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666 J. Phys. Chem. 1991,95, 666-674 Salt Effects on Steady-State Quantum Yields of Ultrafast, Muslon-Influenced,
 

Summary: 666 J. Phys. Chem. 1991,95, 666-674
Salt Effects on Steady-State Quantum Yields of Ultrafast, Muslon-Influenced,
Reversible Photoacid Dissociation Reactions
Ehud Pines, Dan Huppert,
Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University,
Tel Aviv 69978, Israel
and Noam Agmon*,+
Physical Sciences Laboratory, DCRT, and Laboratory of Chemical Physics, NIDDK, National Institutes of
Health, Bethesda, Maryland 20892 (Received: August 30, 1989: In Final Form: July 24, 1990)
The steady-statefluorescencequantum yields of the protonated and deprotonatedformsof 8-hydroxypyrene-l,3,6-trisulfonate
are determined in three solventsas a function of NaN03concentration. The measured quantum yields, and therefore also
the overall dissociation rate coefficients,decreasewith increasing salt concentration.in contrast to Brijnsted`s classical model
for primary salt effects. This is a consequenceof the reversibility and the nearly diffusion-controlled recombination rate
of the proton in the reaction studied. A quantitativeanalysisis based on newly derived results for reversible diffusion-influenced
reactions. Comparison with experimentis made for two screened potentials. Numerical integration with the Debye-Hllckel
potential indicates that it overestimates the salt effect. A second model (the "naiveapproximation") is based on a simple
screened potential from which analytical expressionsfor reaction yields and rates are derived. It shows better agreement
with experiment in the range 20-200 mM salt.
Introduction
Rate and equitibrium coefficients for recombination of solvated

  

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

 

Collections: Chemistry