Thomas Bürgi

Chiral Gold Nanoparticles

Monolayer-protected gold nanoparticles have many appealing physical and chemical properties such as quantum size effects, surface plasmon resonance, and catalytic activity. These hybrid organic-inorganic nanomaterials have promising potential applications as building blocks for nanotechnology, as catalysts, and as sensors. Recently, the chirality of these materials has attracted attention, and application to chiral technologies is an interesting perspective. This minireview deals with the preparation of chiral gold nanoparticles and their chiroptical properties. On the basis of the latter, together with predictions from quantum chemical calculations, we discuss different models that were put forward in the past to rationalize the observed optical activity in metal-based electronic transitions. We furthermore critically discuss these models in view of recent results on the structure determination of some gold clusters as well as ligand-exchange experiments examined by circular dichroism spectroscopy. It is also demonstrated that vibrational circular dichroism can be used to determine the structure of a chiral adsorbate and the way it interacts with the metal. Finally, possible applications of these new chiral materials are discussed.

Chirality in thiolate-protected gold clusters.

Over recent years, research on thiolate-protected gold clusters Au(m)(SR)n has gained significant interest. Milestones were the successful determination of a series of crystal structures (Au102(SR)44, Au25(SR)18, Au38(SR)24, Au36(SR)24, and Au28(SR)20). For Au102(SR)44, Au38(SR)24, and Au28(SR)20, intrinsic chirality was found. Strong Cotton effects (circular dichroism, CD) of gold clusters protected by chiral ligands have been reported a long time ago, indicating the transfer of chiral information from the ligand into the cluster core. Our lab has done extensive studies on chiral thiolate-protected gold clusters, including those protected with chiral ligands. We demonstrated that vibrational circular dichroism can serve as a useful tool for the determination of conformation of the ligand on the surface of the cluster. The first reports on crystal structures of Au102(SR)44 and Au38(SR)24 revealed the intrinsic chirality of these clusters. Their chirality mainly arises from the arrangement of the ligands on the surface of the cluster cores. As achiral ligands are used to stabilize the clusters, racemic mixtures are obtained. However, the separation of the enantiomers by HPLC was demonstrated which enabled the measurement of their CD spectra. Thermally induced inversion allows determination of the activation parameters for their racemization. The inversion demonstrates that the gold-thiolate interface is anything but fixed; in contrast, it is rather flexible. This result is of fundamental interest and needs to be considered in future applications. A second line of our research is the selective introduction of chiral, bidentate ligands into the ligand layer of intrinsically chiral gold clusters. The ligand exchange reaction is highly diastereoselective. The bidentate ligand connects two of the protecting units on the cluster surface and thus effectively stabilizes the cluster against thermally induced inversion. A minor (but significant) influence of chiral ligands to the CD spectra of the clusters is observed. The studied system represents the first example of an intrinsically chiral gold cluster with a defined number of exchanged ligands, full control over their regio- and stereochemistry. The methodology allows for the selective preparation of mixed-ligand cluster compounds and a thorough investigation of the influence of single ligands on the clusters properties. Overall, the method enables even more detailed tailoring of properties. Still, central questions remain unanswered: (1) Is intrinsic chirality a ubiquitous feature of thiolate-protected gold clusters? (2) How does chirality transfer work? (3) What are the applications for chiral thiolate-protected gold clusters? In this Account, we summarize the main findings on chirality in thiolate-protected gold cluster of the past half decade. Emphasis is put on intrinsically chiral clusters and their structures, optical activity, and reactivity.

Intermolecular bonding and vibrations of phenol⋅H2O (D2O)

Extensive ab initio calculations of the phenol⋅H2O complex were performed at the Hartree–Fock level, using the 6‐31G(d,p) and 6‐311++G(d,p) basis sets. Fully energy‐minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowest‐energy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6‐311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra‐ and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol⋅H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β2 intermolecular wag. Furthermore, the H2O torsion τ around the H‐bond axis, and the β2 mode...

Size exclusion chromatography for semipreparative scale separation of Au38(SR)24 and Au40(SR)24 and larger clusters.

Size exclusion chromatography (SEC) on a semipreparative scale (10 mg and more) was used to size-select ultrasmall gold nanoclusters (<2 nm) from polydisperse mixtures. In particular, the ubiquitous byproducts of the etching process toward Au(38)(SR)(24) (SR, thiolate) clusters were separated and gained in high monodispersity (based on mass spectrometry). The isolated fractions were characterized by UV-vis spectroscopy, MALDI mass spectrometry, HPLC, and electron microscopy. Most notably, the separation of Au(38)(SR)(24) and Au(40)(SR)(24) clusters is demonstrated.

