The ligand effect on the isomer stability of Au24(SR)20 clustersElectronic supplementary information (ESI) available. See DOI: 10.1039/c4nr05826g

A key challenge in nanocluster research in particular and nanoscience in general is structure prediction for known compositions. Usually a simple ligand such as a methyl group is used to replace complex ligands in structure prediction of ligand-protected nanoclusters. However, how ligands dictate the energy landscape of such a cluster remains unclear. Here we elucidate the role of the ligand effect on the isomer stability of Au 24 (SR) 20 nanoclusters by computing the relative energy of two isomers (one from the experiment, denoted as the "J" isomer; the other is the best theoretical model, denoted as the "P" isomer) of Au 24 (SR) 20 with dispersion-corrected density functional theory. We find that when R = -CH 3 , the two isomers are equally stable (within 0.13 eV), but for R = -CH 2 CH 2 Ph the P isomer is more stable by 1.6 eV and for R = -CH 2 Ph- t Bu the J isomer is more stable by 1.0 eV. Partition of the total energy into DFT and vdW contributions indicates that the higher stability of the P isomer in the case of R = -CH 2 CH 2 Ph stems from the stronger vdW interactions among -CH 2 CH 2 Ph groups, while the higher stability of the J isomer in the case of R = -CH 2 Ph- t Bu is due to its better capacity to respond to the steric effect of the larger -CH 2 Ph- t Bu groups. This finding confirms that the ligand plays a crucial role in dictating the isomer stability. Density functional calculations show that the relative isomer stability of the Au 24 (SR) 20 nanocluster depends on the ligand and that the stereochemical interaction among ligands is crucial in affecting the relative energetics.