SCIENCE
Electron Transfer Studies Yield Evidence for "Inverted Region" Chemists from Argonne and University of Chicago use pulse radiolysis to study intramolecular electron transfer reactions in solutions
could be slower in very highly exo thermic reactions. Nevertheless, Miller observes, "the whole business is quite impor tant from the point of view of effi cient separation of charges—in pho tosynthesis and other biological processes, for example, and in such fields as the development of photo resists and chemical electronic im Ward Worthy, C&EN Chicago aging technologies." The more exothermic a reaction, the Miller for several years has been faster it goes. Generally speaking. studying long-distance intermolecuNot always, according to chem lar electron transfer in rigid matrices. ists John R. Miller of Argonne Na In that work, fixed intermolecular tional Laboratory and Gerhard L. distances were obtained by freez Closs of the University of Chicago. ing molecules in place in a rigid In studies of intramolecular elec solvent "glass," so that the depen tron transfer reactions, they have dency of electron transfer rates on found that other factors, including distance could be measured. Miller's the solvent, can play important roles, experiments gave indications that and that reaction rates can go down the inverted region did, in fact, exist. even as exothermicity goes up. But the rigid matrix work was Specifically, Closs and Miller say, sort of special. A much more con in a communication to the Journal of vincing case for the existence of the the American Chemical Society [106, inverted region could be made if 3047 (1984)], that they have come up similar results were achieved with with firm evidence of the existence intramolecular electron transfer re of an "inverted region" for electron actions in fluid solutions. Miller pro transfer reactions in solutions. posed to Closs that the two join The inverted region had been pre forces to carry out such a project. dicted by theories of electron trans Closs agreed, and work got under fer first put forth by California In way (funds for the collaborative ef stitute of Technology chemistry pro fort have been provided by the Of fessor Rudolph A. Marcus almost fice of Basic Energy Sciences of the 30 years ago and since elaborated Division of Chemical Sciences, De by many other scientists. That pre partment of Energy, and by the Na diction conflicted with the classical tional Science Foundation). approach, the two chemists note. The first order of business was Furthermore, it appeared to be con the synthesis of suitable compounds tradicted by almost all experiments for study. Research associate Lidia designed to confirm it: The inverted T. Calcaterra was enlisted for this region simply could not be found. phase of the project; Miller and Closs In such systems, Miller explains, note that she also has assisted the the competing return reaction is effort in other important ways. highly exothermic; consequently, The team set out to synthesize a one would expect charge separation series of eight compounds. Each efficiency to be low. However, the compound was to consist of two inverted region theory held out the molecular groups (A and B) having possibility that the "jump back" different electron affinities, sepa 42
June 4, 1984 C&EN
rated by a "spacer" (Sp), a saturated hydrocarbon having a negative elec tron affinity. For the first set of A-Sp-B com pounds, the steroidal 5a-androstane skeleton was chosen as the spacer, and the 4-biphenylyl group as B. The eight A groups were (in order of increasing exothermicity) 2naphthyl, 9-phenanthryl, 1-pyrenyl, 2-hexahydronaphthoquinonyl, 2naphthoquinonyl, 2-benzoquinonyl, 2-(5-chlorobenzoquinonyl), and 2-(5,6-dichlorobenzoquinonyl). In all the compounds, the distances be tween A and Β are identical—on the order of 10 Â, edge to edge. Each of the compounds was dissolved in methyltetrahydrofuran (MTHF) at room temperature. Then the liquid solutions were subjected to pulses of electrons (generated by Argonne's 20-MeV Linac). The Linac is "a beautiful machine," Closs notes, ''because it gives us high current pulses in very short times, on the order of 30 picoseconds. These highly energetic electrons come zooming through the sample. As they do, they kick secondary electrons out of the solvent. These come out with some energy, but they finally thermalize and get solvated. Then the solvated electrons can be captured with any acceptor." An advantage of the pulse radiolysis technique, Closs adds, is that it creates free ions, not close ion pairs. Also, the Linac can generate ions not only in polar solvents like MTHF but also in solvents as nonpolar as hydrocarbons. Regardless of the relative electron affinities of A and B, the solvated electrons are captured by A or Β with almost statistical prob ability. With the model compounds, however, A has a greater electron affinity than B. The result, initially, is a nonequilibrium state. The rate
