Intersystem crossing plays an important role in photochemistry. It is understood to be efficient when heavy atoms are present due to strong spin–orbit coupling, or when strongly bound long-lived complexes are formed that increase the chance of finding the singlet–triplet intersection seam. Here we present evidence for a different intersystem crossing mechanism in the bimolecular reaction of O(3P) with alkylamines. In crossed-beam experiments, product velocity–flux maps are measured for aminoalkyl radicals produced from H abstraction from the methyl group, which also gives OH radicals as co-fragments. The low translational-energy release and isotropic angular distributions of the products indicate that such reactions undergo the formation of a complex before OH and aminoalkyl are produced. However, there is no well on the triplet potential energy surface that could support such a complex. Multi-reference ab initio calculations suggest, instead, that intersystem crossing occurs in the exit-channel region due to the long-range dipole–dipole interaction between the nascent radical product pair coupled with the vanishing singlet–triplet splitting at long range. Intersystem crossing then leads to a deep hydroxylamine well before OH elimination.
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The experimental research (A.G.S.) was supported by the Director, Office of Science, Office of Basic Energy Science, Division of Chemical Science, Geoscience and Bioscience of the US Department of Energy under contract no. DE-SC0017130 with additional support from the ARO under grant no. W911NF-17-1-0099. S.M. was supported by the NSF (CHE-1800171). The authors thank J. M. Bowman for comments on the manuscript and T. Sewell for helpful discussions.
Supplementary Results, Supplementary Figures 1 and 2, key stationary point geometries
This video contains animations of one trajectory starting from the transition state of the direct H-abstraction process to OH + CH3NHCH2 products on the triplet surface, calculated using a Born–Oppenheimer molecular dynamics model at the B3LYP/6–31G(d) level of theory. Colour coding for atoms: carbon (grey), nitrogen (blue), oxygen (red) and hydrogen (white). A few snapshotsfor this trajectory are shown in Fig. 5.
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Nature Chemistry (2019)