Pyridinium Complexes. III. Charge-Transfer Bands of Polyalkylpyridinium Iodides

Edward M. Kosower, Joseph A. Skorcz

Research output: Contribution to journalArticlepeer-review


A study of the first (long wave length) charge-transfer band of twenty different alkylpyridinium iodides under carefully standardized conditions has established that (1) increasing the number of methyl groups raises the transition energy, (2) changing a methyl group from the 3- to the 2- to the 4-position increases the transition energy, (3) varying the nature of the alkyl group from methyl to t-butyl on the 4-position has virtually no effect upon the transition energy and (4) the solvent sensitivity of 1-methylpyridinium iodide is appreciably greater than that of 1-ethyl-4-carbomethoxypyridinium iodide, as might have been expected on the basis of a previously proposed model. The maxima observed varied from 3738 Å. for 1-methylpyridinium iodide to 3253 Å. for 1,2,3,4,5,6-hexamethylpyridinium iodide. Surprisingly, replacement of the 1-methyl by hydrogen moved the charge-transfer band to 3463 Å. Two points favor the interpretation of the effect of alkyl groups as being due to the bond dipole generated by the combination of an sp3 bonding orbital with an sp2 orbital: (1) the effect of the 3-methyl group can be accounted for on the basis of electrostatic stabilization of the ground state and (2) a change in the nature of the alkyl group on the 4-position does not elicit electron-supply dependent on the number of α-hydrogens, as would have been expected for hyperconjugation.

Original languageEnglish
Pages (from-to)2195-2203
Number of pages9
JournalJournal of the American Chemical Society
Issue number9
StatePublished - 5 May 1960
Externally publishedYes


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