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).

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Last Modified: June 28, 2012