Membrane-Organized Chemical Photoredox Systems Page: 4 of 21
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were determined by incubating mixtures of {3,3} and {3,4} ions that were 180-enriched in
the cis-aqua positions, then at timed intervals oxidizing portions completely to {5,5} with
excess Ce4" ion using a flow-mixer and analyzing the isotopic distribution by resonance
Raman spectroscopy (manuscript 2). This method takes advantage of the large isotopic
shift in ruthenyl stretching modes (A4s = 38 cm-1) for Ru=160 and Ru=180 ions. By
measuring the rate of Ru=180 to Ru=160 conversion in the {5,5} ions, we were able to
determine that kex = 7x10-3 s1 for water exchange on {3,3} at 23 C (corresponding to t12
= 99 s) and to establish that the exchange rates for all higher oxidation states were slow
with respect to the rate of catalyzed water oxidation. Thus, oxidation of {3,3} to {3,4}
effectively blocked water exchange, yielding stable solutions of substitution-inert ions
with a defined isotopic composition of coordinated water.
Earlier isotope-labeling studies from our lab and that of Meyer gave similar, but
quantitatively conflicting data; in particular, their data suggested the existence of a
bimolecular pathway involving reaction between two coordination complexes, whereas
our data gave no evidence for this type of pathway, but indicated that 02 formation
involves a single catalyst ion. To resolve this issue, we designed an experimental system
that allows continuous mass spectrometric monitoring of the 02 product (manuscript 4).
From the data obtained, it is clear that there are no bimolecular pathways; however, two
unimolecular pathways were identified, in which 02 is formed from either (i) one
coordinated terminal ruthenyl Ru=O atom in the complex plus one solvent O atom or (ii)
two solvent O atoms. From deuterium kinetic isotope measurements made upon the
catalytically active form of the complex, we have established that the primary H(D)-
isotope effect for water oxidation is anomalously small (KIE = 1.6-1.7) for ruthenyl-4
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Hurst, James K. Membrane-Organized Chemical Photoredox Systems, report, May 15, 2007; United States. (https://digital.library.unt.edu/ark:/67531/metadc888148/m1/4/: accessed May 4, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.