CO2 trapping of selected N-heterocyclic vinylidenes with an NBO mechanistic scrutiny by DFT

Trapping of CO2 is probed by a set of three N-heterocyclic vinylidene minima, from the azole family, including imidazole (1), 1,2,4-triazole (2), and tetrazole (3), at M06-2X/6-311++G** level of theory. Exothermic adsorption of CO2 over 1, 2, and 3 gives the initial CO2-complex, 1(a), 2(a), and 3(a), with no transition state (TS). Consequently, carbenic pi-additions to CO2 (within 1(a), 2(a), and 3(a)) may give reactant-like, three-membered cyclic TSs, which lead to the exothermic formation of 1(b) > 2(b) > 3(b). Alternatively, CO2 intramolecular reactions within 1(a), 2(a), and 3(a) may result in the formation of the product-like, five-membered cyclic TSs, which lead to the endothermic formation of unstable side products 1(c), 2(c,) and 3(c), which revert to 1(a), 2(a), and 3(a), respectively. Hence, this manuscript focuses on the main CO2 trapped products (1(b), 2(b), and 3(b)), and takes advantage of the immense potential of the NBO and AIM data in elucidating their formation provisional mechanisms. Unprecedentedly, contributions of different donor-acceptor interacting orbitals to the formation of each product are assessed on the basis of their second-order perturbation stabilization energy (E-2) values.