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Photochemistry of "Super" Photoacids. 2. Excited-State Proton Transfer in Methanol/Water Kyril M. Solntsev,*,, Dan Huppert, Noam Agmon, and Laren M. Tolbert
 

Summary: Photochemistry of "Super" Photoacids. 2. Excited-State Proton Transfer in Methanol/Water
Mixtures
Kyril M. Solntsev,*,, Dan Huppert, Noam Agmon, and Laren M. Tolbert§
Raymond and BeVerly Sackler Faculty of Exact Sciences, School of Chemistry, Tel AViV UniVersity,
Tel AViV, 69978, Israel, Department of Physical Chemistry and Fritz Haber Research Center,
The Hebrew UniVersity, Jerusalem, 91904, Israel, and School of Chemistry and Biochemistry,
Georgia Institute of Technology, Atlanta, Georgia, 30332-0400
ReceiVed: December 23, 1999; In Final Form: March 6, 2000
Excited-state proton transfer to solvent (PTTS) of 5-cyano-2-naphthol was investigated in methanol/water
mixtures. We have found that the time-resolved fluorescence data fit the solution of the Debye-Smoluchowski
equation for the reversible geminate recombination of ions over the whole range of methanol/water concentration
ratios. The rate constants of the elementary protolytic photochemical processes and their isotope effects were
determined by a simultaneous analysis of the time-resolved fluorescence of the photoacid and its conjugated
anion. The competition between adiabatic protonation and quenching of the excited naphtholate anion by the
geminate proton was observed to diminish sharply near pure methanol. The dissociation rate coefficient near
pure methanol depends on a power of the water concentration, which appears to decrease from 2 (for "ordinary"
photoacids) to below 1 for "super" photoacids.
I. Introduction
Proton transfer, in both ground and excited states, is a
fundamental process in chemistry and biology. The subject of

  

Source: Agmon, Noam - Institute of Chemistry, Hebrew University of Jerusalem

 

Collections: Chemistry