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Synthesis

Bond rotation clocked in picoseconds

October 2, 2006 | APPEARED IN VOLUME 84, ISSUE 40

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Credit: Courtesy of Junrong Zheng
8440scicon_fiso-gau.gif
Credit: Courtesy of Junrong Zheng

Physical chemists have hit upon a technique capable of measuring the rate of isomerization around a C-C bond under typical reaction conditions (Science 2006, 313, 1951). "This is one of the most fundamental processes in chemistry," says lead author Michael D. Fayer of Stanford University. "But it happens so fast that no one's ever been able measure its rate under normal chemical conditions." Fayer and his coworkers managed to clock C-C bond rotation in an ethane derivative in CCl4 at room temperature. They used an ultrafast technique called 2-D infrared vibrational echo spectroscopy to observe the interconversion of the gauche and trans conformations of 1-fluoro-2-isocyanato-ethane (shown; light blue = F, red = O, dark blue = N, gray = C). From the spectrum, the researchers determined the compound's isomerization time constant at 40 picoseconds. Calculations based on the measurement reveal that the time constant for butane is very similar, Fayer says, and is in excellent agreement with purely theoretical calculations performed decades ago. Using the same methods, the team estimated that ethane's isomerization time constant is around 10 picoseconds.

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