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Intersystem crossing in the exit channel

Abstract

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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Acknowledgements

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.

Author information

H.L. and A.K. performed the experiments and analysed the data. H.L. and S.M. performed the theoretical calculations and analysed the results. A.G.S. conceived the experiments and guided the interpretation. H.L., S.M. and A.G.S. co-wrote the paper. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Arthur G. Suits.

Supplementary information

  1. Supplementary Information

    Supplementary Results, Supplementary Figures 1 and 2, key stationary point geometries

  2. Supplementary Movie

    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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Fig. 1: Velocity–flux contour map analysis of aminoalkyl products from reactions of O(3P) with DMA and TMA.
Fig. 2: Key points on the triplet and singlet PESs of the O(3P) + DMA reaction.
Fig. 3: Spin–orbit coupling and high level energy calculations.
Fig. 4: Orbitals participating in the wavefunctions of S0, S1, T1 and T2.
Fig. 5: Snapshots of a representative trajectory.