Theoretical receptor chemistry: Complexation of CO 2 and activation towards hydration by cationic binding sites

Abstract

The geometries, stabilization energies and electronic structures of complexes of carbon dioxide with several cationic binding sites (mainly NH 4 +) have been studied by ab initio computations in order to gain information about the design of receptor molecules for the binding and activation of CO 2. The most stable conformations correspond to complexes in which the ligands are colinear with CO 2. It is possible to construct such complexes when n ⩽ 2. For n = 4, the stability of the complex is greater than that for n = 2 only if ψ > 125°. The activation of CO 2 by complexation has been studied by computing [CO 2, 2 NH 4 +, H 2O] complexes and the paths of approach of a water molecule towards the carbon center of CO 2 in its complexes with two point charges. The complexation of CO 2 increases the positive charge on C and lowers the energy of the antibonding π* orbitals. The most stable complexes give the largest charges on C whereas the other complexes yield lower π*, orbitals. The study of the approach of a water molecule indicates that the strength of its interaction with the CO 2 complex correlates with the lowering of the CO 2 π* orbitals. Thus, the complexes which bind CO 2 most strongly and those which activate it most efficiently have different geometries, requiring the design of different receptor molecules for binding and for activation. The geometry which gives the largest activation is that which best binds the bent CO 2 molecule, i.e. the geometry which binds best the product of nucleophilic addition to CO 2, a carboxylate derivative. An additional study of the approach of H 2O towards CO 2 complexed by a guanidinium group confirms these results and shows that the geometry of this group is particularly well suited for efficient activation.