GERHARD LUDWIG CLOSS

May 1, 1928–May 24, 1992

BY HEINZ D. ROTH

WHEN GERHARD LUDWIG CLOSS succumbed to a massive heart attack on May 24, 1992, the work of one of the great organic chemists came to an untimely end. 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. This biographical memoir provides a welcome opportunity to pay tribute to one of the outstanding chemists of the post-World War II era—and to a friend.

Gerhard Closs was born on May 1, 1928, in Wuppertal-Elberfeld, Germany, a small, bustling city known for its suspended tram (Schwebebahn) and for the pharmaceutical branch of Bayer, one of Germany’s major chemical manufacturers. Before he could complete his high-school education he was pressed into military service as a 16-year-old in 1944. He barely survived the ordeal of war: He was seriously wounded on the eastern front. After the war he completed high school and enrolled in Universität Tübingen. Having received a Ph.D. degree in 1955 for work with Georg Wittig, he joined R. B. Woodward’s group at Harvard for two years.



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GERHARD LUDWIG CLOSS May 1, 1928–May 24, 1992 BY HEINZ D. ROTH W succumbed to a massive heart HEN GERHARD LUDWIG CLOSS attack on May 24, 1992, the work of one of the great organic chemists came to an untimely end. Professor Closs made significant contributions in four areas. He was an early leader in the field of carbene chemistry; he elabo- rated various significant aspects of the photosynthetic pig- ments; he pioneered important applications of magnetic resonance to characterize reaction intermediates; and he elucidated intricate facets of electron transfer chemistry. This biographical memoir provides a welcome opportunity to pay tribute to one of the outstanding chemists of the post-World War II era—and to a friend. Gerhard Closs was born on May 1, 1928, in Wuppertal- Elberfeld, Germany, a small, bustling city known for its sus- pended tram (Schwebebahn) and for the pharmaceutical branch of Bayer, one of Germany’s major chemical manu- facturers. Before he could complete his high-school educa- tion he was pressed into military service as a 16-year-old in 1944. He barely survived the ordeal of war: He was seriously wounded on the eastern front. After the war he completed high school and enrolled in Universität Tübingen. Having received a Ph.D. degree in 1955 for work with Georg Wittig, he joined R. B. Woodward’s group at Harvard for two years. 53

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54 BIOGRAPHICAL MEMOIRS In 1957 he accepted a position as assistant professor at the University of Chicago as a natural products chemist. Gerhard’s early independent studies were assisted by Lieselotte E. Closs (née Pohmer), his wife and most pro- ductive coworker; their collaboration produced 15 publica- tions between 1959 and 1969. Lieselotte also received a Ph.D. degree with Wittig for her classic work on the tran- sient existence of dehydrobenzene.1,2 She did postdoctoral work at MIT. Lieselotte and Gerhard were married on Au- gust 17, 1956, in Cambridge, Massachusetts. Employment opportunities for women scientists were very limited in the 1950s and 1960s. 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 polariza- tion field (see below). Lieselotte re-entered the lab, carried out a few simple but elegant experiments to probe key as- pects, and soon had results sufficient for two “Communica- tions to the Editor.” Gerhard Closs was granted tenure in 1961 and was pro- moted to full professor just two years later. Almost 20 years later he accepted the position of section head in the Chem- istry Division at Argonne National Laboratory, while remain- ing on the Chicago faculty. Although he kept this position for only three years, it significantly influenced the direc- tion of his research in the final decade of his life. The work of Gerhard Closs has been recognized at the University of Chicago and in the scientific community at large. He was appointed the Michelson Distinguished Ser- vice Professor, and his colleagues honored him along with N. C. Yang with a symposium on the occasion of their sixti-

