A theoretical study of the isomerization mechanisms of nitrenes CF3CXN (X = O, NH, CH2): competing intersystem crossing reactions driven by heavy-atom quantum tunneling

Abstract

We simulated the isomerization reactions of the nitrenes CF₃CXN (X = O, NH, CH₂), focusing on two competing pathways: the Curtius(-like) reaction and the cyclization reaction, both involving intersystem crossing from a triplet to a 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 the Curtius(-like) reactions of CF₃CXN (X = O, NH, CH₂) have a temperature-independent plateau for the rate constant at temperatures below 100 K, where the rate constant is governed by atom quantum tunneling from the vibrational ground state of the reactant, with half-lives (ln 2/k_WKB) of 113 hours (X = O), 6.9 × 10¹⁸ years (X = NH), and 2.2 × 10⁵⁷ years (X = CH₂) at 5 K, which indicates that 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 for the rate constant at temperatures below 100 K, with a half-life lower limit (ln 2/k_DVS/WC) of 329 hours, implying that the reaction has the possibility to proceed via atom quantum tunneling at cryogenic temperatures. Also, the cyclization reaction is the dominant pathway in the isomerization of CF₃CCH₂N at temperatures below 300 K. Since cyclization reactions of CF₃CON and CF₃CNHN are endothermic, the two reactions have no temperature-independent plateau for the rate constant. At temperatures below 150 K, the Curtius reaction is dominant for the isomerization of CF₃CON, while at the temperatures above 150 K, cyclization becomes more competitive, and the isomerization mechanism changes to cyclization ↔ reverse cyclization (fast) → Curtius(-like) (rate-limiting step). At temperatures below 200 K, CF₃CNHN can only exist via the formation of triplet nitrene, while at temperatures above 200 K, the isomerization mechanism of CF₃CNHN is an equilibrium of cyclization and reverse cyclization.