Chemists usually study reactions at temperatures of tens or hundreds of kelvin, where reaction rates are averaged over many different energies and initial conditions for collision. But new techniques are now making it possible to produce molecules and trap them at temperatures within one-millionth of a degree of absolute zero. Here, all the thermal averaging is removed; the molecules occupy the lowest possible quantum translational states, and all their motions are completely controllable. On page 853 of this issue, Ospelkaus et al. (1) describe chemical reactions between molecules in this new regime and find that tiny changes, such as flipping the orientation of a single nuclear spin, can have profound consequences for how (and whether) chemical reactions occur.
Hutson, J. M. (2010). Ultracold Chemistry. Science, 327(5967), 788-789. https://doi.org/10.1126/science.1186703