Chiral inversion of gold nanoparticles.

The thiolate-for-thiolate ligand exchange was performed on well-defined gold nanoparticles under an inert atmosphere without any modification of the core size. This reaction is faster than the well-known core etching. Surprisingly, if a chiral thiol is exchanged for its opposite enantiomer, the optical activity in the metal-based electronic transitions is reversed although the form of the CD spectra remains largely unchanged. The extent of inversion corresponds to the overall ee of the chiral ligand in the system. This shows that the chiral arrangement of metal atoms in the metal particle (surface) can not withstand the driving force imposed by the ligand of opposite absolute configuration. If the incoming thiol has a different structure, the electronic transitions in the metal core are slightly modified whereas the absorption onset remains unchanged. These results emphasize the influence of the thiols on the structure of the gold nanoparticles and give insight on the ligand exchange pathways.

On the Thermal Conductivity of Gold Nanoparticle Colloids

Nanofluids (colloidal suspensions of nanoparticles) have been reported to display significantly enhanced thermal conductivities relative to those of conventional heat transfer fluids, also at low concentrations well below 1% per volume (Putnam, S. A., et at. J. Appl. Phys. 2006, 99, 084308; Liu, M.-S. L., et al. Int. J. Heat Mass Transfer. 2006, 49; Patel, H. E., et al. Appl. Phys. Lett. 2003, 83, 2931-2933). The purpose of this paper is to evaluate the effect of the particle size, concentration, stabilization method and particle clustering on the thermal conductivity of gold nanofluids. We synthesized spherical gold nanoparticles of different size (from 2 to 45 nm) and prepared stable gold colloids in the range of volume fraction of 0.00025-1%. The colloids were inspected by UV-visible spectroscopy, transmission electron microscope (TEM) and dynamic light scattering (DLS). The thermal conductivity has been measured by the transient hot-wire method (THW) and the steady state parallel plate method (GAP method). Despite a significant search in parameter space no significant anomalous enhancement of thermal conductivity was observed. The highest enhancement in thermal conductivity is 1.4% for 40 nm sized gold particles stabilized by EGMUDE (triethyleneglycolmono-11-mercaptoundecylether) and suspended in water with a particle-concentration of 0.11 vol%.

Fluxionality and low‐lying transition structures of the water trimer

The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and second‐order Mo/ller–Plesset levels, using the 6‐311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen‐bonded O...O...O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational‐tunneling path; neighboring...

A Highly Configurationally Stable [4]Heterohelicenium Cation

Ein konfigurationsstabiles Heterohelicen: Uber die diastereomeren Salze mit dem phosphorhaltigen Anion binphat gelang die Trennung der Enantiomere eines [4](Hetero)heliceniumkations (siehe Bild: N=violett, O=rot). Die absoluten Konfigurationen wurde durch Messung des Schwingungscirculardichroismus bestimmt. Die Energiebarriere fur die Enantiomerisierung der Verbindung ist deutlich groser als beim [6]Helicen.

Separation of Enantiomers and CD Spectra of Au40(SCH2CH2Ph)24: Spectroscopic Evidence for Intrinsic Chirality

Chirality unveiled: Thiolate-protected Au40(SR)24 clusters were enantioenriched using an HPLC approach. CD spectra show strong mirror-image responses, indicating the intrinsic chirality of a cluster of unknown structure protected with achiral ligands.

Accurate hydrogen-bonding energies between 1-naphthol and water, methanol and ammonia

Accurate hydrogen-bond dissociation energies were determined for gas-phase hydrogen-bonded complexes between 1-naphthol or 1-naphthol-d3 and H2O, CH3OH, NH3 and ND3, using the stimulated emission pumping-resonant two-photon ionization spectroscopy technique in supersonic jets. The hydrogen-bond dissociation energies obtained for the electronic ground state are D0 = 2035 ± 69 cm−1 for 1-naphthol · H2O, 2645 ± 136 cm−1 for 1-naphthol · CH3OH, 2680 ± 5cm −1 for 1-naphthol · NH3 and 2801 ± 14 cm−1 for 1-naphthol-d3 · ND3, respectively. Upon electronic excitation to the S1 state the binding energies increase by approximately 8%.