| Abstract
| - Variable temperature electron paramagnetic resonance (EPR) spectra of spin-labeled Au nanoparticles in toluenesolution were recorded at X-, Q- and W-bands. The chain length of the nitroxide-based spin-labeled ligandand the surrounding alkanethiol ligands was systematically varied. The spectra were analyzed using the NLSLprogram (Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. J. Magn. Reson., Ser. A1996, 120, 155) whichsimulates slow motion spectra using the stochastic Liouville approach. The simulation results show that therotational diffusion parameters for particles (∼2.6 nm Au core diameter) are dominated by the local motionof the ligand. The length of the spin label has the biggest effect on the nitroxide dynamics followed by thechain length of the surrounding ligands. The results are consistent with the loose packing of ligands on thenanoparticle surface. The packing density of ligands further decreases with the increasing distance from theAu surface. The Arrhenius analysis of the dynamic parameters obtained at different temperatures showed anactivation energy for rotational diffusion of ∼2.5−4 kcal/mol, regardless of the nature of the spin label andthe surrounding ligand. This result is also consistent with the very open, disordered structure of the organicligands on the nanoparticle surface in toluene.
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