Quantum Chemical Investigation of Covalent Adduct Formation in Five‐Membered N‐Heterocyclic Carbenes for CO ₂ Capture—Insights From Mechanistic and Topological Studies

ABSTRACT

N‐heterocyclic carbenes (NHCs) have been identified as promising CO ₂ capture agents due to their strong nucleophilicity, structural and reactive tunability, and the immense possibility of the resulting zwitterionic adducts to convert into a plethora of valuable compounds. Thus, in this work, we systematically explored NHC─CO ₂ interactions in 81 five‐membered NHCs comprising pyrrole, pyrazole, imidazole, triazole, and tetrazole family, with a wide range of structural variations and nucleophilicity. The proton affinities (PA) of the NHCs and the interaction energies (Δ E _int ) of their complexes with CO ₂ correlate well with varying NHC structures, classifying these into three groups. The thermodynamic and IR data of the NHC─CO ₂ adduct formation pathway agree well with the characteristics of each group of NHCs. These observations were further verified by the topology of the C─C bond formation in NHC─CO ₂ complexes and adducts through QTAIM analysis. The results show that the presence or absence of N─H…CO ₂ secondary interactions and the orientation of CO ₂ (bent/perpendicular) play a vital role in characterizing the C─C bond during NHC─CO ₂ interaction. These insights should assist in the design, development and exploration of more such NHC‐based CO ₂ capture systems addressing an urgent need for decarbonization, promising a sustainable future.