Electronic Structure Calculations of Monolayer–Protected Gold NanoClusters to Aid Spectroscopic Analysis

The underlying goal of this thesis dissertation is to simulate small monolayer protected clusters (MPCs) via quantum mechanics and molecular mechanics calculations to derive electronic, structural, and dynamical properties. First we specifically focus on monolayers consisting of thiolated oligopeptides and alkanethiols. We construct MPCs with a layer of oligopeptides consisting of α-aminoisobutyric acid units. We use Monte Carlo simulations to determine peptide arrangements and folding. We then perform NMR chemical shifts calculations and compare to experimental measurements to validate the predicted structure. The native -1 oxidation state can be oxidized into 0 and to +1 charge states. The neutral charge state is paramagnetic and behaves quite differently than the anion as manifested by the NMR spectra. The objective is to calculate NMR chemical shits, compare to experiment, and obtain insight into the electronic origin of these shifts. We also use DFT to simulate ENDOR spectra. Finally, we investigate the frontier molecular orbital arrangement in different charge states and determine the HOMO – LUMO energy gap and identify its fingerprint in the optical absorption spectra

Interplay of Charge State, Lability, and Magnetism in the Molecule-like Au₂₅(SR)₁₈ Cluster

Au₂₅(SR)₁₈ (R = −CH₂–CH₂–Ph) is a molecule-like nanocluster displaying distinct electrochemical and optical features. Although it is often taken as an example of a particularly well-understood cluster, very recent literature has provided a quite unclear or even a controversial description of its properties. We prepared monodisperse Au₂₅(SR)₁₈⁰ and studied by cyclic voltammetry, under particularly controlled conditions, the kinetics of its reduction or oxidation to a series of charge states, −2, −1, +1, +2, and +3. For each electrode process, we determined the standard heterogeneous electron-transfer (ET) rate constants and the reorganization energies. The latter points to a relatively large inner reorganization. Reduction to form Au₂₅(SR)₁₈^2– and oxidation to form Au₂₅(SR)₁₈²⁺ and Au₂₅(SR)₁₈³⁺ are chemically irreversible. The corresponding decay rate constants and lifetimes are incompatible with interpretations of very recent literature reports. The problem of how ET affects the Au₂₅ magnetism was addressed by comparing the continuous-wave electron paramagnetic resonance (cw-EPR) behaviors of radical Au₂₅(SR)₁₈⁰ and its oxidation product, Au₂₅(SR)₁₈⁺. As opposed to recent experimental and computational results, our study provides compelling evidence that the latter is a diamagnetic species. The DFT-computed optical absorption spectra and density of states of the −1, 0, and +1 charge states nicely reproduced the experimentally estimated dependence of the HOMO–LUMO energy gap on the actual charge carried by the cluster. The conclusions about the magnetism of the 0 and +1 charge states were also reproduced, stressing that the three HOMOs are not virtually degenerate as routinely assumed: In particular, the splitting of the HOMO manifold in the cation species is severe, suggesting that the usefulness of the superatom interpretation is limited. The electrochemical, EPR, and computational results thus provide a self-consistent picture of the properties of Au₂₅(SR)₁₈ as a function of its charge state and may furnish a methodology blueprint for understanding the redox and magnetic behaviors of similar molecule-like gold nanoclusters

Au25(SEt)18, a Nearly Naked Thiolate-Protected Au25Cluster: Structural Analysis by Single Crystal X-ray Crystallography and Electron Nuclear Double Resonance

X-ray crystallography has been fundamental in discovering fine structural features of ultrasmall gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. We prepared a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochemistry, mass spectrometry, and UV-vis absorption spectroscopy features of similar Au25 clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au25(SEt)18 were fully characterized by 1H and 13C NMR spectroscopy, which confirmed the monolayer's properties and the paramagnetism of neutral Au 25(SEt)18 0. X-ray crystallography analysis of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. Here, we also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact molecular structure and having used such a small ligand allowed us to compare the experimental values of hyperfine couplings with DFT calculations unaffected by structure's approximations or omissions