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55 GERHARD LUDWIG CLOSS eth birthdays. He was awarded the Jean Servas Stas Medal by the Belgian Chemical Society in 1971, the James Flack Norris and A. C. Cope awards by the American Chemical Society in 1974 and 1991, respectively, and the Photochem- istry Prize by the Inter-American Photochemical Associa- tion in 1992. He was elected a member of the National Academy of Sciences in 1974 and the American Academy of Arts and Sciences the following year. The Inter-American Photochemical Association honors his memory with the G. L. Closs Memorial Award, which allows a student to present a research paper at one of its meetings. 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 Lecture- ship at Universität Köln, Germany (where the author first met him), a visiting professorship at Yale (where the ac- quaintance was renewed), regular visits to Bell Laborato- ries, and the Merck Distinguished Lectureship at Rutgers University (where the author was privileged to be his host). Closs was a featured speaker at many national and inter- national congresses and symposia, and his participation at meetings was a highly important contribution to science. He brought to these meetings a keen analytical mind and the command of an unequalled breadth of chemical topics: from subtle details of organic synthesis, to a deep under- standing of mechanistic details, to the intricacies of chemi- cal physics, and a keen chemical intuition. This combina- tion allowed him to probe proposed theories or mechanisms as they were being presented. Few of his peers made more pertinent comments than Gerhard did, or in a more imper- tinent fashion when he felt it necessary. Even accomplished

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56 BIOGRAPHICAL MEMOIRS and experienced lecturers must have felt a tinge of appre- hension when he raised his hand and, on being recognized, uttered his familiar, “I would like to take issue with . . .”. Gerhard Closs relaxed by sailing, and sometimes racing, his sailboat on Lake Michigan for hours, days, or weeks. He relished his fine collection of graphic art, he was stimu- lated by theater performances, including modern and avant- garde plays, and he enjoyed classical music. No matter how hard the author tried, however, Gerhard could not be per- suaded to attend an opera performance. (He had sworn off opera as a teen in the early 1940s, following a performance of Da Ponte and Mozart’s Cosi Fan Tutti in Wuppertal.) During his 35 years at the University of Chicago, Gerhard developed a deep appreciation, even love, for his adopted country. He only bought American cars and “took issue” with many Americans and foreigners alike who dared to criticize the United States in his presence. One afternoon, while attending a Gordon Conference in New Hampshire, Gerhard was interviewed by a local reporter. 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 ap- plies 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 experi- menter who was always probing accepted theories and was never afraid to break new ground. The scientific commu- nity has lost a teacher, mentor, collaborator, and kin spirit, and a few who were privileged have lost a friend. The earliest publications of Gerhard Closs stem from his thesis work with Wittig and describe ylid rearrangements with ring enlargement or contraction, 3 a nd from his postdoctoral training with R. B. Woodward (total synthesis

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57 GERHARD LUDWIG CLOSS of chlorophyll).4 His first independent publications, for ex- ample, a paper on the active constituents of Panaeolus venenosus,5 reflect his being hired as a natural products chemist. As part of the venenosus project the physiological effects of the mushroom were to be tested, and the young assistant professor volunteered for the study. The highly amusing conversation ensuing between Gerhard Closs and his physician and collaborator is part of the Mycologia pub- lication.5 It was often cited at Closs group festivities and never failed to amuse; coworkers fortunate to have obtained a reprint of this paper count it among their prized posses- sions. Gerhard Closs never lost interest in natural products and photosynthetic pigments. Seventeen of his 132 lifetime publications dealt with the chlorophylls; he contributed sig- nificantly to such important topics as linked chlorophyll dimers, photosynthetic reaction centers, and porphyrin metal complexes. Still, his most significant contributions came in three other fields of chemistry, one area for each decade of his professional career. Gerhard Closs’s first major contributions came in the emerging field of carbene chemistry; interestingly, a distant predecessor at the University of Chicago, John U. Nef, was an early champion of divalent carbon chemistry. Alas, Nef’s interpretations of his results are at variance with the ac- cepted definitions and the prevailing understanding in the field since the mid-twentieth century so that his work no longer qualifies as carbene chemistry.6 The actual roots of the carbene field lie in the base- catalyzed hydrolysis of trichloromethane by Geuther in 1862.7 Hine repeated this experiment in 1949 and recognized the reaction as an α-elimination, the consecutive removal of H+ and Cl– from the same carbon, generating dichlorocarbene.8 In 1954 Doering and Hoffman trapped the postulated spe-

