We report the complete X-ray crystallographic structure as determined through single-crystal X-ray diffraction and a thorough theoretical analysis of the green gold Au30(S-tBu)18.\ud While the structure of Au30S(S-tBu)18 with 19 sulfur atoms has been reported, the crystal structure of Au30(S-tBu)18 without the μ3-sulfur has remained elusive until now, though matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS) data unequivocally show its presence in abundance. The Au30(S-tBu)18 nanomolecule not only is distinct in its crystal structure but also has unique temperature-dependent optical properties. Structure determination allows a rigorous comparison and an excellent agreement with theoretical predictions of structure, stability, and optical response

The nature of the ligands dictates the composition, molecular formulae, atomic structure and the physical properties of thiolate protected gold nanomolecules, Aun(SR)m. In this review, we describe the ligand effect for three classes of thiols namely, aliphatic, AL or aliphatic-like, aromatic, AR, or bulky, BU thiol ligands. The ligand effect is demonstrated using three experimental setups namely: (1) The nanomolecule series obtained by direct synthesis using AL, AR, and BU ligands; (2) Molecular conversion and interconversion between Au38(S-AL)24, Au36(S-AR)24, and Au30(S-BU)18 nanomolecules; and (3) Synthesis of Au38, Au36, and Au30 nanomolecules from one precursor Aun(S-glutathione)m upon reacting with AL, AR, and BU ligands. These nanomolecules possess unique geometric core structure, metal-ligand staple interface, optical and electrochemical properties. The results unequivocally demonstrate that the ligand structure determines the nanomolecules' atomic structure, metal-ligand interface and properties. The direct synthesis approach reveals that AL, AR, and BU ligands form nanomolecules with unique atomic structure and composition. Similarly, the nature of the ligand plays a pivotal role and has a significant impact on the passivated systems such as metal nanoparticles, quantum dots, magnetic nanoparticles and self-assembled monolayers (SAMs). Computational analysis demonstrates and predicts the thermodynamic stability of gold nanomolecules and the importance of ligand-ligand interactions that clearly stands out as a determining factor, especially for species with AL ligands such as Au38(S-AL)24.

Au(SR) is one of the most extensively investigated gold nanomolecules along with Au(SR) and Au(SR). However, so far it has only been prepared using aliphatic-like ligands, where R = -SCH, -SCH and -SCHCHPh. Au(SCHCHPh) when reacted with HSPh undergoes core-size conversion to Au(SPh), and existing literature suggests that Au(SPh) cannot be synthesized. Here, contrary to prevailing knowledge, we demonstrate that Au(SPh) can be prepared if the ligand exchanged conditions are optimized, under delicate conditions, without any formation of Au(SPh). Conclusive evidence is presented in the form of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), electrospray ionization mass spectra (ESI-MS) characterization, and optical spectra of Au(SPh) in a solid glass form showing distinct differences from that of Au(S-aliphatic). Theoretical analysis confirms experimental assignment of the optical spectrum and shows that the stability of Au(SPh) is not negligible with respect to that of its aliphatic analogous, and contains a significant component of ligand-ligand attractive interactions. Thus, while Au(SPh) is stable at RT, it converts to Au(SPh) either on prolonged etching (longer than 2 hours) at RT or when etched at 80 °C.

Gold nanomolecules are atomically precise gold thiolate nanoparticles. They show size-dependent optical and electrochemical properties due to the quantization effects. The optical properties are well explored. However, there is still a void in understanding their electrochemical properties and probing higher oxidation states. We report the 22 electronic states of a plasmonic nanocrystal molecule with 329 gold atoms and 84 phenylethanethiolate ligands in a wide electrochemical potential (∼4 V) window. We provide a comprehensive understanding of the electrochemical properties as a function of size and composition of gold nanomolecules. This report also demonstrates that they behave like quantum capacitors and their capacitance varies linearly with size. The effect of ligand monolayer and core composition on the electrochemical properties was demonstrated using a 144-metal atom system. These results would help us design applications using the gold and alloy nanomolecules in photovoltaics, catalysis, sensors, and energy storage devices.

Characterization of p-mercaptobenzoic acid (p-MBA) protected Au102(p-MBA)44 nanomolecules has been so far limited by its water-soluble ligand system. In this work we report the first synthesis and isolation of thiolate-protected organosoluble Au102(SPh-X)44 nanomolecules via one-phase synthesis. Monodispersity of the nanomolecules was confirmed from matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS), and composition was determined from high-resolution electrospray ionization mass spectrometry (ESI-MS). For the first time we report the electrochemical behavior and temperature-dependent optical spectra of Au102(SPh)44. Theoretical simulations on the titled nanomolecules fully validate experimental data and demonstrate the role of electronic conjugation on optical properties.