Disassociation

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Electron and Hydride Addition to Gold (I) Thiolate Oligomers: Implications for Gold–Thiolate Nanoparticle Growth Mechanisms

Electron and hydride addition to Au(I):SR oligomers is investigated using density functional theory. Cyclic and chain-like clusters are examined in this work. Dissociation to Au– ions and Aun(SR)n+1– chains is observed after 2–4 electrons are added to these systems. The free thiolate (SR–) is rarely produced in this work; dissociation of Au– is preferred over dissociation of SR–. Electron affinities calculated in gas phase, toluene, and water suggest that the electron addition process is unlikely, although it may be possible in polar solvents. In contrast, hydride addition to Au(I):SR oligomers yields free thiols and complexes containing Au–Au bonds, which are plausible intermediates for gold nanoparticle growth. The resulting compounds can react to form larger nanoparticles or undergo further reduction by hydride to yield additional Au–Au bonds

Incremental Binding Energies of Gold (I) and Silver (I) Thiolate Clusters

Density functional theory is used to find incremental fragmentation energy, overall dissociation energy, and average monomer fragmentation energy of cyclic gold(I) thiolate clusters and anionic chain structures of gold(I) and silver(I) thiolate clusters as a measure of the relative stability of these systems. Two different functionals, BP86 and PBE, and two different basis sets, TZP and QZ4P, are employed. Anionic chains are examined with various residue groups including hydrogen, methyl, and phenyl. Hydrogen and methyl are shown to have approximately the same binding energy, which is higher than phenyl. Gold–thiolate clusters are bound more strongly than corresponding silver clusters. Lastly, binding energies are also calculated for pure Au25(SR)18–, Ag25(SR)18–, and mixed Au13(Ag2(SH)3)6– and Ag13(Au2(SH)3)6– nanoparticles

The Golden Pathway to Thiolate-Stabilized Nanoparticles: Following the Formation of Gold (I) Thiolate from Gold (III) Chloride

Pathways for the formation of gold thiolate complexes from gold(III) chloride precursors AuCl4– and AuCl3 are examined. This work demonstrates that two distinct reaction pathways are possible; which pathway is accessible in a given reaction may depend on factors such as the residue group R on the incoming thiol. Density functional theory calculations using the BP86 functional and a polarized triple-ζ basis set show that the pathway resulting in gold(III) reduction is favored for R = methyl. A two-to-one ratio of thiol or thiolate to gold can reduce Au(III) to Au(I), and a three-to-one ratio can lead to polymeric Au(SR) species, which was first suggested by Schaaff et al. J. Phys. Chem. B, 1997, 101, 7885 and later confirmed by Goulet and Lennox J. Am. Chem. Soc., 2010, 132, 9582. Most transition states in the pathways examined here have reasonable barrier heights around 0.3 eV; we find two barrier heights that differ substantially from this which suggest the potential for kinetic control in the first step of thiolate-protected gold nanoparticle growth

Theoretical Examination of Solvent and R Group Dependence in Gold Thiolate Nanoparticle Synthesis

The growth of gold thiolate nanoparticles can be affected by the solvent and the R group on the ligand. In this work, the difference between methanol and benzene solvents as well as the effect of alkyl (methyl) and aromatic (phenyl) thiols on the reaction energies and barrier heights is investigated theoretically. Density functional theory (DFT) calculations using the BP86 functional and a triple ζ polarized basis set show that the overall reaction favors methylthiol over phenylthiol with reaction energies of −0.54 and −0.39 eV in methanol, respectively. At the same level of theory, the methanol solvent is favored over the benzene solvent for reactions forming ions; in benzene, the overall reaction energies for methylthiol and phenylthiol reacting with AuCl4− to form Au(HSR)2+ are 0.37 eV and 0.44 eV, respectively. Methylthiol in methanol also has the lowest barrier heights at about 0.3 eV, whereas phenylthiol has barrier heights around 0.4 eV. Barrier heights in benzene are significantly larger than those in methanol

Prediction of Nonradical Au (0)-Containing Precursors in Nanoparticle Growth Processes

