NUI Galway researchers have been confirmed as the first to produce a strange molecule made of nitrogen, writes MARINA MURPHY
IT HAS CAUSED a lot of fuss over the years. Nasa wanted it. The US Air Force wanted it. And indeed, chemists have been trying to isolate it for years.
But now an Irish scientist based at NUI Galway has been proven to have delivered it – it being a molecule containing no less than five nitrogen atoms. “It” also comes with a negative charge and a difficult name, pentazole anion (N5-).
Pentazole is a nitrogen unit that many believe could form the basis of new, energy-intensive rocket fuels that could revolutionise space travel, hence the high level of interest in this molecule.
Prof Dick Butler and his team successfully produced the elusive pentazole anion in their labs in Galway a full seven years ago, but only recently has confirmation of their international success been finally declared.
Such is the fervour with which chemists have gone after pentazole, that when Butler’s team first reported producing it in 2002 there was a backlash from competitors. The potential of pentazole is in the production of high-nitrogen compounds, which have three or more linked nitrogen atoms.
“They are of immense practical interest because they are high-energy molecules,” according to Butler, who is now emeritus professor of chemistry at NUIG.
Nitrogen can form single, double and triple bonds and the energies tied up in these bonds are substantial. This means that when a high-nitrogen compound breaks down, a considerable amount of energy is released.
Very few nitrogen only compounds exist, however, and theorists have spent years predicting which ones might be possible. N5+ was synthesised in 1999. But N5- remained in the realm of conjecture until 2002 when a group at Edwards Air Force in California, reported that they had observed it fleetingly as a gas.
Speculation then began that if N5- could be tamed and combined with N5+, the result would be a stable solid that could deliver twice the punch of the same volume of hydrazine, the nitrogen-rich fuel that propels many spacecraft today.
“The substance N5+N5- would be a remarkable energy source. It would break down to five molecules of nitrogen gas giving out a huge amount of energy per atom and with no pollution,” says Butler. Nitrogen is the main constituent of air and is therefore non polluting.
Producing pentazole in the gas phase basically involved smashing apart molecules containing the N5- unit, but this is of limited use. “In order to be really useful, chemicals have to be under control in solution and preferably in a bottle, rather than floating around,” explains Butler.
Butler and his team had already been working on the problem for 10 years when they hit upon the right conditions to produce it using a chemical route in 2002.
The Galway group’s method involved using a molecule, ArN5, a compound of pentazole produced in the 1950s. Other groups had spent years trying to remove the stabilising Ar (aryl) ring and all had failed.
Galway’s success came by chilling the chemicals to Arctic conditions. “Chemical reactions slow down and eventually stop at very low temperatures,” says Butler. “We successfully removed the Aryl group at -40C and the core N5- and HN5- groups survived for a very short time, possibly only a minute.”
When Butler’s group first reported that they had produced the long-sought-after molecule in solution, there was a furore. After all, every scientist dreams of being able to claim a science first.
Some chemists maintained that what Butler's team produced was actually NO3-. But work published this year by Prof Rod Bartlett at the University of Florida has proved unequivocally that the Galway team did successfully create pentazole in the lab ( The Journal of Physical Chemistry A, 2009, 113, 3197 – 3201).
Butler’s group has considered the potential uses of this tricky molecule. “Although it is possible that N5+ combined with N5- could be a rocket fuel, it would be extremely difficult technically to produce and store large quantities of N5- anion. It is also likely to be highly uneconomical. However one never knows what human ingenuity may achieve,” Butler says.