Electron Self-exchange Dynamics of the Nanoparticle Couple [Au₂₅(SC2Ph)₁₈]^0/1− By Nuclear Magnetic Resonance Line-Broadening

Abstract

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 α-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 α-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 α-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]0/1− self-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.