Mechanistic Insights into the Reactivity and Activation Barrier Origins for CO Capture by Heavy Group-14 Imine Analogues.

Using M06-2X-D3/def2-TZVP, the [2 + 2] cycloaddition reactions of carbon dioxide with the heavy imine analogues (G14 = Group 14 element) were investigated. The theoretical evidence reveals that the nature of the doubly bonded G14=N moiety in heavy imine analogues, (LLG14=N-L), is characterized by the electron-sharing interaction between triplet LLG14 and triplet N-L fragments. Based on our theoretical studies, except for the carbon-based imine, all four heavy imine analogues with Si=N, Ge=N, Sn=N, and Pb=N groups can easily engage in [2 + 2] cycloaddition reactions with CO. Energy decomposition analysis-natural orbitals for chemical valence analyses and the FMO theory strongly suggest that in the CO capture reaction by heavy imine analogues , the primary bonding interaction is the occupied p-π orbital () → vacant p-π* orbital (CO) interaction, instead of the empty p-π* orbital () ← filled p-π orbital (CO) interaction. The activation barrier of the CO capture reactions by molecules is primarily determined by the deformation energy of CO. Shaik's valence bond state correlation diagram model, used to rationalize the computed results, indicates that the singlet-triplet energy splitting of is a key factor in determining the reaction barrier for the current CO capture reactions.