at which the initial ion distribution reaches equilibrium (in this case, with practically all the electrons transferred from Β to A) can be de termined from real-time photomet ric absorption measurements. The observed rate includes intermolecular as well as intramolecular electron transfers. However, what Miller and Closs were looking for was the intramolecular transfer rate, across the known distance between A and B. So they ran similar experi ments with monofunctional (A-Sp and B-Sp) compounds to determine the intermolecular contribution and used the data from those experi ments to determine the intramolec ular rates in the afunctional (A-Sp-B) compounds. According to theory, the electron transfer rate should reach a maxi mum when the free energy change, - A G ° , matches the total reorganiza tion energy, λ, occasioned by the redistribution of the charge on the molecule. The major components of λ are the solvent reorganization energy, Xs, and the change in inter
nal vibration modes, λ ν . In the ex periments with MTHF as solvent, Xs was calculated to be about 0.75 eV with λ ν = 0.45 eV. Thus, again according to theory, the maximum reaction rate should occur when the —AG° of the reaction is about 1.2 eV, with slower rates for reactions of greater or lesser exothermicity. Indeed, that is what Closs and Miller found. For the A = 2-naphthylyl compound (-AG° '= 0.05 eV), the rate constant was about 1.5 X 106 per second. As the electron affinities of the A groups went up, so did the reaction rates, to a maxi mum of at least 2 Χ 109 per second (the upper limit of the measure ment apparatus) for the 2-hexahydronaphthoquinonyl compound, where - A G ° = 1.23 eV. Then—just as the theory had predicted—reac tion rates decreased with increas ing exothermicity, to about 7 X 107 in the case of the 2-(5,6-dichlorobenzoquinonyl) compound, where - A G ° = 2.40 eV. According to Miller and Closs, the maximum rate corresponds to a
Electron transfer rate reaches maximum when - \G° matches total reorganization energy3 Rate constant, k(S- 1 )
Note: Solvent is tetrahydrofuran at 296 K. a Major components of total reorganization energy (λ) are solvent reorganization energy (λ 8 ) and changes in internal vibration modes (λ ν )· In this system, \ a ^ 0.75 eV and λ ν = 0.45 eV
transfer in which the Franck-Condon factors (determined by the degree to which the positions and angles of the nuclei in the molecule either facilitate or h i n d e r the electron transfer) are maximized. In reactions with greater exothermicities, the rate falls off because of the increasing mismatch of the overlaps of the vibrational wave functions. If that's the case, the two scien tists add, the curve should be dis placed if the model compounds are dissolved in a less polar solvent. To test the hypothesis, they ran simi lar experiments, using isooctane in stead of MTHF as solvent. The data from these tests aren't so satisfying, they admit, because some of the re actions were too fast to measure accurately. Also, some of the reac tions probably were complicated by the formation of electronically ex cited states. Nevertheless, the re sults left no doubt that the maxi mum rate occurs at a much smaller —AG° in isooctane than in MTHF and that the rate falloff at high exo thermicity is much greater. Rates of mildly exothermic reactions in creased by factors as large as 103, whereas rates of strongly exother mic reactions decreased by factors as large as 101 8 . Miller and Closs say that their findings provide the first unambig uous evidence of the inverted re gion in electron transfer reactions in s o l u t i o n s . Previous a t t e m p t s failed, they believe, because most studies focused on intermolecular electron transfer reactions, where rates are limited by diffusion. The results also emphasize that electrons can tunnel across insulating barri ers as wide as 10 Â with "astonishing" speed (a few hundred picoseconds) when conditions are right. That speed stands in sharp contrast to the rates observed by Harry B. Gray, head of California Institute of Technology's division of chemistry and chemical engineering. Gray has been studying electron transfer processes in modified protein molecules (C&EN, April 16, page 6). But in his work, rates have ranged from about 0.04 per second to about 30 per second. "We understand why our rates are fast," Closs comments, " b u t we d o n ' t u n d e r s t a n d w h y [Gray's] are so slow." June 4, 1984 C&EN
43
Polymer Blends and Composites in Multiphase Systems I
fWymer Blends
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and Composite»
if
in Multiphase
•
&CI&iiCG
In any case, electron transfer reac tions in solution obviously are in fluenced by factors other than exothermicity and solvent polarity, the focus of the recent work at Argonne and UC. Further experiments, de signed to explore t h e effects of temperature, distance, and molecu lar geometry, are now under way. Ward Worthy, Chicago
