Formation of the N-Iodopyridinium Cation in an Alkane Environment: Mid-Infrared Linear Dichroism Investigation of the Pyridine-Iodine System in Stretched Polyethylene

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 (PE) 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 one or more additional products. On the basis of a comparison with literature spectra, we assigned these transitions to the N-iodopyridinium cation (pyI+). This assignment is consistent with the observed polarization data. Ionic side reactions (py-I2 = pyI+ + I–, py-I2 + I2 = pyI+ + I3–, etc.) are easily observed in binary pyridine-iodine mictures 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 an apolar and aprotic medium like PE. With excess iodine, the peaks assigned to pyI+ dominate the observed mid-infrared spectrum. 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, Roskilde University, June 2006 (in Danish).