Impact of ligands on CO(2) adsorption in metal-organic frameworks: First principles study of the interaction of CO(2) with functionalized benzenes. II. Effect of polar and acidic substituents.

Intermolecular interactions between the CO(2) molecule and a range of functionalized aromatic molecules have been investigated using density functional theory. The work is directed toward the design of linker molecules which could form part of new metal-organic framework materials with enhanced affinity for CO(2) adsorption at low pressure. Here, the focus was on the effect of introducing polar side groups, and therefore functionalized benzenes containing -NO(2), -NH(2), -OH, -SO(3)H, and -COOH substituents were considered. The strongest types of intermolecular interactions were found to be: (i) between lone pair donating atoms (N,O) of the side groups and the C of CO(2) (enhancement in binding energy of up to 8 kJ mol(-1) compared to benzene); and (ii) hydrogen bond interactions between acidic protons (of COOH and SO(3)H groups) and CO(2) oxygen (enhancement of 3-4 kJ mol(-1)). Both of these types of interaction have the effect of polarizing the CO(2) molecule. Weaker types of binding include hydrogen-bond-like interactions with aromatic H and pi-quadrupole interactions. The strongest binding is found when more than one interaction occurs simultaneously, as in C(6)H(5)SO(3)H and C(6)H(5)COOH, where simultaneous lone pair donation and H-bonding result in binding energy enhancements of 10 and 11 kJ mol(-1), respectively.