RESEARCH
Photochemical Theories Gain New Strength Caltech and Wisconsin research supports photochemical role of electron spin and electron distribution mechanisms
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orbitals that aren't occupied since they are at higher energy levels. Electrons in occupied orbitals can be excited to unoccupied ones by absorbing light. During this process, the direction of an electron's spin doesn't change. So in both the ground state and the excited state, the electrons in the molecule have no net angular momentum. Both of these states are called singlets. The ground state is designated So, and the excited states S1? SL,, S;>, and so forth. A molecule, however, will always have another state similar to the excited state, S^ This is one in which the spin of the promoted electron (or the one remaining) is reversed. This molecule will have an electronic magnetic moment and the energy level will be split into three levels in a magnetic field. This is a triplet s t a t e - T , and TL,. Singlet states usually don't go to triplet states, Dr. Hammond says. But the opposite process, T^ to SK, takes place if a molecule is trapped in the T, state. It can be observed as long-lived phosphorescence with light emission. Nonradiative decay of T1 to So also goes much slower than a thermal degradation which conserves spin. An example of the latter case would be SL> to Sj or T2 to T1. Of these states, the lowest triplets are likely candidates as intermediates
in photochemical reactions, Dr. Hammond says. Compared to excited singlets, they have much longer lives. And since triplets are the lowest excited states of molecules, they are probably the intermediates in thermally activated reactions. However, there is no evidence that precludes possibilities of excited singlets also taking part in some very fast reactions. Energy Transfer. To pinpoint the chemical behavior of triplet states, Dr. Hammond, Dr. William Moore, Dr. Peter Leermakers, Nicholas Turro, and Dr. Karl Kopecky developed the energy transfer process to make triplets. A photosensitizer such as benzophenone (which decays by triplet states) is used to absorb the light. The triplet state of the sensitizer then transfers energy to other molecules, converting them to triplet state: ATJ
+ BSo. -> As g + BTi
The method allows selective excitation of B molecules to triplet states. Examples include indirect production of triplets of ethyl pyruvate, diazomethane, and conjugated dienes. When irradiated, ethyl pyruvate decomposes to acetaldehyde, carbon monoxide, and a small amount of carbon dioxide. The reaction must involve a singlet excited state or a longlived triplet, he says. Since it gives a weak, blue phosphorescence in
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New data add support to theories that explain how electrons in molecules behave in photochemical reactions. The theories are electron spin and electron distribution. A research team under Dr. George S. Hammond at the California Institute of Technology is trying to determine which of the various excited states of irradiated reactants are the chemically reactive ones. Results with a variety of organic compounds point to excited states having two electrons with parallel spins as the chemically active intermediates in many photochemical reactions. Dr. Howard Zimmerman's group at the University of Wisconsin is attempting to relate electron distribution in excited states of irradiated reactants to the products of a reaction. Subjecting organic compounds to ultraviolet light gives, in many cases, the products predicted by the electron distribution approach. Organic photochemistry—the study of how visible or UV light affects an organic chemical reaction—hasn't kept pace with ground state organic chemistry. And for good reason. Chemists know the electronic make-up of the reactants used in conventional reactions; under irradiation, though, the reactants are in excited states, have different electronic configurations. This deficiency of organic photochemistry (compared with ground state organic chemistry) is obviously coming to an end, Dr. Hammond and Dr. Zimmerman made clear at the 17th National Organic Chemistry Symposium, held at Indiana University, Bloomington. The symposium was sponsored by the ACS Division of Organic Chemistry. Singlets and Triplets. Organic molecules usually contain an even number of electrons. Normally, these electrons occupy molecular orbitals—either localized or delocalized—in pairs and with opposite spin. But there are always some
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GROUPS. A case of withdrawing groups involves six electrons (solid dots). Promoting an electron (lower arrow) from a molecular orbital that allows the electron to appear at the meta position puts it into an orbital that doesn't. A case of donating groups uses eight electrons (dots plus circles). Excitation moves an electron (upper arrow) from an orbital that doesn't permit meta appearance to one that does to boost meta electron density
frozen ether-pentane-alcohol "glass," the reaction probably goes via a triplet state. To back up this evidence, the Caltech chemists also irradiated the ester in the presence of benzophenone, a photosensitizer. They found that not only does ethyl pyruvate decompose to the same products, but the reaction gives a higher quantum yield than direct irradiation does. The benzophenone triplet that forms probably transfers its energy to the ester, thus excites it to the triplet state. Therefore, the triplet must be involved in the sensitized reaction, Dr. Hammond says. "It is very likely that direct photolysis follows the same route," he concludes. Photosensitization also helped Dr. Hammond's group to study the reaction of methylene in solution. Under direct irradiation, diazomethane forms singlet methylene that is highly reactive, hard to work with. Benzophenone, however, gives this sequence of reactions: (C (; H-) 2 C O - ^ (C (; H 5 ) 2 CO * * - * (C,,H r> ) 2 CO* 3 ( C r , H r J X O * 3 + CH 2 N 2 -» (C l{ H r ,) 2 CO + CHoN.,* 3 CH 2 N 2 * 3 -> C H 2 n + N 2 This approach enabled the research workers to study the decomposition of diazomethane in cyclohexene.
