Hike from one valley to another via a mountain pass, and eventually you'll come to the highest point of your journey, where you'll probably stop briefly to take in the view before descending. That moment serves as a good analogy to one of chemistry's mysteries: the brief transition state that occurs just as molecules in a reaction are about to transform into new chemical species.

Scientists have long regarded transition states as too unstable and fleeting to be observed. But chemists at the Massachusetts Institute of Technology have now measured the energy of a transition state. Just as the height of a mountain pass constrains how long it will take a hiker to reach the summit, energetic properties of a transition state govern how long it takes for chemical reactants to adopt new conformations.

To make the crucial measurements, Joshua Baraban, then a graduate student at M.I.T., and his colleagues excited acetylene with a laser. In this simple reaction, the molecule twists from a linear to a zigzag conformation. Acetylene vibrated predictably as it absorbed light at greater intensities, but in the instant before it went from straight to zigzag, the vibrations stopped, providing a window into the elusive transition state.

“We found that the frequency of the vibrations dips to zero just as the molecule goes over the hump from one conformation to another,” says Baraban, now at the University of Colorado Boulder. By measuring the energy it took until the vibrations paused, the team could characterize the transition state's energetic properties. The results were published recently in the journal Science.

The method also worked to track the transition state of the more complex conversion of hydrogen cyanide to hydrogen isocyanide, found Baraban's colleague Georg Mellau of Justus Liebig University Giessen in Germany. The ability to quantify this brief moment “is important wherever chemistry is important,” Baraban says. For instance, better knowledge of transition states in fuel-combustion reactions could allow scientists to engineer cars with better gas mileage.