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58 BIOGRAPHICAL MEMOIRS cies by addition to cyclohexene, demonstrating its intermo- lecular reactivity.9 CHCl3 + OH– CCl3– + HOH (eq. 1) CCl3– + Cl– :CCl2 (eq. 2) Cl Cl :CCl2 (eq. 3) The development of divalent carbon chemistry involved various facets: substituted carbenes; the notion of spin mul- tiplicity; chemical studies probing carbene reactivity and the stereochemistry of their reactions; and the application of new physical methods (e.g., electron spin resonance, elec- tron nuclear double resonance, chemically induced dynamic nuclear polarization, and optical spectroscopy). Gerhard Closs played a significant role in introducing these new techniques to the study of carbenes. Closs generated chlorocarbene from methylene chloride; addition of the new carbene to alkenes, benzene, or phe- nol gave rise to chlorocyclopropanes,10 tropylium chloride,11 or tropone,12 respectively. Five-membered heterocycles (e.g., pyrrole and indole) reacted with chlorocarbene by ring ex- pansion.13 It is tempting to see in these ring enlargements echoes of his doctoral thesis. :CHCl Cl – + :CHCl N N H

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59 GERHARD LUDWIG CLOSS Additional carbenes arose by reaction of alkyl and benzal halides with organolithium compounds; here the term “carbenoid” was introduced to denote carbenes that appeared to be complexed (i. e., associated) with lithium halide.14 The base-induced α-elimination of chloroalkenes formed cyclopropenes by intramolecular addition of alkenyl- carbenes.15 By the time of his promotion to associate professor he began to ask further-reaching questions; he decided to char- acterize carbenes more thoroughly and, if possible, observe them directly. Spectroscopic techniques available at this time included optical spectroscopy and electron spin resonance. 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. Gerhard Closs was fortunate to have Clyde Hutchison, an expert ESR spectroscopist, as a col- league. They generated diphenylmethylene at cryogenic tem- peratures in benzophenone crystals and observed the first ESR spectrum of a ground state triplet carbene in a single crystal.19 About two weeks before the Chicago group, Edel Wasserman and coworkers at Bell Laboratories generated diphenylmethylene in a glassy matrix.20 The single crystal approach of the Chicago collaborators lent itself to a more detailed analysis and interpretation. Ultimately, electron nuclear double resonance (ENDOR) revealed the detailed structure of this intermediate (see Figure 1).21 Among Closs’s additional ESR studies cyclopentanediyl and trimethylcyclo- propenyl deserve special mention.

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60 BIOGRAPHICAL MEMOIRS FIGURE 1 Structure of diphenylmethylene as derived from ENDOR experi- ments.21 Following the work on the ESR spectroscopy of triplet states Gerhard Closs studied triplet carbenes by optical spec- troscopy,22 including the pioneering time-resolved laser spec- troscopy study of diphenylmethylene.25 Two other groups probed optical spectroscopy of diphenylmethylene indepen- dently,23,24 and only one study had dealt with the applica- tion of time-resolved laser spectroscopy (TRLS) to carbene chemistry26 when Closs and Rabinow’s study of diphenyl- methylene addition to alkenes25 opened the field to studies in other laboratories. Although the limited (µs) time reso- lution of these early studies appears almost primitive com- pared to today’s sophisticated TRLS experiments, the avail- able time resolution was exactly right for the somewhat “sluggish” diphenyl-methylene. Related studies involved cyclopropenes and bicyclo- butanes, newly accessible with his new carbenes, notably the isomerization of 2,4-dimethylbicyclo[1.1.0]butane to butadiene. The conservation of orbital symmetry, a concept developed by Woodward and Hoffmann in the early 1960s,27

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61 GERHARD LUDWIG CLOSS predicts that this reaction will proceed as a concerted [σ2s+σ2a] process with predictable stereochemistry for the migrating carbon centers. Closs and Pfeffer probed the re- arrangement of two 2,4-dimethylbicyclobutanes to two hexadienes and elucidated the steric course of this reac- tion.28 By the mid-1960s Gerhard Closs, an acknowledged ex- pert in carbene chemistry, made his final major contribu- tion to this field, the first application of the chemically induced dynamic nuclear polarization method. In 1967 en- hanced nuclear magnetic resonance (NMR) emission was observed in some chemical reactions,29,30 yet another facet in the rich palette of NMR applications. Because some re- actions giving rise to NMR emission were known free-radi- cal reactions, these effects were explained as electron-nuclear cross relaxation, hence the designation “chemically induced dynamic nuclear polarization” (CIDNP) for the new phe- nomenon. However, this mechanism was soon found want- ing, as an increasing number of effects were incompatible with the cross relaxation mechanism. Gerhard Closs immediately recognized the value of this technique. With his thorough understanding of organic re- action mechanisms and his expertise in the physical prin- ciples underlying magnetic resonance, he was in a unique