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The reaction of singlet methylene gives a number of C 7 products, including 1-, 3-, and 4-methylcyclohexene and norcarane. Triplet methylene gives mainly norcarane and some 3and 4-methylcyclohexene (not separated by analytical methods). Addition of triplet methylene to cis- and trans- 2-butene isn't stereospecific.
The triplet probably adds to double bonds to give a very short-lived biradical—one in which electron spins are unpaired. "And internal rotation before spin inversion takes place would lead to over-all, nonstereospecific addition," Dr. Hammond adds. Electron Distribution. By contrast with the Caltech group, Dr. Zimmer-
CONTRAST. The chemistry of singlet methylene differs from that of triplet methylene. The gas chromatogram at left shows the products of a reaction between cyclohexene and diazomethane with direct photolysis, which produces singlet methylene. The chromatogram on the right shows products from the same but photosensitized reaction. Using a photosensitizer (benzophenone, for instance) gives triplet methylene
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man and his co-workers are using an electron distribution approach to predict what the products of a photochemical reaction will be. In this instance, the two most important kinds of photochemical excitation are n to Pi° and Pi to Pi*. The first t y p e - n to Pi*—occurs when a nonbonding or unshared electron (n) is promoted to an antibonding Pi orbital (Pi*)—as, for example, when ketones absorb light in the 270- to 360-millimicron region. Pi to Pi* excitation takes place when an electron moves from a bonding Pi to an antibonding Pi molecular orbital. This happens when certain molecules are irradiated at wave lengths specific to each compound. Using the Pi to Pi* system, Dr. Zimmerman's group worked out electron densities of both ground state and first excited states for monosubstituted benzenes having either electron withdrawing or electron donating groups. Groups like nitro and cyano attract electrons away from the benzene ring, while methoxy and amino groups tend to donate electrons to the ring. Calculations show that in the ground state, electron withdrawing groups give low electron density in the ortho and para positions. Conversely, donating groups feed electrons to these positions. These conclusions are not new, Dr. Zimmerman points out; they tie in with conventional ground state theory. The excited state is another story, though. Here, calculations indicate that nitro or cyano groups should selectively withdraw electrons from the ortho and meta positions; methoxy and amino groups give electrons to these positions. And other groups give similar results. The Wisconsin group arrives at its conclusions for withdrawing groups this way: Six electrons are involved; promotion of an electron removes it from a molecular orbital that allows it to appear at the meta position, puts it into a molecular orbital where it can't. Net result is low meta density. Donating groups involve eight electrons. Here, excitation removes electrons from an orbital that doesn't permit au electron to appear meta, places it into one where it can. Thus, meta electron density is increased. To test the validity of these predictions, Dr. Zimmerman and Dr. S. Somasekhara irradiated the trityl ethers of m-nitrophenol, p-nitrophenol, 48
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m-cyanophenol, and p-cyanophenol in aqueous dioxane, then observed photochemical solvolysis. "Experimental results bear out our excited state picture," Dr. Zimmerman says. As the theory predicts, the meta isomers would be photochemically more reactive than the para isomers. For instance, 777-nitrophenyl trityl ether—almost completely unreactive in the dark—hydrolyzes rapidly when it's irradiated. But the para isomer, which reacts in the dark, hydrolyzes only a little faster in light. How are the electrons distributed in the excited state? In the first excited state, Dr. Zimmerman explains, ??i-nitrophenyl trityl ether may be approximately pictured as:
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This species readily breaks apart. By contrast, /;-nitrophenyl trityl ether in the excited state looks like this:
•a— Co,
The ether oxygen in this structure isn't as electron deficient. Thus, the para isomer doesn't hydrolyze as easily as the meta isomer. The Wisconsin group also studied a number of photochemical reactions of the n to Pi* type. For example, Dr. Brian Cowley studied the irradiation of //Y/H.v-dypnone oxide in aqueous ethanol, and Dr. David Schuster studied the photochemical dienone rearrangement of 4,4-diphenylcyclohexadienone. The Wisconsin research workers found that these (among other) reactions go through four different processes: • Electronic excitation of the n to Pi* kind. • Some change in bonding (forming or breaking). • Pi to n electron demotion. • Conventional chemical transformation. Again, the experimental results tie in with the predicted ones, Dr. Zimmerman says.
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