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Research Article

Ligand Heterogeneity on Monolayer-Protected Gold Clusters

Supporting Info

Yang Song, Amanda S. Harper,^† and Royce W. Murray*

Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290

Langmuir, 2005, 21 (12), pp 5492–5500

DOI: 10.1021/la0503606

Publication Date (Web): April 30, 2005

†

Present address: Naval Research Laboratory, Washington, DC.

Author to whom correspondence should be addressed. E-mail: rwm@email.unc.edu.

Abstract

This paper describes the effects of oxidative electronic charging of the Au cores of the monolayer-protected clusters (MPCs), Au₁₄₀(S(CH₂)₅CH₃)₅₃ and Au₃₈(SCH₂CH₂Ph)₂₄, on nuclear magnetic resonance (NMR) spectra of their monolayer ligand shells. Previously unresolved fine structure in the ¹³C NMR hexanethiolate methyl and C5 methylene resonances is seen in spectra of solutions of monodisperse Au₁₄₀(S(CH₂)₅CH₃)₅₃ MPCs, reflecting magnetically inequivalent ligand sites. Incremented increases in positive cluster core charge, effected by electrochemical charging, cause the spectral fine structure of the methyl resonance to coalesce, becoming a single peak at the Au₁₄₀³⁺ charge state. The spectral changes are reversible; charging back to the original core charge state regenerates the methyl ¹³C resonance fine structure. Adding an equimolar quantity of a Au(I) thiolate complex, Au^I[SCH₂(C₆H₄)C(CH₃)₃], to an uncharged Au₁₄₀(S(CH₂)₅CH₃)₅₃ MPC solution in d₂-methylene chloride causes partial spectral coalescence. ¹³C NMR spectra of Au₃₈(SCH₂CH₂Ph)₂₄ MPCs exhibit roughly comparable spectral changes upon positive core charging to the ‘0', ‘+1', and ‘+2' states. The NMR results indicate that exchange between magnetically inequivalent sites occurs at rates of 100 to 400 s^-¹, a rate believed to be too fast to be accountable by actual exchanges of ligands between different sites on the Au core. We also describe changes in core electronic spectra of Au₁₄₀(S(CH₂)₅CH₃)₅₃ induced by positive charging, measured using spectroelectrochemistry.