SCIENCE
Photoelectron Spectroscopy Settles Questions About Methylene Sensitive system for studying anions ends controversy about energy levels and geometry of methylene and allows study of other unstable species Rudy M. Baum, C&EN San Francisco
In the past several years, physical chemists interested in the detailed spectroscopy of molecules have turned their attention away from small stable molecules, the study of which has become relatively mundane, toward increasingly exotic species. The spectroscopy of small clusters of metal atoms, van der Waals complexes, and highly reactive, charged and neutral intermediates has become the target of these chemists' elaborate and expensive experimental systems. Those systems often involve entirely new experimental techniques. Just as often, however, they involve combinations of established techniques coupled with new, highly sensitive detectors. An example of the latter is a system for studying the photoelectron spectroscopy of anions developed by University of Colorado chemistry professor W. Carl Lineberger and coworkers. Using the system, the Boulder, Colo., researchers earlier this year laid to rest a major controversy concerning the energy levels of methylene, CH2, a controversy generated largely by Lineberger's earlier studies of that species. Since then, they have obtained detailed photoelectron spectra of several other carbenes, all of which are important intermediates in organic chemistry and all of which have proved relatively intractable to spectroscopic study. As Lineberger points out in one 30
November 26, 1984 C&EN
Lineberger: hot bands the key
of his published papers, "The history of the determination of the geometry and relative energies of the lowest singlet and triplet states of methylene has been a long and colorful o n e / ' Being a small molecule, methylene was amenable to detailed theoretical studies. On the basis of those studies, theorists were the first to suggest that the ground state of methylene is a bent triplet, in which the spins of the two unshared electrons are in the same direction. This contrasts with the more common situation in which the ground state is the singlet and the unshared electrons are paired with spins of opposite direction. Subsequent experimental work confirmed that prediction. The methylene controversy concerned, first, the energy difference between the ground triplet state and the first excited singlet state. Second, it concerned the geometry of the triplet state. Was it bent or was it linear? Arguments could be made either way, Lineberger says. In 1970,
the question of geometry was settled in favor of the bent structure with electron spin resonance studies done by Robert A. Bernheim of Pennsylvania State University, and Edel Wasserman, then of Bell Laboratories. Many i n d e p e n d e n t , ab-initio, quantum mechanical calculations of the singlet-triplet splitting indicated an energy gap between the two states of 9 to 11 kcal per mole. Indirect determinations of the splitting based on photochemical studies, scattering results, and measurements of the heats of formation of singlet and triplet methylene came up with the same range of energies. However, "those were not definitive, direct s t u d i e s / ' Lineberger points out. Photoelectron spectroscopy of the methylene anion, CH2~, provided such a direct measurement. The only problem was that Lineb e r g e r kept coming u p w i t h a singlet-triplet splitting of 19.5 kcal per mole, instead of the predicted 9 to 11 kcal per mole. Photoelectron spectroscopy, particularly of neutral species, is an old technique. Essentially, one uses radiation to knock an electron off the molecule under study, collects the electrons, and measures their energy. In his system, Lineberger studies anions rather than neutral molecules. Working with anions provides a number of advantages. Because they are charged, the anions can be put through a mass spectrometer to select a specific anion for study. In highly reactive neutral species, such as carbenes, the extra electron stabilizes the molecule. In Lineberger's initial studies of methylene, CH 2 ~ was generated in an electric discharge from diazomethane, ketene, or methane. Laser radiation photodetaches the extra elec-
tron to produce methylene in either the singlet or triplet state. The detached electrons have different energies depending on which state their leaving created. Thus, the photoelectron spectrum consists of a number of peaks representing different electron energies. Once the peaks corresponding to the vibrationless level of the ground-state triplet and the first excited singlet are identified, d e t e r m i n i n g the singlet-triplet splitting is a matter of subtraction. Therein, however, lies the rub. Identifying the peak corresponding to singlet methylene is straightforward: It is the largest peak in the spectrum because singlet methylene and CH 2 ~ have almost identical geometries. There are, however, a number of other weaker peaks in the spectrum. One corresponds to triplet methylene in its ground electronic and vibrational state. Others correspond to triplet methylene in different vibrationally excited states. And still others, referred to as hot bands, arise from photodetachment of electrons from electronically or vibrationally excited CH2~. Identifying and discounting the hot bands is the key to the experiment. If a hot band is misidentified as the peak corresponding to the ground-state triplet methylene, the
