Intramolecular Ligand Dynamics in d 15 - ( PPh 3 ) - Capped Gold Nanoparticles Investigated by 2 H NMR

The knowledge of intramolecular dynamics, geometric confinement, and structure of surface-bound ligands are important factors for understanding surface chemistry, catalysis, and design of catalytically optimized nanoparticles. In this report, intramolecular dynamics of perdeuterated triphenylphosphine ligands, d15-(PPh3), bound to the surface of gold nanoparticles (AuNPs) are characterized by pulsed deuteron nuclear magnetic resonance (H NMR) in the solid state. The H quadrupolar echo Fourier transform NMR line shapes indicate fast 180 flip dynamics (π flip) of phenyl rings in surface-bound d15-(PPh3) ligands about the P-C bond at the rate of kg 7 10 s. In contrast, the d15-(PPh3)AuCl complex, a model compound that is also present in this nanoparticle system, does not exhibit measurable π flips on the time scale sampled by the experiment (∼10 s). Its phenyl rings appear rigid from ambient temperature up to 386 K. The experimental HNMR line shape results are consistent with a theoretical model of phenyl rings undergoing rapidπ flips. The deuterons of the phenyl ring experiencing the rapid motion of surface-bound d15-(PPh3) ligands in a high magnetic field exhibit substantially shortened spin-lattice (T1) relaxation times of 44ms in the ortho andmeta positions and 2.5 s for the para position in comparison to the very long T1 time in excess of 10 3 s observed in the rigid d15-(PPh3)AuCl complex. The difference in T1 by several orders of magnitude further confirms the rapid phenyl ring dynamics of ligands bound through gold-phosphorus bond on the surfaces. The observation of uninterrupted rapid motion in phenyl rings suggests the presence of well-separated ligands and availability of low-coordinated surface gold atoms. These parameters are critically important in the catalytic activity of ligand-capped nanoparticles.