The observation of one-dimensional N 2 inside single-walled carbon nanotubes raises the questions, how are the N 2 molecules formed and how do they manage to make their way to this peculiar place? We have used N 15 and C 13 isotope labeled acetonitrile during the synthesis of single-walled carbon nanotubes to investigate this process. The isotope shifts of phonons and vibrons are observed by Raman spectroscopy and X-ray absorption. We identify the catalytic decomposition of acetonitrile as the

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... nitial step in the reaction pathway to single-walled carbon nanotubes containing encapsulated N 2 .