Seixas de Melo J Sérgio, Maçanita António L
Coimbra Chemistry Centre, Department of Chemistry, University of Coimbra , Rua Larga, 3004-535 Coimbra, Portugal.
J Phys Chem B. 2015 Feb 12;119(6):2604-10. doi: 10.1021/jp508782h. Epub 2014 Nov 3.
The prototropic reactions of the first excited singlet state of 3-chloro-4-methylumbelliferone (3Cl4MU), in dioxane:water mixtures (Dx:H2O), were revisited using ps-time-resolved fluorescence techniques. The data response to the dielectric constant of the mixtures revealed the presence of an additional fourth kinetic species, kinetically coupled to the neutral (N*), the tautomeric (T*), and anionic (A(-)) forms of 3Cl4MU, which is assigned to the elusive geminate (A(-)···H(+)) ion pair. From the data analysis, all rate constants of the prototropic and diffusion processes involved were separately evaluated. The results showed that, whenever the geminate ionic pair is not kinetically detected, the evaluated values for deprotonation and protonation rate constants can substantially deviate from the real ones, depending on the efficiencies of pair recombination and dissociation. Finally, the results provide convincing kinetic evidence for the Eigen-Weller mechanism (intermediacy of the geminate ionic pair) in a quasi-aqueous medium, which to our knowledge had not yet been given.
