Metal-ligand interfaces of octanechalcogenol-protected metal nanoparticles as seen by nuclear magnetic resonance.

摘要

The primary focus of this research was the synthesis of alkanethiol and alkaneselenol capped metal nanoparticles (NP) and their characterization, mainly with 13C nuclear magnetic resonance (13C-NMR). The NMR investigations on the bound ligands were used as a guide to a better understanding of the electronic properties of the metal NP and the attendant chemical and physical properties, such as the metal-ligand interaction, as a function of its size, metal type, charge, and the protective ligand used. Synthesis of the NPs followed Brust's popular two-phase approach with the NP core diameter confirmed by TEM. First was the observation of a change in chemical shift, spin-lattice, and spin-spin relaxation due to the difference in the changing core size of an Au NP. This was most notably observed by the varying position of the 13C1 NMR line of a sulfur bound 13C1-1-octanethiol ligand on the Au surface that increases in chemical shift as the core size increases, apparently limited as the particle core diameter approaches 4nm. This can be surmised from the possible contribution of a combination of a pure geometry and a pure quantum size effect. The second observation is the existence of a disulfide monolayer on a Pd NP surface corroborated by NMR and IR evidence. This is surprising because it is contrary to the thiolate monolayer found on an Au NP, although both were synthesized with the same 13C1-octanethiol ligand under identical conditions. Thirdly, through a combined electrochemical NMR (EC-NMR) experiment, we found that for a given charged Au NP as the charge becomes more positive the 13C1 exhibits shorter relaxation times, translating to faster relaxation rates. This could possibly be attributed to changes in the Fermi level of the metal in relation to the HOMO/LUMO of the ligand. The final experiments incorporated a 1-octaneselenol protected Au NP that allowed the first ever observed 77Se NMR of a selenium atom directly bound to a metal NP surface. Compared to the previously observed 13C1 of the octanethiol, the relaxation rates of the 77Se were also found to be much faster which could be attributed to several explanations involving the presence, or absence, of chemical shift anisotropy.