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62 BIOGRAPHICAL MEMOIRS position to elucidate the physical and chemical principles underlying CIDNP. He entered the field with all his vigor. He persuaded Lieselotte to return to the bench for a few well-designed experiments.31,32 In 1969 four back-to-back communications appeared in the Journal of the American Chemical Society, followed quickly by six more, for a total of ten communications in only 20 months. After two CIDNP studies in photoreactions of diphenyldiazomethane31 and benzophenone32 Closs began to probe the actual origin of the spin polarization effects. Recognizing that all CIDNP effects required the involve- ment of radical pairs,33 he developed a theory that could explain the observed polarization and designed elegant ex- periments to probe key features of the theory. Because the polarization changed with the spin multiplicity (µ) of the precursor from which the pair was generated, he suggested CIDNP “as a tool for determination of spin multiplicities of radical pair precursors.”34 He also recognized that the polarization pattern of CIDNP effects depends critically on the relative g factors (∆g) of the paired radicals and illustrated the effect of systematical changes in the g factor difference (∆g) between the inter- acting radicals on the CIDNP spectra (see Figure 2).35 H C6H4-Y •• Y-C6H4 C6H4-Y H H C6H4-Y hν H • • O X-H4 C6 C6H4-X H HO In this context I will share an anecdote about the 1970 American Chemical Society meeting in Houston, Texas, where

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63 GERHARD LUDWIG CLOSS FIGURE 2 CIDNP spectra (benzylic protons) of coupling products gener- ated during the photoreactions of benzaldehyde (X = H) and derivatives (X = Cl, Br) with diarylmethanes (Y = H, Cl, Br).33

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72 BIOGRAPHICAL MEMOIRS One significant aspect of the electron transfer work sug- gested that donor and acceptor are coupled by the interac- tions with the orbitals of the intervening molecular frag- ments, a mechanism referred to as superexchange. This was one of the reasons that caused Gerhard Closs to recon- sider the interaction of two unpaired electron spins in biradicals through the intervening molecular fragment, lead- ing to the concept of spin-correlated radical pairs (see above). Gerhard Closs made significant contributions in three fields of chemistry. 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 ap- proach to research. Once he became interested in a prob- lem he would first master the theoretical background, or develop it himself, as was the case with CIDNP. Then he would identify key features that needed to be verified and design ingenious and decisive experiments to probe the theory. Finally, he would synthesize the selected target mol- ecules and conduct the physical experiments. IN WRITING THIS MEMOIR I have benefited from conversations with several of Gerhard’s students and colleagues, particularly in the difficult task of choosing the “Selected Bibliography” from his many superb publications: Robert A. Moss, who was a student in the carbene years (cf., 1964); John Miller, who inspired Gerhard’s interest in electron transfer and collaborated with him for his final 10 years (cf., 1983, 1984, 1986, 1989, 1990); Jim Norris, an associate and friend at the University of Chicago, who made available a tribute to Gerhard, written for the University of Chicago Chronicle;54 and Piotr Piotrowiak, one of his last students, who bridged the electron trans- fer and biradical projects (cf., 1989, 1992).

