Ground-state proton-transfer tautomer of the salicylate anion

Solutions of sodium salicylate in anhydrous polar solvents exhibit a weak, temperature-dependent absorption band (lambda(max) approximate to 325 nm) l ying in the Stakes gap between the main absorption (296 nm) and the fluores cence 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 tempera ture. After correction for solvent thermal contraction, the temperature-dep endent absorption spectrum of salicylate in acetonitrile solutions reveals a clear isosbestic point (epsilon(310) = 2000 M-1 cm(-1)) characteristic of an equilibrium between two salicylate species with band-maximum extinction coefficients of epsilon(325) = 3400 M-1 cm(-1) and epsilon(296) = 3586 M-1 cm(-1). In acetonitrile at room temperature (298 K) the concentration equi librium constant (minor/major) for the interconversion reaction between the two species is K-298 = 0.11 With Delta H = 1.6 kcal mol(-1) and Delta S = 0.97 cal.mol(-1) K-1. The fluorescence lifetime (4.8 ns in acetonitrile) an d the shape of the fluorescence spectrum are independent of excitation wave length. The fluorescence quantum yield for excitation in the long-wavelengt h shoulder (340 nn) is approximately 60% larger than the yield for excitati on in the main band at 296 nm (phi(340) = 0.29, phi(296) = 0.18) in acetoni trile 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 absorpti on band to the ground-slate tautomer (phenoxide anion form) of the salicyla te anion. Absorption transition moments for both the normal and tautomer fo rms are parallel to the emission transition moment, are electronically allo wed, and are consistent with L-1(b) assignment for both absorbing and emitt ing transitions. The static dipole moments are in the order of mu(N*) great er than or equal to mu(N) > mu(T*) > mu(T) for the normal (N) and tautomer (T) ground and electronic excited states.