NMR Diffusion, Relaxation, and Spectroscopic Studies of Water Soluble, Monolayer-Protected Gold Nanoclusters

NMR spectroscopy and computer modeling were used to characterize tiopronin monolayer-protected gold clusters (MPCs). These MPCs contain gold cores with a distribution of radii ranging from 0.4 to 2.6 nm. NOESY and HMQC spectra yielded assignments for all NMR sensitive nuclei in the tiopronin ligands. DOSY and T 2 experiments provided information about the particle size distribution as a function of proton frequency shift. Further information was obtained from hole-burning and amide-exchange experiments. The spectroscopic data reveal two classes of ligands, a network of hydrogen bonds, and considerable inhomogeneous and homogeneous line broadening. The methyl and methine protons clearly exhibit two components with separations that decrease strongly with the number of bonds separating the proton from the gold core. Spin−echo experiments clearly show that a range of T 2 values is associated with each resonance frequency in both the upfield and downfield components for each type of proton but that the most probable value is larger for the upfield component. Various models that may be consistent with the NMR data and the properties of reported crystal structures were considered. It is suggested that bimodal frequency distributions result from chemical shifts that are associated with a mixture of primarily two gold cluster structure types that differ in the mode of core packing. It is suggested that the Knight shift contributes to the large downfield shift observed for the methine protons in the larger particles.