DFT Study on the NMR Chemical Shifts of Molecules Confined in Carbon Nanotubes

Large magnetic shielding has been experimentally observed on certain organic molecules, regardless of their intrinsically different chemical natures, when they are confined within the carbon nanotubes. We investigated the underlying physics of the shielding effect by employing a series of density functional theory calculations on various molecule-confined-in-CNT systems. Particularly, the effects of the intermolecular interaction and the ring current of the CNTs on the chemical shifts of the confined molecules were investigated in detail. The results reveal that the changes in chemical shift mainly originate from the magnetic shielding induced by the delocalized π electrons. Electronic structure analysis for nonbonded interactions of host–guest and guest–guest indicated that this intermolecular interaction effect on chemical shift is significant for the polarizable molecules. Thus, the NMR responses of the molecules confined in CNTs are different from those of the molecules in other confining environments. Our study thus suggested that the chemical shift can be used as a probe to distinguish the molecules inside and outside of the CNT channels, as well as the type of CNTs (such as metallic and semiconducting).