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73 GERHARD LUDWIG CLOSS NOTES 1. G. Wittig and L. Pohmer. Intermedi ä re Bildung von Dehydrobenzol (Cyclohexa-dienin). Angew. Chem. 67(1955):348. 2. G. Wittig L. Pohmer. Über das Intermediäre Auftreten von Dehydrobenzol Chem. Ber. 89(1956):1334-51. 3. G. Wittig, F. Mindermann, and G. L. Closs. Über Ringerweiterung und Ringverengerung auf der Basis von Ylidisomerisationen. Liebigs Ann. Chem. 594(1955):89-118. 4. R. B. Woodward, W. A. Ayer, J. M. Beaton, F. Bickelhaupt, R. Bonnett, P. Buchschacher, G. L. Closs, H. Dutler, J. Hannah, F. P. Hauck, S. Itô, A. Langemann, E. Le Goff, W. Leimgruber, W. Lwowski, J. Sauer, Z. Valenta, and H. Volz. The total synthesis of chlorophyll. J. Am. Chem. Soc. 82(1960):3800-3802. 5. G. L. Closs and N. W. Gable. Studies toward the isolation of the active constituents of Panaeolous venenosus. Mycologia 11(1959):211- 16. 6. J. U. Nef. J. Liebigs Ann. Chem. 270(1892):267; 280(1894):291; Ü ber das zweiwerthige Kohlenstoffatom. 287(1895):265-59; 298(1897):202. 7. A. Geuther. Über die Hydrolyse des Chloroforms in Basischem Medium. Liebigs Ann. Chem. 123(1862):121. 8. J. Hine. Carbon dichloride as an intermediate in basic hy- drolysis of chloroform. A mechanism for substitution reactions at a saturated carbon atom. J. Am. Chem. Soc. 72(1950):2438-45. 9. W. von E. Doering and A. K. Hoffmann. The addition of dichlorocarbene to olefins. J. Am. Chem. Soc. 76(1954):2162-65. 10. G. L. Closs and L. E. Closs. Syntheses of chlorocyclopropanes from methylene chloride and olefins. J. Am. Chem. Soc. 81(1959):4996- 97. 11. G. L. Closs and L. E. Closs. Addition of chlorocarbene to benzene. Tetrahedron Lett. 10(1960):38-40. 12. G. L. Closs and L. E. Closs. Carbenes from alkyl halides and organolithium compounds. III. Syntheses of alkyltropones from phenols. J. Am. Chem. Soc. 83(1961):599-602. 13. G. L. Closs and G. M. Schwartz. Ring-expansions of pyrrole and indole. J. Org. Chem. 26(1961):2609. 14. G. L. Closs, R. A. Moss, and J. J. Coyle. Steric course of some carbenoid additions to olefins. J. Am. Chem. Soc. 84(1962):4985-86.

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74 BIOGRAPHICAL MEMOIRS 15. G. L. Closs and L. E. Closs. A novel synthesis of cyclopropenes. J. Am. Chem. Soc. 83(1961):1003-1004. Alkenylcarbenes as precur- sors of cyclopropenes. J. Am. Chem. Soc. 83(1961):2015-16. 16. R. G. W. Norrish and G. Porter. Chemical reactions produced by very high light intensities. Nature 164(1949):658. 17. G. Herzberg. The spectra and structures of free methyl and free methylene. Proc. R. Soc. 262A(1961):291. 18. E. Zavoiskii. Paramagnetic relaxation of liquid solutions for perpendicular fields. J. Phys. U. S. S. R. 9(1945):211-16; Spin-mag- netic resonance in paramagnetic substances. J. Phys. U. S. S. R. 9(1945):245; Paramagnetic absorption in solutions in parallel mag- netic fields. Zh. Eksp. Teor. Fiz. 15(1945):253-57. Paramagnetic ab- sorption in some salts in perpendicular magnetic fields. J. Phys. U. S. S. R. 10(1946):170-73. 19. R. W. Brandon, G. L. Closs, and C. A., Hutchison, Jr. Para- magnetic resonance in oriented ground-state triplet molecules. J. Chem. Phys. 37(1962):1878-79. 20. R. W. Murray, A. M. Trozzolo, E. Wasserman, and W. A. Yager. E. P. R. of diphenylmethylene, a ground-state triplet. J. Am. Chem. Soc. 84(1962):3213-14. 21. D. C. Doetschman and C. 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. J. Chem. Phys. 56(1972):3964- 82. 22. G. L. Closs, C. A. Hutchison, Jr., and B. E. Kohler. Optical absorption spectra of substituted methylenes oriented in single crystals. J. Chem. Phys. 44(1966):413-14. 23. W. A. Gibbons and A. M. Trozzolo. Spectroscopy and photoly- sis of a ground-state molecule, diphenylmethylene. J. Am. Chem. Soc. 88(1966):172-73. 24. I. Moritani, S.-I. Murahashi, M. Nishino, K. Kimura, and H. Tsubomura. Electronic spectra of the products formed by the pho- tolysis of diazo compound at 77 K, possibly identified to carbenes. Tetrahedron Lett. 4(1966):373-78. 25. G. L. Closs and B. E. Rabinow. Kinetic studies on diarylcarbenes. J. Am. Chem. Soc. 98(1976):8190-98. 26. I. Moritani, S.-I. Murahashi, H. Ashitaka, K. Kimura, and H. Tsubomura. Flash photolysis of 5-diazo-10,11-dihydrodibenzo [a,d]cycloheptadiene. J. Am. Chem. Soc. 90(1968):5918-19.

