Three-way reactions open new chemical frontiers
“Chemically termolecular reactions” are an entirely new class of chemical reaction that may change our understanding of everything from combustion to planetary atmospheres.
For chemists, discovering a new chemical reaction is an exciting prospect. Each new reaction we discover adds a little more to our understand of how molecules behave, and new reactions may offer better ways to make drugs and advanced materials or to access new energy sources.
New chemical reactions are reported in the scientific literature almost every day, but each reaction falls into one of three elementary classes chemical processes. It’s a rare event, then, to discover an entirely new kind of elementary process, which is what scientists at Columbia University may have just done.
Chemical reactions are usually classified according to the number of molecules that are involved in the breaking or forming of new chemical bonds.
Unimolecular reactions involve one molecule forming a new product, while bimolecular reactions require two molecules to collide, forming one or more new chemical compounds. Both of these are common.
The third class of chemical reactions is known as termolecular association reactions, and they involve three molecules: first two molecules collide, and then a third molecule removes some of the kinetic energy to stabilise the new product.
Chemically termolecular reactions are subtly different from termolecular association reactions, as the third molecule collides with the first pair to form or break new bonds, rather than just absorbing energy. This fourth type of reaction, where all three molecules are involved in bond formation, was thought to be so statistically unlikely that scientists have ignored them for years.
In a new study led by Michael P. Burke at Columbia University, New York, published in Nature Chemistry, chemically termolecular reactions were examined using state of the art computational methods, examining extreme conditions where this process might be viable, like in a lightning strike or during combustion.
“Combustion has always been a launching point for understanding all sorts of other chemical mechanisms,” says Burke.
Burke and co-author Stephen J. Klippenstein found that under these conditions, a reactive type of species called a free radical might associate with a molecule of oxygen. This “ephemeral collision complex” would be stable enough for a second radical to collide with it, resulting in a three-molecule reaction.
“Potentially there could be innumerable reactions from this new class that impact how we model gas phase chemistry, from designing new types of engines to understanding the planetary chemistry responsible for cloud formations, climate change, evolution of pollutants, even perhaps the sequence of reactions that could impact the conditions for extraterrestrial life. Our discovery opens up a whole new world of possibilities.”