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AstroChemistry

Cool Reactions

cool reactionAs a general rule, atoms and molecules–very much like humans–tend to slow down as they get colder, which means less movement and interactions with their peers. Astrochemist Ian Sims and colleagues at the Rennes-based PALMS laboratory,1 together with colleagues from the United Kingdom and the United States, recently showed how some particles shake off this tendency to hibernate, and become more chemically active when getting near the frigid temperatures found in outer space.2

In chemistry, the Arrhenius equation, which was first formulated in the 19th century, states that chemical reactions become faster as temperatures increase. Atoms and molecules need to overcome an “energy barrier” to react with each other, and as Sims explains, “an increase in temperature can give them the impulse to get over that hill.” But the Arrhenius equation does not apply to all reactions. In the frigid vastness of space, interstellar clouds have to be chemically active to become stars. Figuring out which chemicals will react and at what temperatures is difficult: Theory can't answer everything, and low-temperature experiments are both time-consuming and expensive.

To investigate this matter, Sims and his team used the CRESU3 technique in which a low-density gas moves at supersonic speeds through a tube, creating temperatures as low as -263°C. Measuring the reaction of oxygen atoms and different gas phase hydrocarbons called alkenes, they found that the speed of the reactions was connected to the ionization potential of the alkene. If the ionization potential was above a certain threshold, the alkene and oxygen were unable to overcome the energy barrier to react with each other in cold environments. On the other hand, if the ionization potential was just below the threshold, the chemicals could overcome this barrier and react with one another.

“These [atoms and molecules] need a certain amount of time to adjust their configuration to get over the barrier, and they have that time at lower temperatures, “explains Sims. In other words, low temperatures actually aid the reaction process.

With Sims' experiments, and theoretical calculations from his American colleagues, it should now be possible to predict extreme-cold reactions from room-temperature experiments.

 

Mark Reynolds

 

Notes :

1. Laboratoire de physique des atomes, des lasers, des molécules et des surfaces (CNRS / Université Rennes 1).
2. H. Sabbah et al., “Understanding reactivity at very low temperatures: the reactions of oxygen atoms with alkenes,” Science. 317: 102-105. 2007.
3. Cinétique de Réaction en Ecoulement Supersonique Uniforme: Reaction Kinetics in Uniform Supersonic Flow.

Contacts :

Ian Sims
PALMS, Rennes.
ian.sims@univ-rennes1.fr


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