Effects of N-substitution on CO₂ trapping by cyclic vinylidenes at DFT levels

Abstract

Carbenes are anticipated to interact with different greenhouse gases including CH₄, N₂O, CO₂, etc. Here, CO₂ trapping of singlet (s) and triplet (t) 4-vinylidencyclopentene minima (1_s and 1_t) are compared and contrasted with their various N-substituted analogs, with different types of topology (2_s-9_s and 2_t-9_t, respectively), at M06-2X/6–311 +  + G** level of theory. In the first step, spontaneous non-covalent adsorption of CO₂, without any transition state (TS), over singlet 1_s-9_s, and triplet 1_t-9_t, gives exothermic CO₂-adducts 1_s(a)-9_s(a) and 1_t(a)-9_t(a), respectively. In the second step, the adsorbed CO₂ interacts covalently with the carbene moiety of singlet 1_s(a)-9_s(a), and forms reactant-like, three-membered cyclic TSs that convert to their corresponding singlet 1_s(b)-9_s(b) exothermically. Alternatively, CO₂ intramolecular reactions within triplet 1_t-9_t results in formation of the reactant-like TSs, which lead to the exothermic formation of triplet 1_t(b)-9_t(b). Singlet cyclic vinylidenes (1_s-9_s, with trapping energy range, E_trp =  − 19.85 to − 40.34 kcal mol⁻¹) are better CO₂ trappers than their triplet spin isomers (1_t-9_t, with E_trp =  + 1.34 to − 10.13 kcal mol⁻¹). The best singlet and triplet trappers appeared to be Arduengo type 4_s and 4_t with E_trp values of − 40.34 and − 10.13 kcal mol⁻¹, respectively. The ease of CO₂ trapping by the scrutinized singlet and triplet vinylidenes is: 4_s > 7_s > 9_s > 2_s > 5_s > 6_s > 8_s > 1_s > 3_s > 4_t > 3_t > 7_t > 6_t > 5_t > 9_t > 2_t > 8_t > 1_t. The AIM results appear consistent with above trend and our proposed mechanism.

Graphical abstract