Skoltech researchers have theoretically investigated the wide range of molecules that oxygen and carbon atoms can form in addition to the well-known carbon dioxide and carbon monoxide. Compounds of oxygen and carbon are of interest for space research, battery technology, biochemical studies, and — surprisingly — for the development of industrial explosives and rocket fuel. In their study in Materials Today Energy, supported by a Russian Science Foundation grant, the scientists highlight dozens of unheard-of molecules, some of which hold over 75% of the explosive energy of TNT.

High-energy-density materials are compounds that can release a lot of chemical energy per unit mass and can therefore be used as propellants or explosives. Scientists study these materials in the hope of uncovering compounds that pack more energy than the traditional nitrogen-based explosives, such as TNT, and rocket fuels, such as ammonium perchlorate. In their recent study, Skoltech researchers came upon a new explosive chemistry without a trace of nitrogen while examining compounds known as oxocarbons, or carbon oxides.

“The reason why many high-energy-density materials are based on nitrogen chemistry is this. In the course of chemical reactions, nitrogen atoms seek to form the remarkably stable molecule N₂. As the atoms adopt this energetically favorable configuration, vast amounts of energy are released,” said Skoltech MSc student Elizaveta Vaneeva from the Materials Science program, the lead author of the study.

“So there was this idea that since the bond energy in the carbon monoxide molecule CO is even somewhat greater, one could find a compound that decomposes into carbon monoxide plus something else, and that process would release even more energy,” she went on. “As it turned out, some carbon oxides that we studied release up to 81% as much energy as TNT when they decompose into products that include carbon dioxide, not carbon monoxide.”

The team’s analysis uncovered an entire “molecular zoo” numbering some 224 oxocarbons, of which only 78 had been reported in earlier papers and still fewer had been investigated at that level of detail. Among them the scientists identified 32 compounds with explosive potential and some prospect of synthesis. These include C₄O₈ and C₄O₉ and the newly discovered C₆O₁₂ and C₆O₁₃, all of which release 75% or more of the energy of TNT, with the molecule C₄O₉ packing the biggest punch at 81% the equivalent of TNT.

The principal investigator of the study, Distinguished Professor Artem R. Oganov, who heads the Material Discovery Laboratory at Skoltech, commented on the fundamental significance of the work: “This molecular chemistry is unusual. You see, it is commonly held that molecules are understood better than crystals. But what we see is just the opposite. The variability of chemical compositions of crystals is very limited. Molecules, on the contrary, show great diversity. Still, even among molecules not every composition conceivable on paper will actually form in nature, and we have explained why and predicted a number of those that will likely be found.”

The explanation draws on the concept of “magicity,” which is itself an extension of the notion of magic numbers from nuclear physics to the level of molecules. Rather than evaluate the absolute energy contained in the bonds that hold any given molecule together to assess its stability, the researchers compared the energy of a molecule to that of molecules with the closest possible compositions, i.e., with one extra or one missing oxygen or carbon atom. Any compound that proves energetically more favorable than the neighboring configurations is deemed a “magic” molecule, suggesting its relative stability and high likelihood of formation.

Oxocarbons are promising for many applications including advanced energetics, electrode materials for lithium-ion batteries, atmospheric chemistry, and biochemical studies. They are important for the study of combustion products of common hydrocarbon fuels such as kerosene, ethanol, and dimethyl ether. Oxocarbons are also expected to exist in the interstellar medium and in planets, making them a target of astrophysical research. That said, compounds of carbon and oxygen remain insufficiently studied, with the bulk of the data available for several textbook molecules, primarily for carbon dioxide and carbon monoxide. The Skoltech study is a step to remedy that.