It is well known that pyridine (py) and iodine (I₂) form a 1:1 molecular complex, py-I₂. Within the framework of a student project at Roskilde University [1], we have recently investigated the formation and mid-infrared linear dichroism of the complex in a stretched polyethylene matrix. However, the observed spectra displayed not only the expected transitions due to py and py-I₂, but also a third, prominent series of sharp transitions, evidently due to at least one additional product. On the basis of a comparison with literature spectra, these transitions can be assigned to the N-iodo­pyridinium cation, pyI⁺. This assignment is consistent with the observed polarization data.

Ionic side reactions such as py-I₂ → pyI⁺ + I^-, etc., are easily observed in binary pyridine-iodine mixtures and in polar solvents, but not so easily in non-polar media. We are thus surprised to see the apparent efficiency with which the pyI⁺ cation is formed in a non-polar medium like polyethylene. With excess iodine, the peaks assigned to pyI⁺ dominate the observed mid-infrared spectrum (see below). We suspect that a driving force for the ionic reaction is the formation of polyiodide anions in the channels of the polymer medium. Further investigation of the phenomenon is in progress.

[1] H. S. van Deurs, M. L. M. Espersen, R. Nissen, M. H. Rasmussen, C. J. S. Schönbeck, Student Project Report, Roskilde University, June 2006 (in Danish).

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