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| - NMR Diffusion, Relaxation, and Spectroscopic Studies of Water Soluble,Monolayer-Protected Gold Nanoclusters
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| - NMR spectroscopy and computer modeling were used to characterize tiopronin monolayer-protected goldclusters (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. DOSYand T2 experiments provided information about the particle size distribution as a function of proton frequencyshift. Further information was obtained from hole-burning and amide-exchange experiments. The spectroscopicdata reveal two classes of ligands, a network of hydrogen bonds, and considerable inhomogeneous andhomogeneous line broadening. The methyl and methine protons clearly exhibit two components with separationsthat decrease strongly with the number of bonds separating the proton from the gold core. Spin−echoexperiments clearly show that a range of T2 values is associated with each resonance frequency in both theupfield and downfield components for each type of proton but that the most probable value is larger for theupfield component. Various models that may be consistent with the NMR data and the properties of reportedcrystal structures were considered. It is suggested that bimodal frequency distributions result from chemicalshifts that are associated with a mixture of primarily two gold cluster structure types that differ in the modeof core packing. It is suggested that the Knight shift contributes to the large downfield shift observed for themethine protons in the larger particles.
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