Capture of CO 2 by diiodo-N,N-imidazoliumvinylidene, with singlet and triplet minima (1 s and 1 t , respectively), is probed at M06-2X/6-311++G ** level of theory. Ground state appears to be triplet 1 t , which exhibits a rather large singlettriplet energy gap (ΔE s-t = +10.30 kcal mol À1 ). Adsorption of CO 2 over 1 t releases À3.70 kcal mol À1 energy and gives the initial CO 2 adduct, with no transition state (TS). Sequentially, the latter goes through a reactant-like TS, which leads to an internal exothermic attack of the carbene carbon to the adsorbed CO 2 and forms 3 t with conversion energy (E con ) of À6.77 kcal mol À1 . Additional calculations involve endothermic capture of CO 2 by the rather unstable 1 s . The immense potential of the natural bond orbital (NBO) and atom in molecule (AIM) data in elucidating a reaction mechanism is unveiled by employing the electron-pushing technique. Results appear consistent with the proposed mechanism.

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Trapping of CO 2 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 CO 2 over 1, 2, and 3 gives the initial CO 2 -complex, 1 a , 2 a , and 3 a , with no transition state (TS). Consequently, carbenic π-additions to CO 2 (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, CO 2 intramolecular reactions within 1 a , 2 a , and 3 a may result in the formation of the product-like, fivemembered 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 CO 2 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.

Carbenes are anticipated to capture greenhouse gases. We have compared and contrasted trapping of CO 2 by O, S, and Se substituted singlet (s) and triplet (t) N-heterocyclic vinylidene minima (1 s -3 s and 1 t -3 t , respectively), at M06-2X/6-311++G** level of theory. Unlike many carbenes, our triplet minima (1 t -3 t ) appeared as ground states. Hence, we compared their ability to capture CO 2 . The ease of CO 2 trapping by the scrutinized triplet vinylidenes is:The NBO and AIM results appear consistent with the above trend and our proposed mechanism.