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Ligand heterogeneity on monolayer-protected gold clusters
Yang Song1, Amanda S Harper, Royce W Murray
1Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, USA.
Langmuir : the ACS Journal of Surfaces and Colloids
|June 1, 2005
Summary
Oxidative charging of gold cores in monolayer-protected clusters (MPCs) alters nuclear magnetic resonance (NMR) spectra of ligand shells. These spectral changes, observed in gold clusters, are reversible and indicate rapid ligand site exchange.
Area of Science:
- Nanomaterials Science
- Surface Chemistry
- Spectroscopy
Background:
- Monolayer-protected clusters (MPCs) are nanoscale gold particles stabilized by organic ligands.
- The electronic properties of MPCs can be tuned by altering the charge state of their gold cores.
- Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool for characterizing the structure and dynamics of molecules.
Purpose of the Study:
- To investigate the impact of oxidative electronic charging of gold (Au) cores in MPCs on the NMR spectra of their ligand shells.
- To explore the relationship between cluster core charge and ligand dynamics.
- To characterize changes in core electronic spectra induced by positive charging.
Main Methods:
- Electrochemical charging of MPCs (Au140(S(CH2)5CH3)53 and Au38(SCH2CH2Ph)24) to alter core charge states.
- 13C Nuclear Magnetic Resonance (NMR) spectroscopy to analyze ligand shell resonances.
- Spectroelectrochemistry to measure core electronic spectral changes.
Main Results:
- Unresolved fine structure in 13C NMR spectra of Au140(S(CH2)5CH3)53 MPCs revealed magnetically inequivalent ligand sites.
- Positive core charging caused spectral fine structure coalescence, indicating dynamic ligand behavior.
- Spectral changes were reversible upon de-charging, and comparable effects were observed for Au38(SCH2CH2Ph)24 MPCs.
- Ligand site exchange rates were determined to be 100–400 s(-1).
Conclusions:
- Oxidative electronic charging of gold cores significantly influences the NMR spectra of MPC ligand shells.
- The observed spectral changes are attributed to dynamic processes within the ligand shell, likely involving rapid exchange between magnetically inequivalent sites.
- These findings provide insights into the electronic coupling between the gold core and the surrounding ligands in MPCs.