Two quantum effects can enable reactions to take place at energies below the barrier separating reactants from products: tunnelling and intersystem crossing between coupled potential energy surfaces. Here we show that intersystem crossing in the region between the pre-reactive complex and the reaction barrier can control the rate of bimolecular reactions for weakly coupled potential energy surfaces, even in the absence of heavy atoms. For O(3P) plus pyridine, a reaction relevant to combustion, astrochemistry and biochemistry, crossed-beam experiments indicate that the dominant products are pyrrole and CO, obtained through a spin-forbidden ring-contraction mechanism. The experimental findings are interpreted—by high-level quantum-chemical calculations and statistical non-adiabatic computations of branching fractions—in terms of an efficient intersystem crossing occurring before the high entrance barrier for O-atom addition to the N-atom lone pair. At low to moderate temperatures, the computed reaction rates prove to be dominated by intersystem crossing.
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This work has been supported by the Italian MUR (PRIN 2017, grant 2017A4XRCA (V.B.); PRIN 2017, grant 2017PJ5XXX (P.C.); PRIN 2020, grant 202082CE3T (C.P.)) and the Italian Space Agency (‘Life in Space’ project, N.2019-3-U.0 (N.B.,V.B.)). We acknowledge the Scuola Normale Superiore (Internal Funds), the University of Bologna (RFO funds), the Italian MUR (Ministry of University and Research) and Università degli Studi di Perugia (within the programme ‘Department of Excellence−2018−2022−Project AMIS’) and the SMART@SNS Laboratory (http://smart.sns.it) for high-performance computer facilities.
The authors declare no competing interests.
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Recio, P., Alessandrini, S., Vanuzzo, G. et al. Intersystem crossing in the entrance channel of the reaction of O(3P) with pyridine. Nat. Chem. 14, 1405–1412 (2022). https://doi.org/10.1038/s41557-022-01047-3
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