Solvent Assisted Photoacidity [chapter]

Dina Pines, Ehud Pines
Hydrogen-Transfer Reactions  
Proton transfer and electron transfer [33] [34] [35] [36] [37] [38] are among the most common classes of chemical reactions in nature. Similar to electron transfer, the main outcome of a proton transfer reaction is the net transfer of a charge. However, unlike electron transfer, proton transfer reactions involving large organic molecules are usually localized between two donor and acceptor atoms. Proton transfer reactions are inherently reversible in the ground electronic state of the proton
more » ... or and proton acceptor molecules. The inherent reversibility of the proton transfer reaction is usually maintained in the electronic excited-state of photoacids and photobases in aqueous solutions. Most proton transfer reactions involve relatively small changes in the backbone structure of the proton donor and proton acceptor molecules. These ensuing changes fully reverse upon back-transfer of a proton, either by the back-recombination of the dissociated (geminate) proton or following recombination with a proton coming from the bulk solution . Proton transfer is very sensitive to the environment, which usually affects both the yield and the rate of the proton transfer reaction. Aqueous solutions are the most common environment accommodating proton transfer reactions due to the high dielectric constant and extensive hydrogen-bond interactions of the aqueous medium which act both to stabilize charged products and to establish the reaction coordinate along which the proton is transferred. Following the pioneering work of Brønsted and Lowry it has been customary to define proton donors as Brønsted acids and proton acceptors as Brønsted bases [1, 2]: AH (acid) + B (base) . A -(conjugated base) + BH + (conjugated acid) (12.1) where the acid and base molecules prior to proton transfer may either be charged or neutral species. The pK a and pK b scales in aqueous solutions (B=H 2 O, BH + "H + in Eq. (1)) serve to define the extent of acidity and basicity of the proton donor and the proton acceptor, respectively, Eqs. (12.2) and (12.3).
doi:10.1002/9783527611546.ch12 fatcat:5nxspm64lzf6baysvbpdn4mfre