J. Am. Chem. Soc. 1998, 120, 4161-4166
4161
Electron-Transfer Component in Hydroxyl Radical Reactions
Observed by Time Resolved Resonance Raman Spectroscopy†
G. N. R. Tripathi
Contribution from the Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556
ReceiVed January 6, 1998
Abstract: The existence of an electron-transfer pathway in the reaction of •OH radical with aromatic molecules
in water has been established, for the first time, using time-resolved resonance Raman spectroscopy as a
diagnostic tool and p-dimethoxybenzene as a model system. In the currently accepted mechanism, the cation
radical is produced by •OH addition to the ring, followed by loss of OH-. The present work demonstrates that
this process competes with direct electron transfer. A generalized reaction mechanism has been proposed in
terms of potential energy diagrams to explain two-step formation of the cation radical. In this reaction
mechanism, the electron-transfer component and the rate of OH- elimination from the •OH adduct both depend
on the ionization potential (IP) of the molecule. The cation radical yield by electron transfer increases from
6% in p-dimethoxybenzene to 30% in p-anisidine and 85% in p-phenylenediamine. For neutral molecules
•
with IP > 8 eV, the OH addition is the first step in the chemistry, and for IP < 7 eV, it is the electron
transfer. In the intermediate IP range, both processes occur simultaneously.
Introduction
the reaction involves an electron transfer (ET) component, and
what mechanistic implications that would have for the early
chemical events, has never been answered. In the commonly
held reaction mechanism, the •OH radical reacts with aromatic
molecules by addition to the ring.2-9 This behavior contrasts
The oxidation of organic substrates by the •OH radical is one
of the most widely studied reactions because of its central role
in chemistry and biology, organic synthesis, photocatalysis in
aqueous environments, wastewater treatment, and numerous
other chemical processes.1-10 The reaction is used to prepare
radical intermediates in aqueous medium, to investigate their
structure and reactivity by ESR and time-resolved Raman
spectroscopy.9,11 However, the fundamental question of whether
•
with the highly oxidizing nature of the OH radical (E°(•OH/
OH-) ) 1.9 V)12 that should generally favor electron transfer.
The adduct (hydroxycyclohexadienyl) radicals undergo loss of
OH- by reaction with H+ and/or water to form the cation radical
(adduct-mediated electron transfer; AMET). The ET component
in the reaction is difficult to recognize, and can be easily
confused with AMET, for the following reasons: (1) Experi-
mentally, a clear resolution cannot be made between the ET
† The research described herein was supported by the Office of Basic
Energy Sciences, Department of Energy. This is Document No. NDRL-
4044 from the Notre Dame Radiation Laboratory.
(1) See, Biweekly List of Papers on Radiation Chemistry and Photo-
chemistry, pubished by the NDRL Data Center, Radiation Laboratory,
University of Notre Dame, Notre Dame, IN. Farahataziz; Ross, A. B.
Selected Specific Rates of Reactions of Transients from Water in Aqueous
Solution, NSRDS-NBS 59; US Department of Commerce, 1977.
(2) Sonntag, C. v. The Chemical Basis of Radiation Biology; Taylor and
Francis: New York, 1987.
•
and AMET processes, unless the OH addition (diffusion-
controlled) and OH- elimination occur at drastically different
time scales. Obviously, the H+-catalyzed OH- loss (generally
diffusion controlled) must be avoided by making measurements
•
in basic solutions. (2) The OH adducts at the different ring
(3) Tripathi, G. N. R.; Y. Su. J. Am. Chem. Soc. 1996, 118, 2235 and
references therein.
sites decay at different rates, and not necessarily into the cation
radical. Complications arise due to the presence of several
transient species, with overlapping absorption, making identi-
fication and kinetic monitoring of the individual species quite
difficult. To establish the ET component in the reaction, it is
extremely important to select model systems in which the
number of possible •OH adducts is small, and to use a structure-
sensitive technique, such as time-resolved resonance Raman
spectroscopy, for transient identification and kinetic monitoring.
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