Reactions of halides with molecular hydrogen have served as test-beds for theories in quantum chemistry for many years. One area in which the family of reactions has proved useful is in the understanding of the Born–Oppenheimer approximation. It states that because electrons are far smaller than nuclei, and thus move much more quickly, the motions of the two can be treated separately, which simplifies nucleus–electron interactions and makes complicated chemical problems far easier to handle. The approximation does not always hold, however, and now Xingan Wang and co-workers1 have investigated the extent to which the Born–Oppenheimer approximation breaks down in the H2 + Cl reaction.

Atomic chlorine can exist in two possible spin-orbit states: a ground state and a higher energy spin-orbit excited state, Cl*. The Born–Oppenheimer approximation forbids the reaction of Cl* with H2, although previous experimental studies have showed that, under certain conditions, the reaction was indeed possible and occurred preferentially over the reaction with chlorine atoms in the ground state. Now, using crossed-molecular-beam experiments, a collaborative group of scientists from America, China, Germany, Italy and Singapore, have studied the reactivity of ground state and spin-orbit-excited chlorine, Cl*, with H2.

They found that contrary to previous experiments, the contribution by Cl* is minor and thus the reaction mostly follows the path predicted by the Born–Oppenheimer approximation — supporting the implication that the coupling between nuclei and electrons is weak.