The lowest-energy structure of thiolate-group-protected Au38(SR)24 is with ab initio computations. A unique bi-isocahedral Au23 core is predicted for the Au38(SR)24 cluster, consistent with recent experimental and theoretical confirmation of the icosahedral Au13 core for the [Au25(SR)18]- cluster. The computed optical absorption spectrum and X-ray diffraction pattern are in good agreement with experimental measurements. Like the "magic-number" cluster [Au25(SR)18]-, the high stability and selectivity of the magic-number Au38(SR)24 cluster is attributed to high structural compatibility between the bi-isocahedral Au23 core and the 18 exterior staple motifs.

The precise atomic structure of the recently synthesized "magic cluster" Au(20)(SR)(16) is predicted using ab initio calculations and global-minimum searches. The cluster contains a prolate Au(8) core and four level-3 extended staple motifs (-RS-Au-RS-Au-RS-Au-RS-). The simulated optical absorption spectra of the lowest-energy structures are in good agreement with the measured spectrum. The Au(20)(SR)(16) cluster, with a low Au/SR ratio of 1.25:1, may represent a structural evolution from core-free homoleptic clusters [Au(SR)](N) to core-stacked clusters.

A grand unified model (GUM) is developed to achieve fundamental understanding of rich structures of all 71 liganded gold clusters reported to date. Inspired by the quark model by which composite particles (for example, protons and neutrons) are formed by combining three quarks (or flavours), here gold atoms are assigned three ‘flavours' (namely, bottom, middle and top) to represent three possible valence states. The ‘composite particles' in GUM are categorized into two groups: variants of triangular elementary block Au3(2e) and tetrahedral elementary block Au4(2e), all satisfying the duet rule (2e) of the valence shell, akin to the octet rule in general chemistry. The elementary blocks, when packed together, form the cores of liganded gold clusters. With the GUM, structures of 71 liganded gold clusters and their growth mechanism can be deciphered altogether. Although GUM is a predictive heuristic and may not be necessarily reflective of the actual electronic structure, several highly stable liganded gold clusters are predicted, thereby offering GUM-guided synthesis of liganded gold clusters by design.

Atomic structure of a recently synthesized ligand-covered cluster Au(24)(SR)(20) [J. Phys. Chem. Lett., 2010, 1, 1003] is resolved based on the developed classical force-field based divide-and-protect approach. The computed UV-vis absorption spectrum and powder X-ray diffraction (XRD) curve for the lowest-energy isomer are in good agreement with experimental measurements. Unique catenane-like staple motifs are predicted for the first time in core-stacked thiolate-group (RS-) covered gold nanoparticles (RS-AuNPs), suggesting the onset of structural transformation in RS-AuNPs at relatively low Au/SR ratio. Since the lowest-energy structure of Au(24)(SR)(20) entails interlocked Au(5)(SR)(4) and Au(7)(SR)(6) oligomers, it supports a recently proposed growth model of RS-AuNPs [J. Phys. Chem. Lett., 2011, 2, 990], that is, Au(n)(SR)(n-1) oligomers are formed during the initial growth of RS-AuNPs. By comparing the Au-core structure of Au(24)(SR)(20) with other structurally resolved RS-AuNPs, we conclude that the tetrahedral Au(4) motif is a prevalent structural unit for small-sized RS-AuNPs with relatively low Au/SR ratio. The structural prediction of Au(24)(SR)(20) offers additional insights into the structural evolution of thiolated gold clusters from homoleptic gold(I) thiolate to core-stacked RS-AuNPs. Specifically, with the increase of interfacial bond length of Au(core)-S in RS-AuNPs, increasingly larger "metallic" Au-core is formed, which results in smaller HOMO-LUMO (or optical) gap. Calculations of electronic structures and UV-vis absorption spectra of Au(24)(SR)(20) and larger RS-AuNPs (up to ~2 nm in size) show that the ligand layer can strongly affect optical absorption behavior of RS-AuNPs.

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Using the CO oxidation as a chemical probe, we perform a comprehensive ab initio study of catalytic activities of subnanometer gold clusters. Particular attention is placed on 12 different clusters in the size range of Au(16)-Au(35), whose atomic structures in the anionic state have been resolved from previous experiments. Adsorption energies of a single CO or O(2) molecule as well as coadsorption energies of both CO and O(2) molecules on various distinctive surface sites of each anionic cluster and their neutral counterpart are computed. In general, the anionic clusters can adsorb CO and O(2) more strongly than their neutral counterparts. The coadsorption energies of both CO and O(2) molecules decrease as the size of gold clusters increases with the exception of Au(34) (an electronic "magic-number" cluster). Besides the known factor of low coordination site, we find that a relatively small cone angle (<110°) associated with each surface site is another key geometric factor that can enhance the binding strength of CO and O(2). For the subnanometer clusters, although the size effect can be important to the strength of CO adsorption, it is less important to the activation energy. Using Au(34) as a prototype model, we show that strong CO and O(2) adsorption sites tend to yield a lower reaction barrier for the CO oxidation, but they have little effect on the stability of the reaction intermediate. Our calculations support the notion that CO and O(2) adsorption energies on the gold clusters can be an effective indicator to assess catalytic activities of subnanometer gold clusters. This systematic study of the site- and size-dependent adsorption energies and reaction pathways enables a quantitative assessment of the site-size-activity relationship for the CO oxidation on subnanometer gold clusters.

A long-standing aim in molecular self-assembly is the development of synthetic nanopores capable of mimicking the mass-transport characteristics of biological channels and pores. Here we report a strategy for enforcing the nanotubular assembly of rigid macrocycles in both the solid state and solution based on the interplay of multiple hydrogen-bonding and aromatic π − π stacking interactions. The resultant nanotubes have modifiable surfaces and inner pores of a uniform diameter defined by the constituent macrocycles. The self-assembling hydrophobic nanopores can mediate not only highly selective transmembrane ion transport, unprecedented for a synthetic nanopore, but also highly efficient transmembrane water permeability. These results establish a solid foundation for developing synthetically accessible, robust nanostructured systems with broad applications such as reconstituted mimicry of defined functions solely achieved by biological nanostructures, molecular sensing, and the fabrication of porous materials required for water purification and molecular separations.

The structural evolution of negatively charged gold clusters (Au(n)(-)) in the medium size range for n = 27-35 has been investigated using photoelectron spectroscopy (PES) and theoretical calculations. New PES data are obtained using Ar-seeded He supersonic beams to achieve better cluster cooling, resulting in well-resolved spectra and revealing the presence of low-lying isomers in a number of systems. Density-functional theory calculations are used for global minimum searches. For each cluster anion, more than 200 low-lying isomers are generated using the basin-hopping global minimum search algorithm. The most viable structures and low-lying isomers are obtained using both the relative energies and comparisons between the simulated spectra and experimental PES data. The global minimum structures of Au(n)(-) (n = 27, 28, 30, and 32-35) are found to exhibit low-symmetry core-shell structures with the number of core atoms increasing with cluster size: Au(27)(-), Au(28)(-), and Au(30)(-) possess a one-atom core; Au(32)(-) features a three-atom triangular core; and Au(33)(-) to Au(35)(-) all contain a four-atom tetrahedral core. The global searches reveal that the tetrahedral core is a popular motif for low-lying structures of Au(33)(-) to Au(35)(-). The structural information forms the basis for future chemisorption studies to unravel the catalytic effects of gold nanoparticles.