Theoretical study on the isomerization mechanism of nitrene CF3CXN (X = O, NH, CH2): Competing intersystem crossing reactions driven by heavy-atom quantum tunneling

Abstract

We simulated the isomerization reactions of nitrene CF₃CXN (X = O, NH, CH₂), focusing on two competing pathways: the Curtius(-like) reaction and the cyclization reaction, both involving intersystem crossing from triplet to singlet potential energy surface. Using the Wentzel–Kramers–Brillouin (WKB) and weak coupling approximation (WC) methods, in conjunction with B3LYP-D3(BJ)/6-311++G(d,p), we determined the rate constants for these reactions. The results show that Curtius(-like) reaction of CF₃CXN (X = O, NH, CH₂) has a temperature-independent plateau of the rate constant at temperatures below 100 K, where the rate constant governed by atom quantum tunneling from the vibrational ground state of the reactant, with half-lives (ln2/k_WKB) of 113 hours (X = O), 6.9 × 10¹⁸ years (X = NH), and 2.2 × 10⁵⁷ years (X = CH₂) at 5 K, which indicate Curtius(-like) reactions of CF₃CCH₂N and CF₃CNHN are forbidden at temperatures below 100 K. The cyclization reaction of CF₃CCH₂N also has a temperature-independent plateau of the rate constant at temperatures below 100 K, with half-life lower-limit (ln2/k_DVS/WC) of 329 hours, imply that the reaction has the possibility to proceed via atom quantum tunneling at cryogenic temperatures. And the cyclization reaction is dominant pathway in the isomerization of CF₃CCH₂N at temperature below 300 K. Since cyclization reactions of CF₃CON and CF₃CNHN are endothermic, the two reactions have no temperature-independent plateau of the rate constant. At temperature below 150 K, Curtius reaction is dominant for the isomerization of CF₃CON, while at the temperatures above 150 K, cyclization becomes more competitive, and the isomerization mechanism change to cyclization ↔ reverse-cyclization (fast) → Curtius(-like) (rate-limiting step). At temperature below 200 K, CF₃CNHN can only exist in the formation of triplet nitrene. While at temperatures above 200 K, the isomerization mechanism of CF₃CNHN is the equilibrium of cyclization and reverse cyclization.