This density functional theory (DFT) investigation examines the formation of nonradical Au(0) species from the reduction of Au(I) species. The Au(I) complexes of interest are AuCl2–, AuBr2–, AuI2–, AuClPH3, and AuCl(H)SCH3(−), which are precursors for gold nanoparticle and cluster formation. Reaction of two of the Au(I) species with a hydride results in ejection of two of the ligands and formation of Au2 with two ligands still attached. AuX2– (where X = Cl, Br, or I) reactions eject two halides and form Au2X22–. AuClL(−) (where L = PH3, HSCH3, or SCH3–) reactions can eject either chloride, HCl, PH3, HSCH3, or SCH3– and form Au(0)L2q– or Au(0)ClLq– (q = 0, 1, 2). The Au2Cl22– complex can further react with AuCl2–, which forms Au3Cl32– and a chloride anion. The new Au3Cl32– species can then react with AuCl2– or Au2Cl22– or with another Au3Cl32–. Larger clusters can be formed from these precursors. In this work, reactions in both methanol and benzene solvents are considered as models for one-phase and two-phase gold nanoparticle growth processes. Overall, this investigation shows how Au(0)-containing species can be formed without assuming the formation of Au(0) atoms (radical species)

Theoretical investigation of the growth mechanism of gold thiolate nanoparticles

Doctor of PhilosophyDepartment of ChemistryChristine M. AikensThis body of work describes a theoretical study of the growth mechanism of gold thiolate nanoparticles from Au(III) as synthesized in the Brust-Schiffrin method. The Au(III) salt can be reduced to form Au(I) by two thiols or a hydride. Depending on the polarity of the solvent, the Au(I) species will either yield rings and anionic chains, remain in isolation, or create an ionic complex with the phase transfer agent. No matter what form the Au(I) species takes, a second reduction must occur to yield Au(0). If the solvent is polar, such as methanol or water, and the Au(I) species is a ring or anionic chain, then a hydride can reduce the structure and create a gold-gold bond and dissociate a thiol from the structure. The gold atoms involved in the gold-gold bond would have a formal Au(0) oxidation state. However if the Au(I) species can be kept from forming rings or chains in the polar solvent or if the system is in a nonpolar solvent, then two Au(I) species in close proximity in the presence of hydride can react to yield a non-radical Au(0) species. The oxidation of bare gold nanoclusters by thiol will also be examined, such as in the case of SMAD-produced gold nanoparticles. In this process, the gold nanoclusters are initially in the Au(0) oxidation state. However the SR-Au-SR “staple” motifs that are known to passivate gold nanoparticles contain Au(I) species. The adsorption of thiol on various sizes of gold clusters in several charge states will be calculated and the mechanism for the oxidation of Au3 and three-dimensional Au12 will be modeled. The rate-limiting step is found to be the thiol hydrogen dissociation onto the gold cluster. Once this dissociation occurs, the hydrogen can move freely around the surface. Finally, Au25(SH)18- will be investigated as a catalyst for selective hydrogenation of α,β-unsaturated aldehyde. The dependence of the energetics of hydrogen gas dissociation on Au25(SH)18- on the functional and Grimme dispersion correction employed will also be examined

The significance of bromide in the Brust–Schiffrin synthesis of thiol protected gold nanoparticles

The mechanism of the two-phase Brust–Schiffrin synthesis of alkane thiol protected metal nanoparticles is known to be highly sensitive to the precursor species and reactant conditions. In this work X-ray absorption spectroscopy is used in conjunction with liquid/liquid electrochemistry to highlight the significance of Br⁻ in the reaction mechanism. The species [AuBr₄]⁻ is shown to be a preferable precursor in the Brust–Schiffrin method as it is more resistant to the formation of Au(I) thiolate species than [AuCl₄]⁻. Previous literature has demonstrated that avoidance of the Au(I) thiolate is critical to achieving a good yield of nanoparticles, as [Au(I)X₂]⁻ species are more readily reduced by NaBH₄. We propose that the observed behavior of [AuBr₄]⁻ species described herein explains the discrepancies in reported behavior present in the literature to date. This new mechanistic understanding should enable nanoparticle synthesis with a higher yield and reduce particle size polydispersity