An unexpected molecular spin flip

The behavior of small molecules, alone and in reactions with one another, is for the most part well understood. Many quantitative details remain to be worked out, to be sure, and there’s plenty of room for innovative new computational and experimental techniques to illuminate those details (see Physics Today, October 2013, page 15 , and November 2013, page 15 ). Still, given how much we know, it’s highly unusual for two colliding molecules to behave in a qualitatively unexpected way. But in an experiment by the University of Missouri’s Arthur Suits and colleagues, that’s just what happened.

Suits and company were studying the gas-phase reaction of atomic oxygen with dimethylamine to produce a pair of charge-neutral radicals, OH and CH₃NHCH₂. (The product radicals are depicted in the figure.) In at least 90% of the reactions, they observed an intersystem crossing, or radiationless spin flip, from an overall spin-triplet state to a spin-singlet state.

By itself, that’s not so surprising: Intersystem crossings, facilitated by strong spin–orbit coupling, are common in chemical dynamics. What makes the new result unusual is that the intersystem crossing occurs after the molecules have already reacted and are starting to move away from each other. That had never before been observed or even considered: Intersystem crossings take time—typically nanoseconds to milliseconds, although faster crossings are possible under certain conditions—whereas the products of a completed reaction generally fly away from each other in just femtoseconds.

The explanation of that so-called exit-channel intersystem crossing lay in another surprising molecular phenomenon, the roaming pathway, first described by Suits and collaborators in 2004. Sometimes, those researchers found, chemical reaction products don’t immediately separate but instead orbit each other several times before parting. (For more on the discovery and physics of roaming pathways, see the article by Joel Bowman and Arthur Suits, Physics Today, November 2011, page 33 .) A detailed simulation of the oxygen–dimethylamine reaction showed that the product radicals roam around each other for hundreds of femtoseconds before they ultimately separate, and the electron configuration is just right for a subpicosecond spin flip.

Roaming pathways, first recognized in the photoinduced decomposition of formaldehyde molecules, have since been observed in many other systems. The exit-channel intersystem crossing may prove to be similarly general, now that researchers know to look for it. (H. Li et al., Nat. Chem., 2018, doi:10.1038/s41557-018-0186-5 .)