Solvatochromism of Heteroaromatic Compounds: XXI. Ion Pairs of 4-Nitropyrazolide Anion.

The solvatochromism of the long-wave UV absorption band of the contact ion pairs of the 4-nitropyrazolide anion with alkali metal cations in aprotic protophilic solvents with ℇ ≤ 24 was studied and quantitatively described using the Kamlet-Taft empirical solvent parameters. Of two possible types of tight ion pairs differing in the murtual arrangement of the counterions, the solvent stabilizes the pair in which the cation is located in the σ plane of the anion in the vicinity of its nitrogen atoms. Among the factors affecting the quantitative characterisitcs of the solvatochromism of such ion pairs, the major factors are the polarity/polarizability of the solvent and the capability of its molecules for coordination with cations. The peripheral specific solvation of the cation in an ion pair stabilizes its Franck-Condon excited state relative to the ground electronic state. In the quantitative respect, this effect is similar to that characteristic of solvation H complexes. [ABSTRACT FROM AUTHOR]