New research examines a class of reactions between neutral free radicals and neutral molecules that, unlike most reactions, happen rapidly at very low temperatures.

Chemical reactions generally accelerate when temperature increase. However, in today’s issue of Science Magazine [1] a research group from France revealed a class of gas-phase reaction between oxygen atoms and alkenes that just do the opposite – the rate coefficients increase as the temperature is lowered.

Reactions of this sort may be taking place in interstellar clouds, which are difficult to study directly. Hassan Sabbah and colleagues have performed rate measurements between oxygen atoms and ethene, propene, 1-butene, cis-, trans-, and iso– butene over a range of temperatures from 20 to 300 Kelvin. Then they modeled the unusually rapid low-temperature rates, providing a theoretical framework for predicting reaction rates in systems that may be useful for understanding the chemistry of interstellar clouds.

1. “Understanding Reactivity at Very Low Temperatures: The Reactions of Oxygen Atoms with Alkenes”, doi:10.1126/science.1142373