Au₂₅(SEt)₁₈, a Nearly Naked Thiolate-Protected Au₂₅ Cluster: Structural Analysis by Single Crystal X‑ray Crystallography and Electron Nuclear Double Resonance

X-ray crystallography has been fundamental in discovering fine structural features of ultrasmall gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. We prepared a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochemistry, mass spectrometry, and UV–vis absorption spectroscopy features of similar Au₂₅ clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au₂₅(SEt)₁₈ were fully characterized by ¹H and ¹³C NMR spectroscopy, which confirmed the monolayer’s properties and the paramagnetism of neutral Au₂₅(SEt)₁₈⁰. X-ray crystallography analysis of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. Here, we also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact molecular structure and having used such a small ligand allowed us to compare the experimental values of hyperfine couplings with DFT calculations unaffected by structure’s approximations or omissions

Au₂₅(SEt)₁₈, a Nearly Naked Thiolate-Protected Au₂₅ Cluster: Structural Analysis by Single Crystal X‑ray Crystallography and Electron Nuclear Double Resonance

X-ray crystallography has been fundamental in discovering fine structural features of ultrasmall gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. We prepared a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochemistry, mass spectrometry, and UV–vis absorption spectroscopy features of similar Au₂₅ clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au₂₅(SEt)₁₈ were fully characterized by ¹H and ¹³C NMR spectroscopy, which confirmed the monolayer’s properties and the paramagnetism of neutral Au₂₅(SEt)₁₈⁰. X-ray crystallography analysis of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. Here, we also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact molecular structure and having used such a small ligand allowed us to compare the experimental values of hyperfine couplings with DFT calculations unaffected by structure’s approximations or omissions

Interplay of Charge State, Lability, and Magnetism in the Molecule-like Au25(SR)18Cluster

none5Au25(SR)18 (R = -CH2-CH2-Ph) is a molecule-like nanocluster displaying distinct electrochemical and optical features. Although it is often taken as an example of a particularly well-understood cluster, very recent literature has provided a quite unclear or even a controversial description of its properties. We prepared monodisperse Au25(SR)18 0 and studied by cyclic voltammetry, under particularly controlled conditions, the kinetics of its reduction or oxidation to a series of charge states, -2, -1, +1, +2, and +3. For each electrode process, we determined the standard heterogeneous electron-transfer (ET) rate constants and the reorganization energies. The latter points to a relatively large inner reorganization. Reduction to form Au25(SR) 18 2- and oxidation to form Au25(SR) 18 2+ and Au25(SR)18 3+ are chemically irreversible. The corresponding decay rate constants and lifetimes are incompatible with interpretations of very recent literature reports. The problem of how ET affects the Au25 magnetism was addressed by comparing the continuous-wave electron paramagnetic resonance (cw-EPR) behaviors of radical Au25(SR)18 0 and its oxidation product, Au25(SR)18 +. As opposed to recent experimental and computational results, our study provides compelling evidence that the latter is a diamagnetic species. The DFT-computed optical absorption spectra and density of states of the -1, 0, and +1 charge states nicely reproduced the experimentally estimated dependence of the HOMO-LUMO energy gap on the actual charge carried by the cluster. The conclusions about the magnetism of the 0 and +1 charge states were also reproduced, stressing that the three HOMOs are not virtually degenerate as routinely assumed: In particular, the splitting of the HOMO manifold in the cation species is severe, suggesting that the usefulness of the superatom interpretation is limited. The electrochemical, EPR, and computational results thus provide a self-consistent picture of the properties of Au25(SR)18 as a function of its charge state and may furnish a methodology blueprint for understanding the redox and magnetic behaviors of similar molecule-like gold nanoclusters.noneSabrina Antonello;Neranjan V. Perera;Marco Ruzzi;José A. Gascón;Flavio MaranAntonello, Sabrina; Neranjan V., Perera; Ruzzi, Marco; José A., Gascón; Maran, Flavi