A new study reported in the journal Nature Chemistry has filled a significant gap in the scientific understanding of how alcohols are formed and destroyed in space.
According to the study, the rate coefficient for the reaction between the hydroxyl radical and methanol is almost two orders of magnitude larger at 63 Kelvin than previously measured at 200 Kelvin (Orion Nebula by ESO / Digitized Sky Survey 2)
To explain the reaction between the hydroxyl radical and methanol, one of the most abundant organic molecules in space, a team led by Prof Dwayne Heard from the University of Leeds proposes that a quantum mechanical phenomenon known as ‘quantum tunneling’ is revving up the reaction.
Prof Heard’s team found that the rate at which the reaction occurs is 50 times greater at minus 210 degrees Celsius than at room temperature.
It’s the harsh environment that makes space-based chemistry so difficult to understand; the extremely cold conditions should put a stop to chemical reactions, as there isn’t sufficient energy to rearrange chemical bonds. It has previously been suggested that dust grains – found in interstellar clouds, for example – could lend a hand in bringing chemical reactions about. The idea is that the dust grains act as a staging post for the reactions to occur, with the ingredients of complex molecules clinging to the solid surface. However, last year, a highly reactive molecule called the ‘methoxy radical’ was detected in space and its formation couldn’t be explained in this way.
Lab experiments showed that when an icy mixture containing methanol was blasted with radiation – like would occur in space, with intense radiation from nearby stars, for example – methoxy radicals weren’t released in the emitted gases.
The findings suggested that methanol gas was involved in the production of the methoxy radicals found in space, rather than any process on the surface of dust grains. But this brings us back to the problem of how the gases can react under extremely cold conditions.
“The answer lies in quantum mechanics. Chemical reactions get slower as temperatures decrease, as there is less energy to get over the ‘reaction barrier’. But quantum mechanics tells us that it is possible to cheat and dig through this barrier instead of going over it. This is called quantum tunneling,” Prof Heard said.
“We suggest that an ‘intermediary product’ forms in the first stage of the reaction, which can only survive long enough for quantum tunneling to occur at extremely cold temperatures.”
“If our results continue to show a similar increase in the reaction rate at very cold temperatures, then scientists have been severely underestimating the rates of formation and destruction of complex molecules, such as alcohols, in space,” Prof Heard concluded.
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Bibliographic information: Robin J. Shannon et al. Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling. Nature Chemistry, published online June 30, 2013; doi: 10.1038/nchem.1692
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