The charge-transfer band of N-alkylpyridinium iodides in mixed aqueous solvents

Abstract

The charge-transfer band of N-alkylpyridinium iodides has been studied in mixed aqueous solvents. For water–alcohol (up to C₃) mixtures a linear relation between the transition energy corresponding to the band maximum (E^max_T) and the mole fraction of water holds good. [The transition energy denoted by E^max_T is not related to the E_T(30) values of Dimroth et al.(K. Dimroth, C. Reichardt, T. Siepmann and F. Bohlmann, Annalen, 1963, 661, 1) involving a pyridinium betaine as the measure of solvent polarity.] For dipolar aprotic solvents deviation from linearity appears at the organic end. The results have been explained by a suitable model of solvation. The linear correlation for water–alcohol system enables the Z value of water to be re-examined; a value of 91.4 kcal mol^–1 for water has been advocated.