subtraction will result in a singlettriplet splitting that is overstated. However, Lineberger says, hot bands are common in photoelectron spectroscopy and there are proven methods for identifying them. Those methods involve changing the source of the anions because, presumably, different sources produce a different distribution of states of the anion which results in changes in the relative intensity of any hot bands. In the methylene spectrum obtained by Lineberger, all of the peaks remained invariant relative to each other regardless of the source of the CH 2 ~ anion, leading the researchers to the conclusion that the singlet-triplet splitting was 19.5 kcal per mole. There things stood for a number of years. Lineberger and coworkers continued efforts to refine their technique. A new generation of methylene spectroscopic studies by researchers such as Richard N. Zare of Stanford University, Yuan T. Lee of the University of California, Berkeley, and Kenneth M. Evenson of the National Bureau of Standards continued to come up with consistent evidence for the smaller value of singlet-triplet splitting. Despite the fact that photoelectron spectroscopy is direct, one chemist familiar with the history of
the controversy says that "if it were not for Carl's reputation as a meticulous e x p e r i m e n t e r , t h e d i s p u t e would have ended long ago. The mass of data on the other side was too strong." Lineberger had recognized that using a flowing afterglow ion source could provide crucial data on the presence or absence of hot bands in his spectra. In such a source, reactant molecules are mixed with an inert carrier gas flowing at near sonic speed. An electron beam creates ions that relax vibrationally and rotationally to the temperature of the carrier gas. Further downstream, a second reactant is added to the flow. Those molecules react with the ions to produce the species to be studied. That species also relaxes to the temperature of the carrier gas. The advantage of the afterglow ion source over creation of anions in an electric discharge is that the chemistry is well controlled and the selected anions are known to be cold. The disadvantage is that only a very small number of anions can be collected into a beam for study. Lineberger's early efforts to use such a source did not produce a signal. In work supported by the National Science F o u n d a t i o n a n d the American Chemical Society Petroleum Research Fund, Lineberger, post-
Flowing afterglow ion source eliminates hot bands in photoelectron spectra of CH2 Counts, x 1 0 4 10
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Six peaks appear in spectrum of methylene anion generated in a gas discharge ion source leading to conclusion that peak A corresponds to the ground-state triplet of methylene
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Peaks A, B, and C disappear in spectrum of methylene anion generated in a flowing afterglow ion source, indicating that they are hot bands and that peak D corresponds to triplet methylene November 26, 1984 C&EN
31
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November 26, 1984 C&EN
Science doctoral fellows Doreen G. Leopold and Amy E. Stevens, and graduate students Kermit K. Murray and Charles S. Feigerle recently built a new spectrometer sensitive enough to be used with a flowing afterglow source and restudied methylene with it. The spectrometer they built has about 1000 times greater sensitivity and about 10 times greater resolution than their previous system. Nothing particularly magical contributed to those increases. A factor of about 100 increase in sensitivity was achieved by using a multichannel array detector that allows them to look at many electron energies simultaneously. Technical adjustments of the system's operating parameters account for the remainder of the sensitivity increase. For the electron energies involved, the system is probably the most sensitive of its kind, Lineberger says. " I n terms of studying unstable systems, it provides a unique capability/' When the new system was applied to methylene, three of the previously invariant lines in the photoelectron spectrum simply disappeared [/. Chem. Phys., 81, 1048 (1984)]. One of the peaks that had been considered part of the vibrational progression clearly was the peak that corresponded to triplet methylene. The subtraction process yielded a singlet-triplet splitting of about 9 kcal per mole. In conjunction with research from other laboratories, it was clear that the triplet ground state is a bent molecule, but that low levels of vibrational excitation result in a large-amplitude motion that has a linear molecule as its equilibrium position. Thus, one experiment essentially ended a controversy that had been ongoing for a decade. Lineberger is continuing to pursue his studies of methylene. His work coupled with research in other laboratories presages that "methylene is going to be one of the best understood of all small molecules," he says. "Worrying about the energetics and structure of methylene has been a unique experience for my group," Lineberger adds. "For a long time, physical chemistry was an area that was somewhat removed from the
rest of chemistry. In a sense, we never worried about real molecules. Methylene represents a merger of a variety of things. It has few enough electrons and atoms that theorists can do accurate calculations on it. It is small enough that it is amenable to some of the modern experimental tools of physical chemists. Yet it also is involved in real organic chemistry." The technique developed by the researchers is useful for studying many other unstable species of chemical interest. Already, photoelectron spectra have been obtained for the simple halocarbenes—CHF, CHC1, and CHI—as well as more complicated diradicals such as vinylidene and benzyne. "It provides an avenue to see structures that you really have no direct way of seeing with other techniques," Lineberger says. "Methylene has been fun, but in the long run the importance of the technique is the breadth of species one can study with it." •
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