Combined experimental and computational 1H NMR study of water adsorption onto graphenic materials

Abstract

The effects caused by the interaction with graphene-like layers on the ${}^1$H NMR spectra of water molecules adsorbed onto porous carbon materials were investigated by a combination of shielding calculations using density functional theory (DFT) and ${}^1$H NMR experiments. Experimental ${}^1$H NMR spectra were recorded for different water-containing carbon materials (activated carbons and milled graphite samples); the ${}^1$H NMR signals due to adsorbed water in these materials showed a strong shielding effect caused by the electron currents present in the graphene-like layers. This effect was enhanced for activated carbons prepared at high heat treatment temperatures and for milled graphite samples with short milling times, evidencing that the structural organization of the graphene-like layers was the key feature defining the magnitude of the shielding on the ${}^1$H nuclei in the water molecules adsorbed by the analyzed materials. The DFT calculations of the shielding sensed by these ${}^1$H nuclei showed an increased interaction with the graphitic layers as the distance between these layers (representing the pore size) was reduced. A continuous decrease of the ${}^1$H NMR chemical shift was then predicted for pores of smaller sizes, in good agreement with the experimental findings. These calculations also showed a large dispersion of chemical shifts for the several ${}^1$H nuclei in the water clusters, attributed to intermolecular interactions and to shielding variations within the pores. This dispersion, combined with the effects due to the locally anisotropic diamagnetic susceptibility of graphite-like crystallites, are the main contributions to the broadening of the ${}^1$H NMR signals associated with water adsorbed onto porous carbon materials.