Impact of ligands on CO2 adsorption in metal-organic frameworks: First principles study of the interaction of CO2 with functionalized benzenes. I. Inductive effects on the aromatic ring

Intermolecular interactions between the CO2 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 CO2 adsorption at low pressure. Two classes of substituted benzene molecules were considered: (i) with halogen substituents (tetrafluoro-, chloro-, bromo-, and dibromobenzene) and (ii) with methyl substituents (mono-, di-, and tetramethylbenzene). In the benzene-CO2 complex, the main interaction is between the delocalized pi aromatic system and the molecular quadrupole of CO2. Halogen substituents have an electron-withdrawing effect on the ring which destabilizes the pi-quadrupole interaction. Weak halogen-bond and hydrogen bondlike interactions partially compensate for this, but not to the extent that any significant enhancement of the intermolecular binding energy is observed. Methyl groups, on the other hand, have a positive inductive effect which strengthens the CO2-aromatic interaction by up to 3 kJ mol(-1) in the case of tetramethylbenzene. Weak hydrogen bondlike interactions with methyl H also contribute to the stability of the complexes.