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tokens: We are grateful to Ewine van Dishoeck for suggesting this study and we thank her, Simon Bruderer, and François Lique for valuable discussions. We thank the computer and communication department (CCZ) of the Faculty of Science of the Radboud University for computer resources and technical support. The work is supported by The Netherlands Organisation for Scientific Research, NWO, through the Dutch Astrochemistry Network, and in part by the National Science Foundation under grant No. NSF PHY11-25915. N.B. is supported in part by NSF grant PHY-1505557. K.M.W. and P.C.S. acknowledge support from NASA grants NNX12AF42G and NNX13AF42G. I.K. and W.F.T. acknowledge funding from the European Union Seventh Framework Programme FP7-2011 under grant agreement no. 284405.