Biographical Memoirs Volume 84 (2004) / Chapter Skim
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Pages 53-82
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From page 53... ...
Professor Closs made significant contributions in four areas. He was an early leader in the field of carbene chemistry; he elaborated various significant aspects of the photosynthetic pigments; he pioneered important applications of magnetic resonance to characterize reaction intermediates; and he elucidated intricate facets of electron transfer chemistry.
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From page 54... ...
University regulations against nepotism prohibited wives from holding paid positions in the same department as their husbands; therefore, Lieselotte could only work as an unpaid volunteer. The availability of a skilled coworker proved especially fortuitous when Gerhard entered the chemically induced dynamic nuclear polarization field (see below)
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From page 55... ...
In 1981 he also was honored as chairman of the Gordon Research Conference on free radical reactions and in 1990 the Gordon Research Conference on radical ions, and by many distinguished lectureships in the United States, Canada, Japan, and Europe. Among these were the Bayer Lectureship at Universität Köln, Germany (where the author first met him)
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From page 56... ...
Asked what he thought of consumers who bought foreign-made articles, he quickly voiced his disapproval and then added, having spotted the reporter's Japanese-made camera, "and that applies also to you." With the death of Gerhard Closs the chemical sciences lost a most formidable champion, a practitioner of the highest intellectual standards, a keen mind, and a skilled experimenter who was always probing accepted theories and was never afraid to break new ground. The scientific community has lost a teacher, mentor, collaborator, and kin spirit, and a few who were privileged have lost a friend.
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From page 57... ...
Gerhard Closs never lost interest in natural products and photosynthetic pigments. Seventeen of his 132 lifetime publications dealt with the chlorophylls; he contributed significantly to such important topics as linked chlorophyll dimers, photosynthetic reaction centers, and porphyrin metal complexes.
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From page 58... ...
3) The development of divalent carbon chemistry involved various facets: substituted carbenes; the notion of spin multiplicity; chemical studies probing carbene reactivity and the stereochemistry of their reactions; and the application of new physical methods (e.g., electron spin resonance, electron nuclear double resonance, chemically induced dynamic nuclear polarization, and optical spectroscopy)
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From page 59... ...
Optical spectroscopy had received a recent boost by the advent of flash photolysis in 1949-50.16 Herzberg observed the emission spectra of the parent methylene, CH2, and its isotopomers CHD and CD2, in 1961.17 Electron spin resonance (ESR) spectroscopy was a later development,18 but by 1953 organic free radicals or radical ions had been studied.
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From page 60... ...
Although the limited (µs) time resolution of these early studies appears almost primitive compared to today's sophisticated TRLS experiments, the available time resolution was exactly right for the somewhat "sluggish" diphenyl-methylene.
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From page 61... ...
Closs and Pfeffer probed the rearrangement of two 2,4-dimethylbicyclobutanes to two hexadienes and elucidated the steric course of this reaction.28 By the mid-1960s Gerhard Closs, an acknowledged expert in carbene chemistry, made his final major contribution to this field, the first application of the chemically induced dynamic nuclear polarization method. In 1967 enhanced nuclear magnetic resonance (NMR)
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From page 62... ...
Recognizing that all CIDNP effects required the involvement of radical pairs,33 he developed a theory that could explain the observed polarization and designed elegant experiments to probe key features of the theory. Because the polarization changed with the spin multiplicity (µ)
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From page 63... ...
G E R H A R D L U D W I G C L O S S 63 FIGURE 2 CIDNP spectra (benzylic protons) of coupling products generated during the photoreactions of benzaldehyde (X = H)
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From page 64... ...
products showed polarization of opposite sign,36 and that protons with hyperfine coupling constants of opposite sign show CIDNP effects of opposite sign.36 The Radical Pair Theory emerging from the work of the Chicago and Leiden groups is now generally accepted and can explain the vast majority of all nuclear spin polarization effects. In more than 20 additional publications Closs and collaborators elucidated additional facets of the theory of CIDNP or dealt with applications to new problems.
