Rotational Motion of Ligand-Cysteine on CdSe Magic-Sized Clusters

Structure and dynamics of organic ligand capping on the surface of clusters significantly influence the property and function of the clusters. Recently, we revealed that, in cysteine-capped (CdSe)34 magic-sized cluster (CdSe-Cys), ligand-cysteine has two capping structures, namely, monodentate ligand-cysteine with sulfur–cadmium bond and bidentate one with sulfur– and nitrogen–cadmium bonds. In this work, we examine the motion of ligand-cysteine capping on (CdSe)34 by performing solid-state 2H nuclear magnetic resonance spectroscopy in CdSe-Cys with the deuterated methanediyl group (−CD2−). 2H quadrupolar Carr–Purcell–Meiboom–Gill (QCPMG) and quadrupolar-echo spectra of CdSe-Cys were measured. Temperature dependence of line width and spin–lattice relaxation time of spikelets in the QCPMG spectra suggested that monodentate ligand-cysteine undergoes molecular motion. The quadrupolar-echo spectra were a sum of a motionally narrowed peak at the center of the spectra and a typical 2H Pake pattern, which are assigned to monodentate and bidentate ligand-cysteine, respectively. The line-shape analysis for the 2H quadrupolar-echo spectra obtained at 0–40 °C revealed that the CH2 group in monodentate ligand-cysteine undergoes pseudoisotropic rotation by the combination of two intramolecular rotations around the Cd–S and S–C bonds with the activation energy of ∼19 kJ/mol. Further, no evidence was found for the exchange of the two capping structures.