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75 GERHARD LUDWIG CLOSS 27. R. B. Woodward and R. Hoffmann. The Conservation of Orbital Symmetry. Weinheim: Verlag Chemie, 1971. 28. G. L. Closs and P. E. Pfeffer. The steric course of the thermal rearrangements of methylbicyclobutanes. J. Am. Chem. Soc. 90(1968):2452. 29. J. Bargon, H. Fischer, and U. Johnsen. Kernresonanz- Emissionslinien w ä hrend rascher Radikalreaktionen. I. Aufnahmeverfahren und Beispiele. Z. Naturforsch. A. 22(1967):1551- 55. 30. J. Bargon and H. Fischer. Kernresonanz-Emissionslinien während rascher Radikalreaktionen. II. Chemisch induzierte dynamische Kernpolarization. Z. Naturforsch. A 22(1967):1556-60. 31. G. L. Closs and L. E. Closs. Induced dynamic nuclear spin polarization in reactions of photochemically and thermally gener- ated triplet diphenylmethylene. J. Am. Chem. Soc. 91(1969):4549-50. 32. G. L. Closs and L. E. Closs. Induced dynamic nuclear spin polarization in photoreductions of benzophenone by toluene and ethylbenzene. J. Am. Chem. Soc. 91(1969):4550-52. 33. G. L. Closs. A mechanism explaining nuclear spin polariza- tions in radical combination reactions. J. Am. Chem. Soc. 91(1969):4552- 54. 34. G. L. Closs and A. D. Trifunac. Chemically induced nuclear spin polarization as a tool for determination of spin multiplicities of radical-pair precursors. J. Am. Chem. Soc. 91(1969):4554-55. 35. G. L. Closs, C. E. Doubleday, and D. R. Paulson. Theory of chemically induced nuclear spin polarization. IV. Spectra of radical coupling products derived from photoexcited ketones and aldehydes. J. Am. Chem. Soc. 92(1970):2185-86. 36. R. Kaptein. Chemically induced dynamic nuclear polarization in five alkyl radicals. Chem. Phys. Lett. 2(1968):261. 37. G. L. Closs and C. E. Doubleday. Determination of the aver- age singlet-triplet splitting in biradicals by measurement of the magnetic field dependence of CIDNP. J. Am. Chem. Soc. 95(1973):2735-36. 38. G. L. Closs. Low-field effects and CIDNP of biradical reac- tions. In Chemically Induced Magnetic Polarization: Theory, Technique, and Applications, vol. 34, NATO Advanced Study Institutes Series, eds. L. T. Muus, P. W. Atkins, K. A. McLauchlan, and J. B. Pedersen, pp. 225-56. Dordrecht, Holland: D. Reidel, 1977. 39. G. L. Closs and M. S. Czeropski. Observation of a CIDNP pumped nuclear Overhauser effect: A caveat for the interpretation of CIDNP spectra. Chem. Phys. Lett. 45(1977):115-16.

