Pyridinium Complexes. IV. The Effect of N-Substituents on the Position of the Iodide Charge-transfer Band

The positions of the iodide charge-transfer bands for a series of substituted l-X-benzyl-4-carbomethoxypyridinium ions in cis-l, 2-dichloroethylene are shown to be correlated by the equation E_T/2.303RT = ρ₃σ in which σ is the Hammett constant and ρ₃, is a sensitivity parameter comparable to the usual ρ. In the present case, ρ₃ = — 1.70, consistent with the large effect of substitution expected for a charge-transfer band. It is suggested that an intramolecular charge-transfer band (i.c.t.) is observed as a shoulder at 3090 Å. in solutions of l-(4-methoxybenzyl)-4-carbomethoxypyridinium ion (as the perchlorate).