NMR spectral parameters of open- and closed-shell graphene nanoflakes: Orbital and hyperfine contributions

Graphene nanoflakes have attracted a growing interest owing to their tunable and unique electronic, optical, and magnetic properties. In particular, recent breakthroughs in the on-surface synthesis and characterization of graphene nanoflakes exhibiting π -magnetism have shown their promising great potential for spintronic applications. In this context, a theoretical investigation on the relative energetic stability, 13 C nuclear magnetic resonance (NMR) chemical shifts, magnetically induced currents, hyperfine shifts, and hyperfine coupling constants of graphene nanoflakes of hexagonal and triangular shape has been performed using the density functional theory (DFT). The role played by the size, shape, and atomic site position in the flake on the 13 C isotropic chemical shift is thoroughly examined. As a general trend, considering only the orbital contribution, sites from the innermost region of the flake present lower chemical shifts than the ones close to the border and, for large enough systems, such values tend to converge to roughly the graphene one. For the open-shell flakes, the hyperfine shifts and coupling constants exhibit oscillatory behavior, with opposite signs for adjacent sites. The magnitude of these parameters is progressively reduced with the increase in the distance from the edge, where the largest values of excess spin density are concentrated. ● NMR parameters of graphene nanoflakes are investigated through DFT based simulations. ● Flakes of different sizes and shapes (coronoids and triangulenes) are analyzed. ● Spin-state energy splittings and spin ground state are calculated for triangulenes. ● 13 C NMR chemical shifts for coronoids show good correlation with the flake size and atomic site position. ● Spin density and hyperfine shifts show oscillatory behavior for adjacent sites in triangulenes.