Pyridinium Complexes. I. The Significance of the Second Charge-Transfer Band of Pyridinium Iodides

A second charge-transfer absorption band appears in the spectra of many pyridinium iodide complexes in non-polar solvents. Its relationship to the first charge-transfer absorption band² is shown by parallel behavior with respect to solvent variation and within limits, to change in the substituents on the pyridinium ring. The difference, ΔTE, between the transition energies for the two bands varies between 15.9 and 28.6 kcal./mole. However, ΔTE for 1-methylpyridinium iodide in chloroform is 21.8 kcal./mole. Franck and Scheibe⁴ interpret the ultraviolet spectrum of iodide ion as a photoionization process, producingan iodine atom and a “solvated electron.” Two bands are observed with ΔTE 21.2 kcal./mole, corresponding closely to the expected difference for electronic transitions leading to an iodine atom, which has two low-lying energy states, ²P½ and ²P###, separated by 21.74 kcal./mole (0.943 ev.) as determined by analysis of iodine spectra. The close correspondence of the ΔTE value for 1-methylpyridinium iodide in chloroform to the ΔTE value expected for production of an iodine atom is direct evidence for the charge (electron)-transfer nature of the excitation in the pyridinium iodide. In addition, the position for the charge-transfer band of 1-methylpyridinium iodide in water can be derived from a plot of transition energies against Z, a standard of solvent polarity.² The position thus derived (2561 Å.) demonstrates that the charge-transfer process is greatly facilitated by the 1-methylpyridinium ion, for iodide ion in water has a maximum at 2259 Å. A complete description for the transition is thus: MPy⁺⁺####MPy·I·. Systematic variations in ΔTE values are found for different types of iodide charge-transfer spectra, for which no rationalization is readily apparent.