Vibrational Normal Modes of Small Thiolate-Protected Gold Clusters

Abstract

This report addresses a density functional theory study of the vibrational normal modes of small-sized thiolate-protected gold clusters. Calculated far-infrared (far-IR) and lowfrequency Raman spectra of established thiolate-protected gold clusters (4,12,18, 19,20, 24, and 25 Au atoms) show characteristic peaks in the range of 20−350 cm −1 that can be attributed to the breathing mode of the gold core, Au−S−Au bending modes and Au−S stretching modes. It was found that as cluster size increases the Au 13 breathing mode of Au 25 cluster emerges. This study reveals both systematic and specific characteristics that may provide a basis for identification ("fingerprinting") by far-IR and low-frequency Raman spectroscopy of the many diverse gold− thiolate cluster compounds.

Acoustic vibrations of metal nano-objects: Time-domain investigations

“…Size-independent vibrational frequencies around 2.2 THz were detected in Au l (SR) m thiol-protected gold nanoclusters smaller than 2 nm [177]. These results are consistent with atomistic simulations showing an important impact of the surface bound ligand molecules, with a non-monotonic variation of the quasi-breathing mode frequency for thiolateprotected 18-25 gold atom clusters [187]. In these systems, the breathing motion is almost restricted to their 13-atom gold core making its frequency independent of the gold atom number, in contrast to the results for free clusters [28].…”

Structural Information on the Au–S Interface of Thiolate-Protected Gold Clusters: A Raman Spectroscopy Study

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 we can observe is the Au-S-C bending at 175 cm -1 which was also reported by Murray et al 14 between 220 and 350 cm -1 ; the latter at higher wavenumbers and with very low intensities. 12 In the experimental spectra (Figure 1) it can be clearly seen that the ligand shell strongly influences the (Figure 1b) can directly be compared to the Raman spectrum calculated by DFT. 12 One has to bear in mind though that in the calculations the S-CH3 ligand was used, whereas in the experiment the 2-PET ligand was present, which could lead to changes in the spectra due to the different reduced masses of the normal modes and steric effects We calculated the vibrational spectrum of Au38(SCH3)24.…”

“…12 In the experimental spectra (Figure 1) it can be clearly seen that the ligand shell strongly influences the (Figure 1b) can directly be compared to the Raman spectrum calculated by DFT. 12 One has to bear in mind though that in the calculations the S-CH3 ligand was used, whereas in the experiment the 2-PET ligand was present, which could lead to changes in the spectra due to the different reduced masses of the normal modes and steric effects We calculated the vibrational spectrum of Au38(SCH3)24. Figure 2 shows the Raman spectrum of the low wavenumber range and a schematic presentation of characteristic vibrations.…”

“…DFT calculations on small thiolate-protected gold clusters, with [Au25(SCH3)18] -having been so far the biggest system considered, predict different far infrared (FIR) and low frequency Raman spectra for different cluster sizes. 12 For the [Au25(SCH3)18] -cluster, the vibrations of the metal core (including its breathing mode) are predicted below 200 cm -1 , a S-CH3 bending mode at 174 cm -1 , a Au-S stretching mode at 242 cm -1 and the breathing mode of the staples at 269 cm -1 . Generally, the calculations predict a richer spectrum with more bands for Raman as compared to far infrared.…”

“…Additionally we performed a calculation on Au38(SCH3)24 in analogy to the series of small clusters calculated by Tlahuice et al 12 for supplementary comparison with the experimental spectra.…”