Description
Acid-base reactions in aqueous solution, where protons are exchanged between acid and base molecules directly or via the hydrogen-bonded water solvent molecules, are particularly well-suited test beds for studying proton transfer reactions. We have chosen naphthol-type photoacids to initiate proton transfer towards an organic or inorganic base base imidazole in aqueous solution. We report on femtosecond O K-edge spectroscopic measurements at the BESSYII and European XFEL free-electron laser facilities that inform about the changes in electronic structure from the photoacid to its conjugate base form, when the first step of proton release from the acid occurs. In addition to that, we have measured changes in the N K-edge spectra monitoring the transient response of the base converting in its conjugate acid form, i.e. the final step of the proton transfer process. We thus reveal, with femtosecond soft X-ray spectroscopy, how local electronic structures change during proton exchange reactions. Interestingly, we observe on a time scale of 50 fs electronic structural dynamics in on-contact photoacid-base pairs directly upon electronic excitation, for both the photoacid and the base molecules. We ascribe this to charge distribution changes in the full photoacid-base complex. Interestingly the completion of a full proton transfer event with a net charge separation event occurs rather for on-contact neutral acid-base complexes on a time scale of 1 ps. This can be understood when taking into account the solvent shell rearrangement time scale that must accommodate the charge separation.
