Chemistry at the threshold: Unexpected products, unusual mechanisms, and generally weird things that happen near the energetic threshold for a reaction

Reactions that occur near the energetic threshold can be very different to those where energy is in excess. In a series of experiments, we have used a narrow linewidth laser to provide a small organic molecule, acetaldehyde (CH
₃
CHO), with a precise amount of energy, and used laser and imaging based techniques to probe the reaction products. In this paper I shall present three different examples of "weird chemistry" that happens at threshold: i) two fragments of acetaldehyde, CH
₃
and HCO almost separate but are trapped in their mutual van der Waals well. Here they orbit each other and produce different chemical products; ii) selectively deuterated acetaldehyde is observed to undergo facile and unexpected H/D exchange before the reaction is complete; and iii) as soon as there is enough energy to break more than one bond, this channel becomes dominant, but the mechanism for this dominance is as yet unexplained. Near-threshold chemistry is not an esoteric, narrow window, but one that is prevalent in chemistry more generally. Examples in atmospheric and combustion chemistry will be presented.