Reactions between neutral species

Rapid Neutral-Neutral Reactions at Low Temperatures?

Before the work of our group initiated here in Rennes, it was widely believed that rate constants for neutral-neutral reactions would become vanishingly small at the low temperatures of dense interstellar clouds, owing to the presence of substantial energetic barriers on the minimum energy path connecting reactants to products (case (a) above). Processes involving charged species, such as ion-molecule reactions, are generally barrierless, along with a limited number of reactions between two neutral radicals (case (b)), and these processes were assumed to dominate the chemistry of low temperature environments. In the early 1990s, a joint project was initiated between the Rennes group, directed at the time by Bertrand Rowe, and the gas kinetics and dynamics group of Ian W.M. Smith at Birmingham, to study the kinetics of neutral-neutral reactions down to very low temperatures using the CRESU technique. The first results on reactions of the CN radical shown to the right, demonstrated that rate constants for reactions between radicals and molecules could remain rapid or even increase at low temperatures, and therefore be of importance in low temperature environments such as dense interstellar clouds and planetary atmospheres.

Rate constants - Rate constants k as a function of temperature, T, displayed on a log-log scale for reactions of the CN radical with some simple hydrocarbons. I. R. Sims, et al., Chem. Phys. Lett. 211, 461 (1993).

The Descartes Prize

These results and subsequent developments in Rennes and Birmingham resulted in the award of one of the first Descartes Prizes of the European Union in 2000, for a project entitled “Chemistry Close to Absolute Zero”. The Descartes Prize “is awarded for outstanding scientific or technological results from European collaborative research.”

And today ?

Research on neutral-neutral reactions at very low temperatures continues in Rennes, spurred on by technical innovations and the demand for data from astrochemical modellers and planetary scientists. Recently, for example, the group have measured the rate constant for the reaction F + H2 → HF + H down to 11 K (M. Tizniti, S. D. Le Picard, F. Lique, C. Berteloite, A. Canosa, M. H. Alexander, and I. R. Sims, Nature Chemistry 6, 141 (2014)). This has enabled astronomers to link observations of HF in the interstellar medium by the Herschel Space Observatory with the abundance of H2, and hence effectively the total mass of interstellar objects such as molecular clouds.

. - Herschel and Aquila. Credit: ESA and SPIRE & PACS consortia, Ph. André (CEA Saclay) for Gould’s Belt Key Programme Consortia