Ab initio methods are used to probe the effect of fluorosubstitution upon the excited state proton transfer process in the OH⋯N hydrogen bond contained in the model system salicylaldimine. Two different sites are chosen for substitution, one nearby to the nitrogen on the cycle that contains the intramolecular H-bond, while the other position is located on the aromatic ring close to the oxygen. Many of the computed effects on the proton transfer potential can be considered inductive in nature; the electronegative F makes the proximate N or O atom a stronger acid/weaker base and thereby moderates the preferred position of the bridging proton. The magnitude of the perturbation diminishes as the site of substitution is further removed from the H-bond. This principle also controls the manner in which F affects the geometry and strength of the intramolecular H-bond in both the enol and keto tautomers. There are a number of stretches and contractions that occur in bonds that do not border on the H-bond, patterns that can be understood via the bonding patterns in the orbitals involved in the excitation. Whereas these notions apply fairly consistently to the ground state and excited ππ∗ singlet and triplet, there are a number of anomalous patterns that emerge in the View the MathML source∗ state. In general, the effects of fluorosubstitution are smaller in magnitude than the changes that occur in the proton transfer properties as a result of electronic excitation.
- excited state,
- proton transfer,