On the structure of the Au18(SR)14 cluster

Abstract

First principles calculations are used for a systematic search of the lowest-energy (most-stable) structure of the recently synthesized Au 18 (SR) 14 cluster. A comparison of the calculated optical absorption and electronic circular dichroism spectra, which are highly sensitive to the cluster structure and chirality, with the experimental spectra of the glutathione-protected gold cluster, Au 18 (SG) 14 , is used to discriminate between low-energy isomers of the Au 18 (SR) 14 (R = CH 3 ) cluster. From the good agreement between calculated and measured spectra, it is predicted that the structure of the Au 18 (SR) 14 cluster consists of a prolate Au 8 core covered with two dimer (SR-Au-SR-Au-SR) and two trimer (SR-Au-SR-Au-SR-Au-SR) motifs. These results provide additional evidence on the existence of longer trimer motifs as protecting units of small thiolated gold clusters.Studies on ligand-protected metal clusters became an important field of research due to their interesting properties and promising applications in nanotechnology. 1 Despite the tremendous progress achieved during the past fifteen years on the synthesis and physicochemical characterization of small (o2 nm) thiolateprotected gold clusters, several issues of scientific and technological interest still remain unknown. 1 One of the challenges is to develop and improve the synthetic chemistry for synthesizing a series of size-discrete Au n (SR) m clusters with high purity and high yield, followed by their crystallization and X-ray total structure determination, 1 as was the case for (n,m) = (25,18), (38,24), and (102,44). 2-4 A precise knowledge of the cluster atomic structure is a fundamental step toward a full understanding of other physicochemical properties of thiolated gold clusters. [1][2][3][4] Gold clusters covered by glutathione, Au n (SG) m , and other related ligands had been synthesized and size-separated by Tsukuda's group. 5,6 High mass resolution and accurate mass calibration in their electrospray ion mass spectrometry (ESI-MS) data allowed the exact determination of the cluster chemical composition. 5,6 The nine smallest compounds were assigned to cluster sizes with n : m ratio: 10 : 10, 15 : 13, 18 : 14, 22 : 16, 22 : 17, 25 : 18, 29 : 20, 33 : 22, and 39 : 24, respectively. 5,6 Interestingly, the most abundant compound with enhanced stability against unimolecular decomposition was found to be the Au 18 (SG) 14 cluster, 5 although it also had been shown that the 25 : 18 clusters exhibit the highest stability against core etching. 7More recently, Dass' group has synthesized a mixture of thiolated gold clusters, Au n (RS) m (R = CH 2 CH 2 Ph), in the Au 16 -Au 31 size range using a surfactant-free synthesis method. 8 Again, the most abundant cluster size in the mixture, detected by matrix-assisted laser desorption time of flight mass spectrometry (MALDI-TOF-MS), corresponded to the Au 18 (SR) 14 cluster. 8 It is expected that the peculiar stability and higher relative abundance of the 18 : 14 cluster should motivate its crystallizat...

Superatom Networks in Thiolate‐Protected Gold Nanoparticles

“…A probably geometric structure of [Au 20 (SR) 16 ] has been predicted by Pei et al [21] using DFT calculations in good agreement with the experiment in optical spectra and HOMO-LUMO gap. The predicted structure can be viewed as a prolate Au 8 14 ] cluster [20] and [Au 24 (SR) 20 ] cluster [22] were also theoretically predicted in good agreement with the experimental optical spectra and HOMO-LUMO gaps, and the prolate Au 8 core was further confirmed in the two compounds.…”

“…DFT predictions have successfully located the structures for many of the synthesized compounds in agreement with the experimental X-ray diffraction and/or UV/Vis spectra, for example, [Au 40 (SR) 24 ] 2À , [17] [Au 144 (SR) 60 ], [18] [Au 12 (SR) 9 ] + , [19] [Au 18 (SR) 14 ], [20] [Au 20 (SR) 16 ], [21] [Au 24 (SR) 20 ], [22] and [Au 19 (SR) 13 ]. [23] All of these Au-SR clusters show geometric shell closure following the "divide-and-protect" model and have molecule-like electronic structures with a relatively large gap between the highest occupied-lowest unoccupied molecular orbitals (HOMO-LUMO).…”

“…Using the SVB model, the 23-center 14-electron (23c-14e) bi-icosahedral Au core of [Au 38 (SR) 24 ] is shown to be a superatomic molecule (super F 2 ) in electronic shells. [28] Exceptions were also found for some other clusters such as [Au 18 (SR) 14 ], [Au 20 (SR) 16 ], and [Au 24 (SR) 20 ]. [10][11][12] The three compounds have both four free valence electrons in the gold core, which disagrees with the counts of magic numbers in the superatom model, but are electronically very stable with large HOMO-LUMO gaps in about 1.6 eV, 2.1 eV, and 1.5 eV, respectively.…”

“…In summary, we proposed a SAN model to explain the electronic stability of the 4e compounds [Au 18 (SR) 14 ], [Au 20 (SR) 16 ], and [Au 24 (SR) 20 ]. As the products of self- assembly, the experimentally produced Au-SR clusters must be shell-closed species in terms of both their geometric and electronic structures.…”

“…Besides the four crystallized Au-SR compounds, there are also a variety of Au-SR compounds which are experimentally isolated and measured by mass and/or optical spectra but not crystallized yet, such as [Au 44 (SR) 28 ] 2À , [7] [Au 144 (SR) 60 ], [8] [Au 12 (SR) 9 ] + , [9] [Au 18 (SR) 14 ], [10] [Au 20 (SR) 16 ], [11] [Au 24 (SR) 20 ], [12] [Au 40 (SR) 24 ], [13] [Au 19 (SR) 13 ], [14] and [Au 68 (SR) 34 ]. [15] Structures of the experimentally isolated Au-SR compounds can be theoretically predicted by density functional theory (DFT) calculations.…”