This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under grant LE 2221/15-1 and project 500728527. Computations were performed with computing resources granted by RWTH Aachen University under project rwth0087 and rwth1400. We thank Maxim Papusha for contributing his valuable expertise in optimizing transition states using the Nudged Elastic Band method.