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76 BIOGRAPHICAL MEMOIRS 40. G. L. Closs and M. S. Czeropski. Amendment of the CIDNP phase rules. Radical pairs leading to triplet states. J. Am. Chem. Soc. 99(1977):6127-28. 41. G. L. Closs and R. J. Miller. Laser flash photolysis with NMR detection. Microsecond time-resolved CIDNP: Separation of gemi- nate and random-phase processes. J. Am. Chem. Soc. 101(1979):1639- 41. 42. G. L. Closs and E. V. Sitzmann. Measurements of degenerate radical ion-neutral molecule electron exchange by microsecond time- resolved CIDNP. Determination of relative hyperfine coupling con- stants of radical cations of chlorophylls and derivatives. J. Am. Chem. Soc . 103(1981):3217-19. 43. G. L. Closs and R. J. Miller. Application of Fourier transform- NMR spectroscopy to submicrosecond time-resolved detection in laser flash photolysis experiments. Rev. Sci. Instrum. 52(1981):1876- 85. 44. R. W. Fessenden and R. H. Schuler. Electron spin resonance studies of alkyl radicals. J. Chem. Phys. 39(1963):2147-95. 45. Y. Sakaguchi, H. Hayashi, H. Murai, and Y. J. I’Haya, CIDEP study of the photochemical reactions of carbonyl compounds show- ing the external magnetic field effect in a micelle. Chem. Phys. Lett. 110(1984):275-79; Y. Sakaguchi, H. Hayashi, H. Murai, Y. J. I’Haya, and K. Mochida. CIDEP study of the formation of cyclohexadienyl- type radicals in the hydrogen abstraction of triplet xanthone. Chem. Phys. Lett. 120(1985):401-405. 46. G. L. Closs, M. D. E. Forbes, and J. R. Norris, Jr. Spin-polar- ized electron paramagnetic resonance spectra of radical pairs in micelles. Observation of electron spin-spin interactions. J. Phys. Chem. 91(1987):3592-99. 47. C. D. Buckley, D. A. Hunter, P. J. Hore, and K. A. McLauchlan. Electron spin resonance of spin-correlated radical pairs. Chem. Phys. Lett. 135(1987):307-12. 48. G. L. Closs, M. D. E. Forbes, and P. Piotrowiak. Spin and reaction dynamics in flexible polymethylene biradicals as studied by EPR, NMR, and optical spectroscopy and magnetic field effects. Measurements and mechanisms of scalar electron spin-spin cou- pling. J. Am. Chem. Soc. 114(1992):3285-94. 49. R. A. Marcus. The theory of oxidation-reduction reactions in- volving electron transfer. I. J. Chem. Phys. 24(1956):966-78; The theory

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77 GERHARD LUDWIG CLOSS of oxidation-reduction reactions involving electron transfer. II. Ap- plications to data on the rates of isotopic exchange reactions. J. Chem. Phys. 26(1957):867-71. The theory of oxidation-reduction re- actions involving electron transfer. III. Applications to data on the rates of organic redox reactions. J. Chem. Phys. 26(1957):872-77. Theory of electrochemical and chemical electron-transfer processes. Can. J. Chem. 37(1959):155-63. 50. D. Rehm and A. Weller. Kinetics of fluorescence quenching by electron and h-atom transfer. Israel J. Chem. 8(1970):259-71. 51. J. R. Miller, J. V. Beitz, and R. K. Huddleston. Effect of free energy on rates of electron transfer between molecules. J. Am. Chem. Soc. 106(1984):5057-68. 52. G. L. Closs and J. R. Miller. Intramolecular long-distance elec- tron transfer in organic molecules. Science 240(1988):440-47. 53. G. L. Closs, M. D. Johnson, J. R. Miller, and P. Piotrowiak. A connection between intramolecular long-range electron, hole, and triplet energy transfers. J. Am. Chem. Soc. 111(1989):3751-53. 54. J. M. Norris. Gerhard Closs, 1928-1992. University of Chicago Chronicle 12(11), 1993.

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78 BIOGRAPHICAL MEMOIRS SELECTED BIBLIOGRAPHY 1960 With L. E. Closs. Carbenes from alkyl halides and organolithium compounds. I. Synthesis of chlorocyclopropanes. J. Am. Chem. Soc. 82:5723-28. 1961 With L. E. Closs. Alkenylcarbenes as precursors of cyclopropenes. J. Am. Chem. Soc. 83:2015-16. 1963 With L. E. Closs. Carbon orbital hybridizations and acidity of the bicyclobutane system. J. Am. Chem. Soc. 82:2022-23. 1964 With R. A. Moss. Carbenoid formation of arylcyclopropanes from olefins, benzal bromides, and organolithium compounds and from photolysis of aryldiazomethanes. J. Am. Chem. Soc. 86:4042-53. 1965 With R. L. Brandon, C. E. Davoust, C. A. Hutchison, Jr., B. E. Kohler, and R. Silbey. Electron paramagnetic resonance spectra of the ground-state triplet diphenylmethylene and fluorenylidene mol- ecules in single crystals. J. Chem. Phys. 43:2006-16. 1969 With L. E. Closs. Induced dynamic nuclear spin polarization in re- actions of photochemically and thermally generated triplet diphenylmethylene. J. Am. Chem. Soc. 91:4549-50. With L. E. Closs. Induced dynamic nuclear spin polarization in pho- toreductions of benzophenone by toluene and ethylbenzene. J. Am. Chem. Soc. 91:4550-52. A mechanism explaining nuclear spin polarizations in radical com- bination reactions. J. Am. Chem. Soc. 91:4552-54. With A. D. Trifunac. Chemically induced nuclear spin polarization as a tool for determination of spin multiplicities of radical-pair precursors. J. Am. Chem. Soc. 91:4554-55.

