Unprecedented molecule could revolutionize engine performance

Clean energy generation is one of today’s main challenges. The search for potential alternatives to fossil fuels has already led us to the development of technologies such as biofuels and solar energy. However, how can a simple molecule, once considered impossible to produce, change our outlook?

Starting from the Basics: What is a Fuel?

From a chemical point of view, a fuel is a molecule that stores chemical energy and is capable of undergoing a reaction that releases this energy in the form of heat and/or work. Thus, an ideal fuel not only releases a lot of energy but also produces a large amount of gas, causing system expansion that can be harnessed, for example, in pistons to generate thrust, as in conventional engines that burn gasoline or ethanol.

Environmental Problems of Traditional Combustion

However, the combustion products of these compounds are responsible for several environmental issues, the most well-known being the intensification of the greenhouse effect. Therefore, the perfect molecule for this task should convert entirely into gas, in a highly exothermic reaction and without generating harmful byproducts.

Hydrogen vs. Nitrogen as Fuels

Many efforts have focused on hydrogen-powered engines, since its combustion releases only water as a product. However, the use of H₂ has severe technical limitations, such as its energy density, which requires about five times more hydrogen volume to generate the same energy as gasoline.

Therefore, the ideal element for the task is nitrogen, since high-nitrogen compounds often react by releasing large amounts of nitrogen gas (N₂​) — an inert and safe molecule — along with energy. This is the principle behind the use of famous explosives such as TNT and nitroglycerin.

Groundbreaking Discovery: How Researchers Managed to Stabilize N6

The challenge of using nitrogen lies in the fact that, although this element is abundant in the atmosphere as N₂​ (dinitrogen), no natural allotrope (another molecule containing only nitrogen) is known. Researchers such as the German chemist K. O. Christe have spent decades N₅[P(N₃)₆], N₅[B(N₃)₄], and N₂H₅N₅, substances with great potential as high-density energetic materials. These molecules contain ions such as the cation N₅+​ and the anions N₃−​ and N₅−​.

The obstacle was converting these ideas into a neutral molecule made only of N: when attempting to bond these ions, the structure would instead rearrange its bonds and expel N₂, leading to immediate decomposition. The practical outcome: the chemical structure itself favored decomposition into dinitrogen. Thus, for decades, obtaining a neutral molecule containing only nitrogen was considered nearly impossible.

To solve this problem, Peter Schreiner and colleagues published a simple synthesis at room temperature in the journal Nature, starting from AgN₃​ and chlorine or bromine gas, obtaining the unprecedented N₆​, which was then cryogenically captured in argon at 10 K or in liquid nitrogen.

Why N6​ May Revolutionize Energy (and What Still Prevents Its Use)

According to calculations presented by the authors, the energetic potential of N₆ is 2.2 times greater than that of TNT. Despite the excitement, the biggest limitation is the instability of N₆​ under ambient conditions. The article states that the new compound has a half-life of 132 years at 77 K (−196∘C), but only 35.7 milliseconds at 298 K (25∘C).

Another challenge is developing a method for controlled decomposition, allowing gradual energy release, which is indispensable for applications in engines and generators. In short, although N6​ is not yet a ready-to-use fuel, Schreiner’s synthetic route demonstrates that it is possible to obtain unprecedented neutral nitrogen compounds — and it can serve as a starting point for new substances, such as N₁₀​.

Learn More: What are Allotropes?

Allotropes are different forms of the same element, resulting from different atomic arrangements. Classic examples include O2​/O3​ (oxygen/ozone) and graphite/diamond (carbon).

For nitrogen, the most stable and common molecular form is dinitrogen (N₂​), a diatomic gas highly stable due to the strong N≡N triple bond. This stability explains why atmospheric nitrogen is chemically inert. However, under high pressures and temperatures (∼110 GPa and 2000 K), new structures can form, such as polymeric nitrogen (cg-N), in which each atom bonds to three neighbors in a cubic network similar to diamond. N₆​ represents a new step in this exploration.