Binuclear copper complex binds oxygen HEW C D . H a n , Editor t Polytechnic Institute of New York Describes new and exciting research activities in polymer blends and com posites. Looks at three aspects of multiphase polymer systems: com patibility and characterization of poly mer blends; rheology, processing, and properties of heterogeneous polymer blends; and polymer com posites. CONTENTS GPC Use in Determination of Polymer-Poly mer Interaction Parameters · Polymer Blend Exhibiting Upper and Lower Critical Solution Temperature Behavior » Phase Equilibria in Polymer Melts by Melt Titration · Comparison of Miscible Blend Binary Interaction Param eters Measured by Different Methods · Com patibility Studies of Poly(vinylidene fluoride): Blends Usina Carbon-13 NMR • Effect of Mo lecular Weight on Blend Miscibility: Study by Excimer Fluorescence · Structure-Property Relationships of Polystyrene/Poly(vinyl methyl ether) Blends · Segmented Orientation in Multicomponent Polymer Systems · Proper ties and Morphology of Poly(methyl methacrylate)/Bisphenol A Polycarbonated Blends · Compatibility of Random Copolymer of Vary ing Composition with Each Homopolymer · Rheological Behavior of Blends of Nylon with Chemically Modified Polyolefin · Controlled Ingredient-Distribution Mixing · Mechanical Behavior of Polyolefin Blends · Model Studies of Rubber Additives in High-Impact Plastics · Reinforcements of Butadiene-Acrylonitrile Elastomer by Carbon Black · Fatigue Crack Propagation in Short-Fiber Reinforced Plas tics · Prediction and Control of Fiber Orienta tion in Molded Parts * Pulling Force and Its Variation in Composite Materials Pultrusion · Effect of Hygrothermal Fatigue on PhysicalMechanical Properties and Morphology of Graphite/Epoxy Laminates
Researchers at the State University of New York, Albany, have prepared, characterized, and for the first time confirmed spectroscopically a syn thetic binuclear copper complex that binds molecular oxygen as perox ide [/. Am. Chem. Soc, 106, 3372 (1984)]. The work represents a step toward better understanding of how copper-containing enzymes bind and activate dioxygen in biological systems. The research was carried out by SUNY associate professor of chemis try Kenneth D. Karlin, postdoctoral fellow Yilma Gultneh, and gradu ate students Richard W. Cruse and Jon C. Hayes. SUNY associate pro fessor of chemistry Jon A. Zubieta collaborated on the cristallograph ie studies. The work was supported by the National Institutes of Health. Such synthetic copper complexes are the focus of current research in many laboratories in the hope that understanding them can help eluci date the activity of a number of
Complex binds oxygen as peroxide +1
Based on a symposium sponsored by the Division of Polymer Chemistry of the American Chemical Society Advances in Chemistry Series 206 286 pages (1984) Clothbound LC 83-24362 ISBN 0-8412-0783-6 US & Canada $59.95 Export $71.95 Order from: American Chemical Society Distribution Office Dept. 01 1155 Sixteenth St., N.W. Washington, DC 20036 or CALL TOLL FREE 800-424-6747 and use your ViSA, MasterCard, or American Express credit card.
44
June 4, 1984 C&EN
^
Ν
Py-/
Ο
Cu*
Ν
Cu*
"Py Note: Py = 2-pyridyl
?
YPy
Py
(Q)
copper-containing enzymes. Hemocyanin, for instance, binds and trans ports oxygen; tyrosinase and dopa mine β-hydroxylase are monooxygenases that incorporate oxygen into organic substrates. Such reactivity also has potential practical applica tion in development of synthetic systems for oxidation reactions. In previous research, Karlin and coworkers prepared a compound that binds two copper ions to form a binuclear copper(I) complex. Each copper ion is coordinated to the three nitrogens of a tridentate ligand consisting of two pyridine groups linked to an aminomethyl nitrogen. The two tridentate groups are linked via the aminomethyls meta to each other on a phenyl ring [/. Am. Chem. Soc, 106, 2121 (1984)]. This complex reacts with dioxygen to form a doubly bridged copper(H) complex with specific hydroxylation of the aromatic ring. Removal of the two copper ions yields a phenol which forms the basis of the recent ly characterized complex. The phenol binds two copper(I) ions to form a new binuclear com plex. That complex reacts w i t h dioxygen. Resonance Raman spec troscopic studies of the complex being carried out by Edward I. Solo mon and coworkers at Stanford Uni versity using isotopically labeled dioxygen confirm that the bound species is peroxide. The orientation of the oxygen atoms cannot yet be determined. Karlin points out that the observed spectrum is not similar to that of oxygen bound to hemocyanin, which suggests that the mode of binding is different. However, he adds, "we have demonstrated for the first time that one can produce a synthetic system in which oxygen is bound as peroxide." "One can make an analogy with work in the 1970s on iron porphy rin systems," Karlin says. "That is, where models were designed to mimic the oxygen binding in hemo globin. It is an important step toward a better understanding of the way oxygen is bound, in this case in copper-containing biologi cal systems, and that will give us insights into the way copper en zymes utilize oxygen for oxidation of organic substrates." Rudy Baum, San Francisco
