Vibrational Coupling Modulation in n-Alkanethiolate Protected Au₂₅(SR)₁₈⁰ Clusters

Abstract

We have studied, both experimentally and theoretically, the Raman vibrational spectra of a series of n-alkanethiolate protected Au25(SC n H2n+1)18 clusters, with n = 2, 3, 4, 5, 6, 8, 10, 12, and 14. The C–H stretching region of the infrared spectra reveals that, while shorter chains are flexible, longer chains are more ordered with a propensity toward extended all-trans conformation. The different behavior of long and short chains is also reflected in the low-frequency Raman spectra of the clusters, which are broadened for the longer chains due to interchain interactions and formation of bundles. The experimental low-frequency modes in the Raman spectra, associated with Au–S stretching vibrations, change drastically and in an apparently unsystematic way as a function of chain length. For example, a band around 320 cm–1 associated with tangential Au–S stretching character shifts up in frequency, then down and then up again as the carbon chain is increased. DFT calculations reveal that this behavior is due to a nonlinear coupling of this mode to torsional and bending modes of the alkyl chain. The frequencies of these modes strongly depend on the chain length and, as a consequence, also their coupling with the Au–S stretching modes, which explains the erratic behavior of this band in the spectra. This behavior is well described by calculations on a mimic cluster model that considers only one staple motif. For the ethanethiolate-protected cluster, the entire cluster was included in the calculation of the Raman spectrum, and this allowed for the first time to compare directly experimental and calculated Raman spectra of the same cluster. Furthermore, our study shows that the entire ligand has to be considered for the calculation of the low frequency vibrations of the Au–S interface, as this spectral region is sensitive to coupling with low-frequency ligand modes.

Ultrafast Relaxation Dynamics of Au38(SC2H4Ph)24 Nanoclusters and Effects of Structural Isomerism

“…Previous investigation on temperature dependent absorption spectra of Au 38Q reveals that the phonon frequency of the surface -S-Au-S-Au-S-staples is around 400 cm -1 . 37,44 The lower frequencies of phonons for both isomers indicate that the coherent phonons observed here should originate in the Au 23 core. Different phonon frequencies have been reported for gold nanoclusters with different structures.…”

Revisiting the conformational adsorption of L‐ and D‐cysteine on Au nanoparticles by Raman spectroscopy

“…The vibrational study was performed by means of Raman spectroscopy instead of SERS to avoid misleading band assignment, reassuring Raman spectroscopy as a powerful analytical technique that can provide key information on the structure and bonding processes of the adsorbate molecules on metal surfaces in the solid and liquid phase. [4,40,41] The high quality of our Raman spectroscopy data provides very well resolved bands with higher definition than previous SERS results, [34][35][36][37] contrary to what can be expected when studying systems that involve metal surfaces. This quality data allowed us to determine the structural differences that the Au NP metallic surface induces on the cysteine molecule upon adsorption.…”

“…Our method produced colloidal neutral clusters in high yield with high affinity to the medium (H 2 O), allowing the attachment of the zwitterionic biosurfactant cysteine on their surface, conferring stability versus aggregation and coalescence that may cause particle growth. The vibrational study was performed by means of Raman spectroscopy instead of SERS to avoid misleading band assignment, reassuring Raman spectroscopy as a powerful analytical technique that can provide key information on the structure and bonding processes of the adsorbate molecules on metal surfaces in the solid and liquid phase . The high quality of our Raman spectroscopy data provides very well resolved bands with higher definition than previous SERS results, contrary to what can be expected when studying systems that involve metal surfaces.…”

Synthesis and Properties of Monolayer-Protected Cox(SC2H4Ph)m Nanoclusters

“…However, when the Raman spectra of cobalt clusters are compared those of monolayer protected thiolated metal clusters several similarities are apparent. [52][53][54][55] Thiol ligands in similar environments or configurations (like staples) should exhibit similar vibrational properties for Co and Au, but the frequencies may be slightly shifted due to different masses and force constants. Raman spectroscopy of gold clusters have focused on the Au-S interface and both experimental and theoretical studies were reported.…”