to calculate ab-initio from the laws of quantum mechanics the properties of molecules or crystals, in particular under conditions of pressure and temperature exotic.

Nothing extreme in Uspex’s new success, as one can be convinced by reading the press release from Skoltech on this subject and from which we take up some excerpts by translating them. We can also see that the published discovery was already announced in the video below dating from 2021.

“Magic” numbers in nuclear and molecular physics

To understand what it is all about, it is necessary to know that hydrocarbons are molecules with atoms of carbon (C) forming a framework on which are stuck hydrogen atoms (H). These are therefore alkanes,alkenes or even cyclic molecules, aromatic or not, as chemists say in their jargon, such as cyclanes (hydrocarbons comprising one or more cycles of carbon atoms united by single bonds) or benzene.

These frameworks have the general formula C_notH_mfor example CH₄ for methane or C₆H₆ for benzene. Combinations of these frameworks on which one can graft atoms ofoxygen and D’nitrogen to form various chemical functions will be found at the base of all organic chemistry. Understanding these molecules is a key to predicting their reactions and therefore to being able to predict how to synthesize them for various applications.

However, as the lead author of the published article, Sergey Lepeshkin, explains: An organic chemistry textbook with its disorderly multitude of different molecules is scary. No one knows why some molecules exist and others don’t. You can learn a lot about each specific molecule if you draw its chemical structure using “sticks” to represent the bonds between atoms, but in reality many concepts related to this are still a matter of debate and unanswerable. not to the question “Why?”. “Sticks” are nothing more than a convenient abstract tool, whereas the stringent stability criterion for compounds is energy. »

As Artem Oganov already explained in the video above, it is possible to transpose concepts used in nuclear physics to quantum chemistry in order to study the stability of hydrocarbon molecules. Thus, a kind of map has been drawn up for a long time giving the energies connections of the nuclei of various elements with their isotopes and some have been found to be particularly stable to the point that they have been called “magic” nuclei with “magic” numbers of protons or of neutrons.

These numbers were discovered during the 1940s, but using Schrödinger’s equation to account for what is known as the shell model of nuclei, the analogue of the model with electronic shells for atoms, the Nobel Prize in Physics Maria Goeppert Mayer was able to predict them theoretically. It is therefore a similar performance that Uspex has made possible by showing that there are magic hydrocarbons with particular numbers ofelectrons for these molecules. A molecule is “magical” if it is more stable than a set of its neighbors in chemical space.

For this, Uspex was used to calculate the bond energies of molecules different from each other by one atom more or less, and to draw up with the results a map of the bond energies similar to what Artem Oganov already had made with his colleagues and which concerned oxides of siliconmaps similar to that of atomic nuclei known for decades.

In total, very many hydrocarbons which can contain up to 20 carbon atoms and 42 hydrogen atoms have been used.

In Skoltech’s press release, Artem Oganov comments on the results obtained, explaining that: ” We now have all hydrocarbon chemistry on one card. Remarkably, the stability ridges represent the homologous series in our textbooks, which are essentially series of compounds with regular changes in composition, structure and properties. The map clearly shows which molecules are easy to synthesize and which can form spontaneously and accumulate in large concentrations. For example, the map clearly explains why some of the compounds exist in atmospheres planets and interstellar space, in the flames and in the deposits of oil. Finally, the map can predict the molecules yet to be found. Of great interest are those which already exist, but which are not “magical”. The most striking examples include cyclopropane which chemists consider unstable due to strained bonds with non-optimal angles, butadiene which is known to be very active and whose propensity to polymerization is used in the industrial production of rubberand an emblematic molecule, cyclobutadiene, which took a good 30 years to synthesize. »