Rudolph Marcus

Experimental surprises and their solution in theory (Lecture + Discussion)


Abstract

From time to time phenomena are uncovered that are highly puzzling and are a rich source for theoretical study. Several examples drawn from my own experience are described together the theories to which they led. (i) In the field of electron transfer reactions, in its early stage, many isotopic exchange reactions mostly in water were studied in which the process was an electron transfer between two ions in different valence states, e.g., Fe²+ + Fe³+. Some reactions were fast and others were slow and none of the existing theories were correct. In turn, a detailed theory1 based on the Franck-Condon principle and the law of conservation of energy explained these differences in rates and led to specific predictions that were later confirmed experimentally. (ii) A more recent example is an unusual isotope effect in ozone formation in the stratosphere and the laboratory, the “mass-independent fractionation” of isotopes. Standard theory needed a modification, such as a nonstatistical (“non-RRKM”) addition. It led to the interpretation of the effects of pressure and temperature on the isotopic fractionation.² (iii) Another example is "on-water" catalysis. Shaking two organic reactants with excess water led surprisingly to a large reaction rate increase in some cases and none in others. The explanation is based upon surface OH-groups that are not hydrogen-bonded.³ A similar catalysis was found with metal oxide nanoparticles enriched in surface OH-groups. (iv) The list of novel phenomena includes also the intermittency of fluorescence (“blinking”) observed in single particles of semiconductors³, dye-coated semiconductors and some fluorescent proteins. Apart from the fundamental scientific interest in these systems, they may be applied in solar energy conversion and biosensing. However, it is clear that any optimization of technical devices rests on a profound understanding of the underlying phenomena and, if possible, on their description in simplified analytical terms.
1. Review: Marcus RA, Sutin N, BBA 811, 265 (1985). 2. Gao, Y, Marcus RA, Science, 293, 259 (2001) 3. Jung, Y, Marcus, RA, J Am Chem Soc 129, 5492 (2007) 4. Tang J, Marcus RA, J. Chem Phys, 123, 204511 (2005)


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