NCSA NEWS

A delicate balance

By Trish Barker, NCSA

Story posted May 10, 2007

In order to effectively harness the energy created by networks of nitrogen atoms, scientists need to add stability to these systems without sacrificing too much energy output.

An old saying insists that "where there's smoke there's fire." And basic chemistry asserts that where there's fire, there must be oxygen. Everything from car engines to rockets relies on the presence of oxygen to spark the release of energy. So what happens in environments -- from deep space to the deep ocean -- where oxygen is scarce or entirely absent?

While carbon fuels are powerless in oxygen-starved environments, nitrogen fuels face no such restriction. A network in which nitrogen atoms are joined by single bonds readily decomposes in an exothermic reaction, releasing a burst of energy. Most of these compounds are highly unstable, however, making it impossible to harness their energy to create viable high energy density materials (HEDMs).

"If it's unstable, you have one application, and that's to create an explosion," says Douglas Strout, an associate professor of chemistry at Alabama State University.

Strout's research team, which includes undergraduate students gaining valuable hands-on experience, uses computational simulation in an effort to identify nitrogen molecules that provide a substantial energy payout but that are stable enough to be practical power sources. The team recently used NCSA's IBM p690 system, called Copper, to simulate the decomposition of various molecules in which nitrogen atoms were replaced with potentially stabilizing atoms, such as carbon and halogens (chlorine and fluorine). Their simulations identified promising compounds for experimental study, and their results were published in The Journal of Physical Chemistry A in July 2006.

Balancing energy with stability

Any molecule composed solely of nitrogen (N_x) is potentially a rich source of energy. However, theoretical modeling of many N_x molecules has shown that they dissociate too easily and are therefore too unstable to be practical fuel sources. So Strout and other researchers have turned their efforts toward finding molecules that combine energy-rich nitrogen with other stabilizing atoms.

"If you trade away nitrogen content, you're trading energy. Hopefully you can find high energy content and high stability," Strout says.

In previous computational simulations, Strout and his collaborators examined a variety of all-nitrogen molecules -- N₁₂, N₁₄, N₁₆, N₁₈, N₂₄, N₃₀, and N₃₆. While the simulations provided clues about which structures add stability to nitrogen molecules (triangles seem to help, and cylindrical shapes are better than spheres), the molecules still weren't stable enough to be practical HEDMs.

So with the most recent calculations, Strout started with an all-nitrogen molecule, N₁₂, and altered it in several ways that might increase its stability. The six variations Strout and his students examined were N₁₄H₄²⁺ (N₁₂ plus two nitrogen atoms), N₁₄F₄²⁺ (fluorine), N₁₄Cl₄²⁺ (chlorine), N₁₂C₂H₄ (carbon), N₁₂C₂F₄ (carbon and fluorine), and N₁₂C₂Cl₄ (carbon and chlorine). They calculated the stability and energy-output of these six molecules using two methods (Hartree-Fock and the more accurate fourth-order perturbation theory) and NCSA's computational resources.

"We're talking about molecules that don't even exist yet in the laboratory," Strout says, so theoretical modeling saves the time, expense, and hazard of synthesizing and analyzing these substances in the lab.

Calculating energies

All of the molecules Strout's group analyzed have a similar cage structure of nitrogen-to-nitrogen bonds at their heart. The team calculated the energies for the breaking of each N-to-N and C-to-N bond for each molecule; the bond that can be broken with the lowest energy is the weak link where the explosion of energy from the molecule is likely to begin.

For the hydrogen-nitrogen compound and the two halogen-containing molecules, calculations using the fourth-order perturbation theory pointed to the NN1 bonds as the weakest, with the lowest dissociation energies. The bond-breaking energy for N₁₄H₄²⁺ (30.6 kcal/mol) indicates that it might be stable enough to be a high-energy density material, while the even lower energies found for the halogen ions might make them too unstable to be practical.

Next, Strout's team considered whether adding carbon atoms to these molecules might create greater stability without the loss of too much energy. For the three carbonated ions, the researchers found that the weakest bond was now the NN3; in all three cases, the bond-breaking energy for the NN3 link was greater than 35 kcal/mol, meaning the molecules should have the necessary stability for high-energy density materials.

Now that Strout had found several molecules with sufficient stability, the next step was to look at their potential energy output. Calculations showed that the substance that provided the best energy return per unit of mass (2.2 kcal/gram) was the N₁₂C₂H₄; the halogens in the other molecules added mass without providing more energy, or greater stability.

While Strout's research has provided valuable insight into how networks of nitrogen atoms can be combined with other stabilizing substances, he cautioned that there is still more work to be done.

"Don't expect to be putting these molecules in your gas tanks any time soon," he said. "The applications will be less commonplace and more obscure than that."

Now that promising molecular structures have been identified through computational simulation, experimental chemists can direct their efforts toward synthesizing and testing the most promising potential HEDMs.

This research is supported by the National Science Foundation and the National Institutes of Health.

Team members
Roshawnda Cottrell
Jacqueline Jones
Ami Gilchrist
Danielle Shields
Douglas Strout