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J. Phys. Chem. C 2008, 112, 13976–13981
Electron Self-exchange Dynamics of the Nanoparticle Couple [Au25(SC2Ph)18]0/1- By Nuclear Magnetic Resonance Line-Broadening Joseph F. Parker, Jai-Pil Choi,a Wei Wang,b and Royce W. Murray* Kenan Laboratories of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599 ReceiVed: June 26, 2008
Proton nuclear magnetic resonance (NMR) was used to measure the rate constant and activation energy barrier for electron self-exchanges of the phenylethanethiolate-protected nanoparticle couple [Au25(SC2Ph)18]0/1-. The thiolate ligand R-methylene proton resonances of electrolytically prepared CD2Cl2 solutions of the oxidized (Au250) and reduced (Au251-) nanoparticles exhibit characteristic chemical shifts and line-shapes. That for the R-CH2 protons in Au250 is shifted ∼2 ppm downfield from Au251- and has an increased line-width reflecting the odd electron count of the nanoparticle core. Solution mixtures of Au250 and Au251- exhibit further peak broadening and intermediate values of R-CH2 proton chemical shifts, effects quantitatively consistent with an electron self-exchange process in the fast-exchange regime. Analysis of changes in peak broadening at varied total nanoparticle concentration and at varied temperatures produces a rate constant for [Au25(SC2Ph)18]0/1self-exchange of 3.0((0.1) × 107 M-1s-1 at 22 °C and an activation barrier energy EA ) 25.0 ((1.5) kJ/mol. This barrier energy is much larger than the calculated estimate of outer-sphere reorganization energy, implying the presence of a significant inner-sphere reorganization energy. The latter is confirmed by a detected difference in the Raman Au-S bond stretch energies of Au250 and Au251- nanoparticles. Introduction Gold monolayer protected clusters (MPCs) are a class of novel materials consisting of a core of gold atoms surrounded and stabilized by a shell of organic ligands, typically thiolates. Au nanoparticles with core diameters
