Ground-State Proton-Transfer Tautomer of the Salicylate Anion

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 ( 310 ) 2000 M-1 cm-1) characteristic of an equilibrium between two salicylate species with band-maximum extinction coefficients of 325 ) 3400 M-1 cm-1 and 296 ) 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 ) 0.11, with ∆H ) 1.6 kcal mol-1 and ∆S ) 0.97 cal‚mol-1 K-1. The fluorescence lifetime (4.8 ns in acetonitrile) and the shape of the fluorescence spectrum are independent of excitation wavelength. The fluorescence quantum yield for excitation in the long-wavelength shoulder (340 nm) is approximately 60% larger than the yield for excitation in the main band at 296 nm (æ340 ) 0.29, æ296 ) 0.18) in acetonitrile at room temperature. These results are consistent with assignment of the shoulder band to the proton-transfer tautomer of the salicylate anion. Electronic structure calculations support assignment of the 325 nm absorption band to the ground-state tautomer (phenoxide anion form) of the salicylate anion. Absorption transition moments for both the normal and tautomer forms are parallel to the emission transition moment, are electronically allowed, and are consistent with Lb assignment for both absorbing and emitting transitions. The static dipole moments are in the order of μ(N*) g μ(N) > μ(T*) > μ(T) for the normal (N) and tautomer (T) ground and electronic excited states.