Guanidine as a strong CO2 adsorbent: a DFT study on cooperative CO2 adsorption

Among the various carbon capture and storage (CCS) technologies, the direct air capture (DAC) of CO2 by engineered chemical reactions on suitable adsorbents has attained more attention in recent times. Guanidine (G) is one of such promising adsorbent molecules for CO2 capture. Recently Lee et al. (Phys. Chem. Chem. Phys., 2015, 17, 10925-10933) reported an interaction energy (Delta E) of -5.5 kcal mol(-1) for the GMIDLINE HORIZONTAL ELLIPSISCO2 complex at the CCSD(T)/CBS level, which was one of the best non-covalent interactions observed for CO2 among several functional molecules. Here we show that the non-covalent GMIDLINE HORIZONTAL ELLIPSISCO2 complex can transform to a strongly interacting G-CO2 covalent complex under the influence of multiple molecules of G and CO2. The study, conducted at M06-2X/6-311++G** level density functional theory, shows Delta E = -5.7 kcal mol(-1) for GMIDLINE HORIZONTAL ELLIPSISCO2 with an NMIDLINE HORIZONTAL ELLIPSISC distance of 2.688 angstrom while almost a five-fold increase in Delta E (-27.5 kcal mol(-1)) is observed for the (G-CO2)(8) cluster wherein the N-C distance is 1.444 angstrom. All the (G-CO2)(n) clusters (n = 2-10) show a strong N-CO2 covalent interaction with the N-C distance gradually decreasing from 1.479 angstrom for n = 2 to 1.444 angstrom for n = 8 approximately equal to 9, 10. The N-CO2 bonding gives (G(+))-(CO2-) zwitterion character for G-CO2 and the charge-separated units preferred a cyclic arrangement in (G-CO2)(n) clusters due to the support of three strong intermolecular OMIDLINE HORIZONTAL ELLIPSISHN hydrogen bonds from every CO2. The OMIDLINE HORIZONTAL ELLIPSISHN interaction is also enhanced with an increase in the size of the cluster up to n = 8. The high Delta E is attributed to the large cooperativity associated with the N-CO2 and OMIDLINE HORIZONTAL ELLIPSISHN interactions. The quantum theory of atoms in molecules (QTAIM) analysis confirms the nature and strength of such interactions, and finds that the total interaction energy is directly related to the sum of the electron density at the bond critical points of N-CO2 and OMIDLINE HORIZONTAL ELLIPSISHN interactions. Further, molecular electrostatic potential analysis shows that the cyclic cluster is stabilized due to the delocalization of charges accumulated on the (G(+))-(CO2-) zwitterion via multiple OMIDLINE HORIZONTAL ELLIPSISHN interactions. The cyclic (G-CO2)(n) cluster formation is a highly exergonic process, which reveals the high CO2 adsorption capability of guanidine.