Curtius rearrangement is the elimination of N2 from carbonyl azides RC(O)N3 to form isocyanates RNCO. Two mechanisms, viz., stepwise and concerted have been proposed in the literature for this reaction. The stepwise mechanism involves the formation of a nitrene RC(O)N by elimination of N2 followed by an intramolecular rearrangement of the nitrene to form the isocyanate. The concerted mechanism is a single-step pathway forming the N2 + RNCO products directly. Previous experimental and theoretical studies have indicated that the mechanism is usually concerted for thermal reactions and both stepwise and concerted are preferred under photochemical conditions. In the present work, we investigated the mechanism of Curtius rearrangement of two carbonyl azides with different substituents (R = CH3 and F). Atomic level reaction mechanisms were studied using chemical dynamics simulations under thermal reaction conditions. Classical trajectories were generated on-the-fly at the density functional B3LYP/6-31+G∗ level of electronic structure theory with similar initial conditions for both the molecules. Simulation results showed a dominant concerted mechanism for CH3C(O)N3 and the operation of both the mechanisms for FC(O)N3. The fluorocarbonyl nitrene FC(O)N had an appreciable lifetime before undergoing intramolecular rearrangement to form the isocyanate. In a small number of trajectories, the product isocyanate produced via the concerted dissociation of FC(O)N3, isomerized back to the nitrene form. Copyright © 2020 American Chemical Society.