Stretching stops scission

Crossed-beam experiments have shown that, counterintuitively, breaking the C–H bond during the F + CHD₃ reaction is impeded by its vibrational excitation

Credit: © 2009 AAAS

Mode-selective chemistry — the ability to influence reaction rates and products by exciting specific bonds in molecules — has recently caught the attention of many chemists because of its potential practical and technological importance. Previous studies and chemical intuition suggest that vibrationally exciting a bond will increase the chances of it breaking on reaction

Now, a team led by Kopin Liu at the Academia Sinica in Taipei have used crossed-beam scattering experiments to study¹ the F + CHD₃ reaction with and without exciting the C–H bond using an infrared laser. The team have shown that exciting the C–H bond effectively blocks the production of HF + CD₃ rather than the expected increase in reaction rate. This results in the other possible reaction pathway — which leads to DF and CHD₂ — dominating

The excited C–H bond is not just a spectator, even though it does not react. Its excitation is still able to affect the dominant pathway by altering the vibrational energy of the DF product. Although some mechanistic detail is lacking, the results are related to the reaction having an early transition state barrier — the transition state resembles the reactants more than the products.