Kinetics and mechanism of photoinduced and thermal proton-transfer processes in o-hydroxybenzaldehyde and o-hydroxyacetophenone. A remarkable temperature dependence of the reaction rate

Abstract

Time-resolved electronic emission and absorption spectroscopy of photoexcited solutions of o-hydroxybenzaldehyde (OHBA) and o-hydroxyacetophenone (OHAP) are reported. When these molecules are brought to their first excited single state an adiabatic excited-state phototautomerization reaction takes place and subsequently the tautomer in its lowest electronic triplet state and in its ground state may be observed as transient species. The return of the ground-state tautomer in solutions in aprotic solvents to the stable form of the molecule may be catalysed by concentrations of alcohols as small as 10^–4 mol dm^–3. The catalysis involves a protonation and a subsequent deprotonation step. The rate of the uncatalysed reaction in aprotic solvents exhibits a remarkable temperature dependence. The rate constant behaves in an Arrhenius manner, but with a negative activation energy in the high-temperature range and a normal positive activation energy in the low-temperature range. The explanation for this behaviour involves a fast unimolecular dynamic equilibrium between two forms of the ground-state tautomer. In one form there is an intramolecular hydrogen bond, and in the other form this bond is broken. Only the form with the hydrogen bond leads directly to the stable form of the molecule. In the other form of the tautomer the proton donor and acceptor sites are far apart as a consequence of rotational isomerization, which accompanies the breaking of the intramolecular hydrogen bond. It is shown that a dynamic equilibrium between two rotameric forms, as in the case of OHBA, may explain the small pre-exponential factors for the observed normal Arrhenius behaviour of a number of other intramolecular proton-transfer reactions.