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79 GERHARD LUDWIG CLOSS 1972 With C. E. Doubleday. 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. J. Am. Chem. Soc. 94:9248-49. 1973 With C. E. Doubleday. Determination of the average singlet-triplet splitting in biradicals by measurement of the magnetic field de- pendence of CIDNP. J. Am. Chem. Soc. 95:2735-36. 1975 With S. G. Boxer. Nuclear magnetic resonance of photoexcited trip- let states. I. The measurement of the rate of degenerate singlet- triplet exchange for anthracene in solution. J. Am. Chem. Soc. 97:3268-70. 1976 With B. E. Rabinow. Kinetic studies on diarylcarbenes. J. Am. Chem. Soc. 98:8190-98. 1979 With S. L. Buchwalter. Electron spin resonance and CIDNP studies on 1,3-cyclopentadiyls. A localized 1,3 carbon biradical system with a triplet ground state. Tunneling in carbon-carbon bond formation. J. Am. Chem. Soc. 101:4688-94. 1981 With E. V. Sitzmann. Measurements of degenerate radical ion-neu- tral molecule electron exchange by microsecond time-resolved CIDNP. Determination of relative hyperfine coupling constants of radical cations of chlorophylls and derivatives. J. Am. Chem. Soc. 103:3217-19. With R. J. Miller. L aser flash photolysis with NMR detection. Submicrosecond time-resolved CIDNP: Kinetics of triplet states and biradicals. J. Am. Chem. Soc. 103:3586-88. 1983 With L. T. Calcaterra and J. R. Miller. Fast intramolecular electron

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80 BIOGRAPHICAL MEMOIRS transfer in radical ions over long distances across rigid saturated hydrocarbon spacers. J. Am. Chem. Soc. 105:670-71. 1984 With J. R. Miller and L. T. Calcaterra. Intramolecular long-distance electron transfer in radical anions. The effects of free energy and solvent on the reaction rates. J. Am. Chem. Soc. 106:3047-49. 1986 With L. T. Calcaterra, N. J. Green, K. W. Penfield, and J. R. Miller. Distance, stereoelectronic effects, and the Marcus inverted re- gion in intramolecular electron transfer in organic radical an- ions. J. Phys. Chem. 90:3673-83. 1988 With J. R. Miller. Intramolecular long-distance electron transfer in organic molecules. Science 240:440-47. 1989 With M. D. Johnson, J. R. Miller, and N. S. Green. Distance depen- dence of intramolecular hole and electron transfer in organic radical ions. J. Phys. Chem. 93:1173-76. With M. D. Johnson, J. R. Miller, and P. Piotrowiak. A connection between intramolecular long-range electron, hole, and triplet energy transfers. J. Am. Chem. Soc. 111:3751-53. With N. Liang and J. R. Miller. Correlating temperature depen- dence to free energy dependence of intramolecular long-range electron transfers. J. Am. Chem. Soc. 111:8740-41. 1990 With N. Liang and J. R. Miller. Temperature-independent long- range electron transfer reactions in the Marcus inverted region. J. Am. Chem. Soc. 112:5353-54. 1992 With M. D. E. Forbes and P. Piotrowiak. Spin and reaction dynam- ics in flexible polymethylene biradicals as studied by EPR, NMR, and optical spectroscopy and magnetic field effects. Measurements and mechanisms of scalar electron spin-spin coupling. J. Am. Chem. Soc. 114:3285-94.

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