Substituent effects and isotope effects on the rate coefficients and equilibrium constant for opening an intramolecular hydrogen bond

Proton removal from the intramolecularly hydrogen-bonded monoanions of substituted 4,6-bis(phenylazo)resorcinols in buffered 70% (v/v) Me2SO-H2O involves opening of the hydrogen bond followed by proton transfer between open form and buffer. At low buffer concentration the proton transfer is rate-limiting but opening of the hydrogen bond becomes rate-limiting at higher buffer concentration. Typical values of the rate coefficients for opening (k1) and closing (k-1) of the hydrogen bond are k1 3.8 ± 0.6 x 104 s-1 and k-1 ca. 8 x 107 s-1 found for 4,6-bis(phenylazo)resorcinol monoanion. The rate coefficients and the corresponding equilibrium constant are almost unaffected by substituents and by changing the solvent to 70% (v/v) Me2SO-D2O. (author)