STRUCTURE AND CHAIN DYNAMICS OF ALKANETHIOL-CAPPED GOLD COLLOIDS

The structure and dynamical behavior of short and long chain alkanethi ols, CH3(CH2)(7)SH and CH3(CH)(17)SH, and of a hydroxyalkanethiol, HO( CH2)(16)SH, adsorbed onto gold nanoparticles were studied by variable temperature solid-state C-13 NMR spectroscopy. In both the solution an d solid state, the resonances of the first three carbons next to the s ulfur headgroup disappear upon binding to the gold, indicating a stron g interaction with the surface. A C-13-enriched sample, CH3(CH2)(12)C H2SH/gold, displays a broad resonance centered at 42 ppm for the carbo n next to the sulfur headgroup. Whereas the solid-state C-13 shifts of CH3(CH2)(7)SH/gold are essentially the same as in solution, the methy lene carbons of CH3(CH2)(17)SH and HO(CH2)(16)SH/gold shift, downfield by 4.5 ppm in the solid state, indicating that the chains crystallize into an extended all-trans conformation. The high conformational orde r, along with reduced methylene proton line widths in the CH3(CH)(17)S H/gold sample, indicates that the chains are undergoing large-amplitud e motions about their long axes. Molecular mobility increases toward t he unbound ends which have a higher population of gauche conformers. R elaxation measurements show the coexistence of motionally restricted a ll-trans chains and a smaller population of liquid-like conformational ly disordered chains in CH3(CH2)(17)SH/gold at room temperature. The t wo types of chains are proposed to arise from close packing of the gol d colloidal spheres, resulting in interstitial spaces and regions wher e chains of neighboring colloids can interdigitate to produce ordered domains. Phase transitions of the thiol-capped gold nanocrystals, whic h are detected by differential scanning calorimetry, are shown to invo lve a reversible disordering of the alkyl chains.