13C NMR spectroscopy of 13C1-labeled octanethiol-protected Au nanoparticles: shifts, relaxations, and particle-size effect.

We report here an investigation of metal-ligand interactions in nanoparticles with 13C NMR, using a labeled 13C1-octanethiol, a protecting ligand for self-assembled monolayer (SAM) systems, in which close proximity of the 13C1 to the metal surface serves as an effective probe for the changing electronic environment. Several remarkable results have been obtained: as the metal core size increases from 1.6 to 4.0 nm, the 13C1 spectrum is downshifted from 40.5 to 53 ppm, and the spin-spin relaxation rate, T2-1, increases while the spin-lattice relaxation ratio decreases. Although the spin-lattice relaxation may be due to particle tumbling and ligand motion in the liquid state, the main source of the spin-spin relaxation, and NMR shift, is most possibly due to the changing electronic properties of the metal core.