Reactions in Solution: 1968-1976
Electron Transfer and the Experimental Confirmation of Marcus Theory
It has been said that the simplest chemical reaction is oxidation or
reduction, representing the loss or gain of electrons by an atom or
molecule. Electron transfer, therefore, is a quintessential prototype system
for chemical reaction theory, and was a
primary theme of research in the Chemistry Department from its earliest
days. "Marcus theory," for which Professor Rudolph A. Marcus received
the 1992 Nobel
Prize in Chemistry, among other predictions, relates the rate constant
of an oxidation-reduction cross-reaction, k12, to two self-exchange rates
constants, k11 and k22 thus:
k12 ≈
(k11k22K12f12)1/2.
Where K12 is the equilibrium constant for the
reaction and f12 is a constant
usually close to unity. This is the famous "Marcus cross-relation". Starting
in this period, and extending into the 1980s, experimental testing of Marcus
theory and demonstration of this relationship by workers in the Chemistry
Department, and its application to biologically related systems resulted in
the publication of four important papers:
"Electron Transfer Reactions with Unusual Activation Parameters: A
Non-Anomaly and a Treatment of Reactions Accompanied by Large Entropy
Decreases" R. A. Marcus and N. Sutin, Inorg. Chem. 14, 213-216
(1975).
"Application of Electron-Transfer Theory to Several Systems of Biological
Interest" R. A. Marcus and N. Sutin in: Antennas and Reaction Centers of
Photosynthetic Bacteria, M.E. Michel-Beyerle, ed., Springer-Verlag,
Berlin, W. Germany (1985) p. 226-233.
"Electron Transfer in Chemistry and Biology" R. A. Marcus and N. Sutin,
Biochim. Biophys. Acta 811, 265-322 (1985).
"The Relation Between the Barriers for Thermal and Optical Electron Transfer
Reactions in Solution" R. A. Marcus and N. Sutin, Comm. Inorg. Chem.
5, 119-133 (1986).
Experiments Using the Laser T-Jump Method
Electron transfer is just one case of a class of reactions with rates that
can be too
rapid to be measured by conventional chemical means. The
1967 Nobel Prize
for Chemistry was bestowed on Eigen, Norrish and Porter for their
contributions to the understanding of rapid chemical reactions and for the
development of methods to measure submicrosecond chemical reaction rates.
One of these methods was the T-jump technique, in which the
temperature of a solution was raised abruptly, disturbing the concentrations
of species participating in a chemical equilibrium. Species concentrations
then relaxed back to equilibrium values, and the rates and amplitudes of
relaxation provided insight into chemical and physical processes. There were
(and still are) a multitude of techniques (absorption and emission of light,
absorption of ultrasound, dielectric relaxation, electron spin relaxation,
nuclear magnetic relaxation, etc.) for monitoring the time
development of species concentrations. The limitation on the time response
of all these techniques was most often the inability to heat the solution
quickly, usually by discharging an electrical capacitor across a solution.
This also required a sufficiently high salt concentration in the solution
so that some chemical systems could not be studied.
The invention of the laser in the late 1950s led to rapid growth of the use
of lasers in research applications during the 1960s and 1970s. Absorption by
the solvent of a powerful, short (a few tens of nanoseconds or shorter)
laser pulse would be an ideal source of "instant" heating of a solution.
However, powerful pulsed laser sources available in the late 1960s did not
produce output at wavelengths suitable for absorption by water, the solute
of choice. Researchers in the Chemistry Department solved this problem by
shifting the 1.06 mm output of a Nd:YAG laser,
which is not absorbed by water, to 1.41 mm, which
is strongly absorbed by water. The wavelength of the laser pulses was
shifted using stimulated Raman scattering in liquid N2 (see Ref.
1
below for an explanation). Pioneering experiments were performed in the
Chemistry Department studying the equilibrium between high- and low-spin
bis[hydrotris-(pyrazolyl)borate]-iron(II),2 the triiodide system,3
I2 +I- 1
I3-,
and
the dimerization of proflavin and ethidium bromide.4 Other
unpublished experiments studied the melting of polynucleotides.
1. "Kinetic Studies of Very Rapid Reactions in Solution" G.W. Flynn and N.
Sutin, in Chemical and biochemical Applications of Lasers C.B.
Moore, ed. Academic Press (1974) pp. 309-338.
2. J.K. Beattie, N. Sutin, D.H. Turner, and G.W. Flynn J. Amer. Chem.
Soc. 95 2052 (1973).
3. D.H. Turner, G.W. Flynn, N. Sutin, and J.V. Beitz, J. Amer. Chem. Soc.
94 1554 (1972).
4. D.H. Turner, G.W. Flynn, S.K. Lundberg, L.D. Faller, and N. Sutin
Nature 293 215 (1972).
Last Modified: January 31, 2008
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