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Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling


Understanding the abundances of molecules in dense interstellar clouds requires knowledge of the rates of gas-phase reactions between uncharged species. However, because of the low temperatures within these clouds, reactions with an activation barrier were considered too slow to play an important role. Here we show that, despite the presence of a barrier, the rate coefficient for the reaction between the hydroxyl radical (OH) and methanol—one of the most abundant organic molecules in space—is almost two orders of magnitude larger at 63 K than previously measured at 200 K. We also observe the formation of the methoxy radical product, which was recently detected in space. These results are interpreted by the formation of a hydrogen-bonded complex that is sufficiently long-lived to undergo quantum-mechanical tunnelling to form products. We postulate that this tunnelling mechanism for the oxidation of organic molecules by OH is widespread in low-temperature interstellar environments.

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Figure 1: Schematic potential energy surface for the reaction between OH and methanol based on the calculations of Xu and Lin8 with all energies given in kJ mol−1 relative to the reagents.
Figure 2: Temperature dependence of the rate coefficient k1 for the reaction of OH radicals with methanol, plotted in Arrhenius form together with a theoretical calculation.
Figure 3: Removal of hydroxyl (OH) radicals and production of methoxy (CH3O) radicals in the reaction of OH with methanol at 82 K.
Figure 4: Variation of the rate coefficient for the reaction of OH with methanol as a function of total gas density at 82 ± 4 K.
Figure 5: Rate coefficients for the dissociation back to reactants (black) and the product-forming channel (red) as a function of energy within the initially formed OH–methanol complex.


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This work received funding from the Natural Environment Research Council through the provision of a PhD studentship (R.J.S.) and for A.G. The National Centre for Atmospheric Science provides funding for M.A.B. and D.E.H. The authors thank the mechanical and electronics workshops within the School of Chemistry, the University of Leeds, for technical assistance.

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R.J.S., M.A.B., A.G. and D.E.H. conceived and designed the experiments and analysed the data. R.J.S. and M.A.B. performed the experiments. R.J.S., M.A.B. and D.E.H. co-wrote the paper.

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Correspondence to Dwayne E. Heard.

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The authors declare no competing financial interests.

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Shannon, R., Blitz, M., Goddard, A. et al. Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling. Nature Chem 5, 745–749 (2013).

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