Probes of Nonreactive Environment. III. Theoretical Aspects of Pyridinium Iodide Charge-Transfer Absorption

Pyridinium iodide salts possess iodide to ring charge-transfer c-t absorption two c-t bands are generally observed in nonpolar solvents. The existence of two bands is important for the researcher in detection because the absolute and relative energies of these bands could be a useful indication of changes to ring substituents and environment surrounding the ring. A question exists, however, as to whether the higher energy c-t band arises from an excited state of the donor or from a higher energy state of the acceptor. Self- consistent extended Huckel calculations have been performed for the unsubstituted and the 2-, 3-, and 4-cyano and acetyl pyridinium ions. The calculated difference between the energy of the first lowest and second next highest vacant molecular orbital of the pyridinium ion is in good agreement with the observed separation of the two c-t bands in methylene chlorine. This supports other experimental results, which indicate that the second c-t band does not arise from the excited state doublet P12 of the iodine atom. These and other results of the calculation are discussed in terms of various experimental parameters.