Ground-State Proton-Transfer Tautomer of the Salicylate Anion

Abstract

Solutions of sodium salicylate in anhydrous polar solvents exhibit a weak, temperature-dependent absorption band (λmax ≈ 325 nm) lying in the Stokes gap between the main absorption (296 nm) and the fluorescence band (396 nm, acetonitrile). This weak, longer wavelength absorption band is hardly observable in aqueous solution, but its intensity increases with temperature and increases with polarity in anhydrous organic solvents in the order of ethanol < acetonitrile < dimethyl sulfoxide at room temperature. After correction for solvent thermal contraction, the temperature-dependent absorption spectrum of salicylate in acetonitrile solutions reveals a clear isosbestic point (e310 = 2000 M-1 cm-1) characteristic of an equilibrium between two salicylate species with band-maximum extinction coefficients of e325 = 3400 M-1 cm-1 and e296 = 3586 M-1 cm-1. In acetonitrile at room temperature (298 K) the concentration equilibrium constant (minor/major) for the interconversion reaction between the two species is K298...