Bending or Breaking the Rules?

The Born-Oppenheimer approximation, which states that electronic and nuclear motion can be treated separately in quantum mechanical calculations, is a key simplifying step in quantum mechanics that underlies the way we think about molecules. But does it really hold in all bimolecular reactions? In his Perspective, Manolopoulos compares the theoretical study of Alexander et al. with earlier experimental work on the reaction of chlorine with molecular hydrogen to form hydrochloric acid. According to theory, all is well for the Born-Oppenheimer approximation but the experiments suggest otherwise. It remains to be seen how the discrepancy will be resolved.