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From page 65... ...
The existence of cross relaxation complicates the interpretation of CIDNP results, particularly in experiments designed to derive electronic structures of free radicals or radical ions from their polarization pattern. FIGURE 3 Normalized intensity versus magnetic field of aldehyde proton signals obtained by photolysis of cycloalkanones and a bicyclic ketone (BC)
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From page 66... ...
The Closs group applied this technique to determine the spin density distribution in radical ions of chlorophyll and derivatives. The CIDNP intensities 1 µs after a photoinduced electron transfer reaction revealed the signs and relative magnitudes of hyperfine coupling constants43 in good agreement with ENDOR spectroscopy results.
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From page 67... ...
free radical scavenger is designated C.41 In the final decade of his life Gerhard Closs became interested in several aspects of chemically induced dynamic electron polarization (CIDEP) , a phenomenon related to CIDNP and discovered several years earlier.44 Essentially all CIDEP effects observed in the 20 years following the discovery could be explained by the radical pair or the triplet mechanism or by a combination of both.
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From page 68... ...
Closs's time-resolved electron spin resonance studies culminated in several elegant studies of electron spin polarized polymethylene biradicals in solution, in which throughbond interactions were illuminated.48 Simulating the shape and time dependence of CIDEP spectra from acyl-alkyl and alkyl-alkyl polymethylene biradicals by perturbation theory yielded the electron spin-spin interaction, J, and the endto-end contact rate, which is inversely related to the lifetime. It is remarkable that Gerhard Closs was still breaking new ground at the onset of his seventh decade.
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From page 69... ...
Although at variance with the existing theories, the results were rationalized based on an ad hoc theory.50 Similar results were observed in additional systems; all attempts to verify the predicted inverted free energy dependence of electron transfer rates met with failure. Miller and coworkers had studied the (charge neutral)
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From page 70... ...
In the collaboration that ensued and continued to Gerhard Closs's death donor and acceptor were linked by a rigid steroid spacer in molecules of the type A-Sp-biphenyl. The electron transfer rates observed for the radiolytically generated monoanions of these systems showed a striking deviation from the classical Brųnsted relationship (see Figure 5)
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From page 71... ...
This consideration spawned an illuminating, unprecedented comparison of the rates of hole, electron, and triplet energy transfer.53 Further collaboration was cut short by his untimely death. FIGURE 5 Intramolecular rate constants as a function of free energy change in 2-methyloxacyclopentane solution at 296 K
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From page 72... ...
He was an early leader in the field of carbene chemistry, he pioneered applications of magnetic resonance to characterize reaction intermediates, and he elucidated intricate facets of electron transfer chemistry. He will be remembered for the depth and breadth of his understanding and for his rigorous, all-encompassing approach to research.
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From page 74... ...
A., Hutchison, Jr. Paramagnetic resonance and electron nuclear double resonance studies of the chemical reactions of diphenyldiazomethane and of diphenylmethylene in single 1,1-diphenylethylene crystals.
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From page 75... ...
Induced dynamic nuclear spin polarization in reactions of photochemically and thermally generated triplet diphenylmethylene.
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From page 76... ...
Spin-polarized electron paramagnetic resonance spectra of radical pairs in micelles. Observation of electron spin-spin interactions.
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From page 77... ...
:867-71. The theory of oxidation-reduction reactions involving electron transfer.
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From page 78... ...
Induced dynamic nuclear spin polarization in re actions of photochemically and thermally generated triplet diphenylmethylene.
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From page 79... ...
Chemically induced dynamic nuclear spin polarization derived from biradicals generated by photochemical cleavage of cyclic ketones, and the observation of a solvent effect on signal intensities.
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From page 80... ...
Temperature-independent long range electron transfer reactions in the Marcus inverted region.
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