Infra-red spectroscopy of size selected Au25, Au38 and Au144 ligand protected gold clusters

Through the discovery of ligand protected metal clusters with cores of a precise number of atoms, the exploration of the third dimension of the periodic table for fundamental research and also for applications has become less remote. So far, the exact number of metal atoms in the core has been determined unambiguously only using mass spectrometry and single crystal X-ray diffraction. Gold clusters protected by 2-phenylethanethiol ligands, for instance, show distinct magic numbers that correspond to either electronic or geometric shell closings. For efficient control of their synthesis simple-to-use in situ spectroscopies are required. In the specific case of Au25(SCH2CH2Ph)18 clusters (1) we found a distinct shift of the aromatic C-H stretching band from 3030-3100 cm(-1) to below 3000 cm(-1) whose origin is discussed as an electronic interaction of the aromatic rings of the ligands with each other or with the gold core. This IR-feature is specific for Au25; the spectra of Au38(SCH2CH2Ph)24 (2) and Au144(SCH2CH2Ph)60 (3) clusters do not show this distinct shift and their IR-spectra in the C-H stretching regime are similar to that of the bare ligand. This significant change in the IR spectrum of Au25(SCH2CH2Ph)18 is not only of fundamental interest but also allows for in situ determination of the purity and monodispersity of the sample using FTIR spectroscopy during synthesis.