Science 337, 1066–1069 (2012)

The chief driver of chemistry in the Earth's atmosphere is the electronic excitation of molecules by light, which kick starts many important processes by photodissociating small molecules. Vibrationally excited molecules also exist in the atmosphere, but current thinking assumes that they quickly relax to equilibrium before being involved in any subsequent chemistry. Now, a team led by David Glowacki from the University of Bristol and Paul Seakins from the University of Leeds have shown that vibrationally excited molecules can play a major role in the outcome of some bimolecular reactions.

They studied the reaction of OH with acetylene, and the subsequent reaction with oxygen of the adduct that they form. Acetylene and OH react to produce a vibrationally excited hydroxyvinyl radical, of which there are two possible, and interconvertible, conformers — with the hydroxyl group taking a position either cis or trans to the nascent radical orbital. The conformers are produced in a 50:50 ratio, but on thermal relaxation this changes to 78:22 in favour of the cis form. Both adducts react with oxygen, but they give different products: the cis isomer produces ethane-1,2-dione and OH, whereas the trans isomer reacts to give formic acid and a formyl radical. Therefore, if the onward reactions were occurring after the vibrational excitation had been quenched, as is universally assumed in such systems, the product yields would reflect the thermally equilibrated 78:22 cis:trans ratio of the adducts.

Glowacki, Seakins and colleagues measured the yield of OH — produced from the reaction of the cis adduct with oxygen — in an O2/N2 atmosphere at various temperatures, pressures and O2:N2 ratios. The yields that they observed showed that oxygen was reacting with the hydroxylvinyl adducts before vibrational relaxation was complete. Using a computational model to assess their results, they calculated that under atmospheric conditions, approximately 25% of the acetylene–OH adducts reacted while still vibrationally excited.