Cyclobutane ring opening can follow two paths
Transition-state mechanism
Diradical mechanism
Science [266, 1359 (1994)], attempts to answer a number of outstanding questions concerning the role of diradicals in organic reactions. Those questions persist despite a long history of indirect experiments and theoretical calculations aimed at understanding the mechanisms of such reactions. "One of the most well-studied reactions, both theoretically and experimentally, is the ring opening of cyclobutane to yield ethylene or the reverse addition of two ethylene molecules to form cyclobutane," says chemistry professor Ahmed H. Zewail of California Institute of Technology, who carried out the research with graduate students Soren Pedersen and Jennifer L. Herek. This reaction can follow two distinct pathways. One is a concerted path through a transition state in which two bonds break simultaneously. The other route is a two-step mechanism beginning with breakage of one bond to produce tetramethylene as a diradical intermediate. The distinction between the two mechanisms may seem esoteric, but it has crucial implications for understanding reaction dynamics. Diradical intermediates exist in a relatively shallow potential well between the reactants and products and are longer lived than transition states. As such, the dynamics of the diradicals' nuclear motions— vibration and rotation—determine the outcome of the reaction. Previous experiments and theory have not been able to distinguish definitively between the two mechanisms. The Caltech chemists use femtosecond chemistry to probe such reactions by using precursors (cyclopentanone, cyclobutanone, and their derivatives) to generate the intermediate species. They use femtosecond pulses, separated by known and variable delays also measured in femto-
seconds, to initiate the reaction and monitor the dynamics of a given mass species by ionization. The experiments provide a wealth of information, but the most fundamental data they supply are the lifetimes of different diradical species. For example, the tetramethylene diradical that results from photoinduced elimination of CO from cyclobutanone has a lifetime of 720 femtoseconds. By contrast, the trimethylene diradical produced by elimination of CO from cyclobutanone has much faster dynamics, with a lifetime of only 120 femtoseconds. The methyl-substituted tetramethylene diradical, in which the methyl groups are expected to stabilize the diradical, has a much longer lifetime of 1.4 picoseconds. John E. Baldwin, an organic chemistry professor at Syracuse University, calls the Caltech experiment a "technical slam dunk" and the information it provides "fabulous." Ideas about diradicals "until now have been based on indirect evidence that lots of good people have woven together into something like a coherent picture," he notes. "With this result, we have very strong evidence to say that we're not just dealing with a convenient fiction. These entities have finite lifetimes." Zewail tells C&EN that "These studies of reaction intermediates involving diradicals provide a real-time picture of nuclear motions and structural changes during the reaction." He adds that "The approach should be general for the examination of other reactive intermediates common in organic chemistry." Rudy Baum
Lab errors cited as top DNA profiling concern The potential for laboratory errors in DNA profiling is so great that it dwarfs other issues plaguing the controversial forensic technique, some experts say. But other experts dismiss the question of laboratory error rates as relatively unimportant. Some two dozen experts aired such conflicting views at a public meeting at
the National Academy of Sciences, called to discuss statistical evaluation and interpretation of forensic DNA evidence. The meeting was held by the National Research Council's (NRC) Committee on DNA Forensic Science, which is preparing a report updating a 1992 NRC report on forensic DNA profiling. The 1992 report endorsed use of DNA profiling in criminal court cases, but defense attorneys have seized upon ambiguities in the report to argue that it did not endorse such use. The new panel's report, due out next summer, is meant to end such misinterpretation. The new report also is supposed to resolve a continuing controversy over how to calculate the odds that a defendant's DNA might match incriminating evidence by chance (C&EN, Oct. 10, page 8). Many of those testifying called the dispute over such calculations purely academic. They urge the panel to delineate the relative merits and shortcomings of different approaches and leave it to individual courts to decide which approach to embrace. But it is irrelevant whether the odds of a chance match are calculated as one in 10,000 or one in a million if the profiling laboratory errs in one out of every 100 samples, Jonathan J. Koehler, assistant professor of behavioral decision-making at the University of Texas, Austin, told the meeting. Koehler notes by analogy that a shortstop who throws impeccably but routinely boots the ball is only as good as his fielding percentage. "The theoretical promise of DNA profiling will be swamped in practice by our limitations as human scientists," he explains. "When jurors are told the probability of a match, they should also be told the rate" at which the lab makes false matches, he says. David Reiser, a public defender in Washington, D.C., also expresses concern about lab errors. "We need a lot more data to know if labs are really as proficient as they claim to be," he adds. However, Federal Bureau of Investigation agent Harold A. Deadman Jr. says the FBI does not believe it is the panel's task to address the issue of errors. Forensic scientists have been concerned about—and have developed means of addressing—the possibility of mixing up samples and other errors since well before the introduction of DNA profiling technology, he notes. Pamela Zurer NOVEMBER 28,1994 C&EN
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