Optical Spectroscopy of the Au4 + Cluster: The Resolved Vibronic Structure Indicates an Unexpected Isomer

Direct probing of low-energy intra d-band transitions in gas-phase cobalt clusters

The interplay between constituent localized and itinerant electrons of metal clusters defines their physical and chemical properties. In turn, the electronic and geometrical structures are strongly entwined and exhibit strong size-dependent variations. Current understanding of low-energy excited states of metal clusters relies on stand-alone theoretical investigations and few comparisons with measured properties, since direct identification of low-lying states is lacking hitherto. Here, we report on the measurement of low-lying electronic transitions in cationic cobalt clusters using infrared photofragmentation spectroscopy. Broad and size-dependent absorption features were observed within 0.056 - 0.446 eV, well above the energies of the sharp absorption bands caused by cluster vibrations. Complementary time-dependent density functional theory calculations reproduce the main observed absorption features, providing direct evidence that they correspond to transitions between electronic states of mainly d-character, arising from the open d-shells of the Co atoms and the high spin multiplicity of the clusters.

Photochemical properties of a potential interstellar dust precursor: the electronic spectrum of Si3O2. Phys Chem Chem Phys 2023. [PMID: 37365971 DOI: 10.1039/d3cp02693k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Silicon oxide compounds are considered as precursors for silicon-based interstellar dust grains which consist mainly of silica and silicates. Knowledge of their geometric, electronic, optical, and photochemical properties provides crucial input for astrochemical models describing the evolution of dust grains. Herein, we report the optical spectrum of mass-selected Si₃O₂⁺ cations recorded in the 234-709 nm range by means of electronic photodissociation (EPD) in a quadrupole/time-of-flight tandem mass spectrometer coupled to a laser vaporization source. The EPD spectrum is observed predominantly in the lowest-energy fragmentation channel corresponding to Si₂O⁺ (loss of SiO), while the higher-energy Si⁺ channel (loss of Si₂O₂) provides only a minor contribution. The EPD spectrum exhibits two weaker unresolved bands A and B near 26 490 and 34 250 cm^-1 (377.5 and 292 nm) and a strong transition C with a band origin at 36 914 cm^-1 (270.9 nm) which shows vibrational fine structure. Analysis of the EPD spectrum is guided by complementary time-dependent density functional theory (TD-DFT) calculations at the UCAM-B3LYP/cc-pVTZ and UB3LYP/cc-pVTZ levels to determine structures, energies, electronic spectra, and fragmentation energies of the lowest-energy isomers. The cyclic global minimum structure with C_2v symmetry determined previously by infrared spectroscopy can explain the EPD spectrum well, with assignments of bands A-C to transitions from the ²A₁ ground electronic state (D₀) into the 4th, 9th, and 11th excited doublet states (D_4,9,11), respectively. The vibronic fine structure of band C is analyzed by Franck-Condon simulations, which confirm the isomer assignment. Significantly, the presented EPD spectrum of Si₃O₂⁺ corresponds to the first optical spectrum of any polyatomic Si_nO_m⁺ cation.

Free-Energy Landscape and Isomerization Rates of Au4 Clusters at Finite Temperatures

In metallic nanoparticles, the geometry of atomic positions controls the particle's electronic band structure, polarizability, and catalytic properties. Analyzing the structural properties is a complex problem; the structure of an assembled cluster changes from moment to moment due to thermal fluctuations. Conventional structural analyses based on spectroscopy or diffraction cannot determine the instantaneous structure exactly and can merely provide an averaged structure. Molecular simulations offer an opportunity to examine the assembly and evolution of metallic clusters, as the preferred assemblies and conformations can easily be visualized and explored. Here, we utilize the adaptive biasing force algorithm applied to first-principles molecular dynamics to demonstrate the exploration of a relatively simple system, which permits a comprehensive study of the small metal cluster Au₄ in both neutral and charged configurations. Our simulation work offers a quantitative understanding of these clusters' dynamic structure, which is significant for single-site catalytic reactions on metal clusters and provides a starting point for a detailed quantitative understanding of more complex pure metal and alloy clusters' dynamic properties.

An ion mobility mass spectrometer coupled with a cryogenic ion trap for recording electronic spectra of charged, isomer-selected clusters

Infrared and electronic spectra are indispensable for understanding the structural and energetic properties of charged molecules and clusters in the gas phase. However, the presence of isomers can potentially complicate the interpretation of spectra, even if the target molecules or clusters are mass-selected beforehand. Here, we describe an instrument for spectroscopically characterizing charged molecular clusters that have been selected according to both their isomeric form and their mass-to-charge ratio. Cluster ions generated by laser ablation of a solid sample are selected according to their collision cross sections with helium buffer gas using a drift tube ion mobility spectrometer and their mass-to-charge ratio using a quadrupole mass filter. The mobility- and mass-selected target ions are introduced into a cryogenically cooled, three-dimensional quadrupole ion trap where they are thermalized through inelastic collisions with an inert buffer gas (He or He/N₂ mixture). Spectra of the molecular ions are obtained by tagging them with inert atoms or molecules (Ne and N₂), which are dislodged following resonant excitation of an electronic transition, or by photodissociating the cluster itself following absorption of one or more photons. An electronic spectrum is generated by monitoring the charged photofragment yield as a function of wavelength. The capacity of the instrument is illustrated with the resonance-enhanced photodissociation action spectra of carbon clusters (C_n ⁺) and polyacetylene cations (HC_2nH⁺) that have been selected according to the mass-to-charge ratio and collision cross section with He buffer gas and of mass-selected Au₂ ⁺ and Au₂Ag⁺ clusters.

Metal-to-Metal Distance Modulated Au(I)/Ru(II) Cyclophanyl Complexes: Cooperative Effects in Photoredox Catalysis

The modular synthesis of Au(I)/Ru(II) decorated mono- and heterobimetallic complexes with π-conjugated [2.2]paracyclophane is described. [2.2]Paracyclophane serves as a rigid spacer which holds the metal centers in precise spatial orientations and allows metal-to-metal distance modulation. A broad set of architectural arrangements of pseudo -geminal, -ortho, -meta, and -para substitution patterns were employed. Metal-to-metal distance modulation of Au(I)/Ru(II) heterobimetallic complexes and the innate transannular π-communication of the cyclophanyl scaffold provides a promising platform for the investigations of structure-activity relationship and cooperative effects. The Au(I)/Ru(II) heterobimetallic cyclophanyl complexes are stable, easily accessible, and exhibit promising catalytic activity in the visible-light promoted arylative Meyer-Schuster rearrangement.

Near-Infrared Spectrum of the First Excited State of Au2. Chemistry 2021;27:15074-15079. [PMID: 34423877 PMCID: PMC8596823 DOI: 10.1002/chem.202102542]

Au₂⁺ is a simple but crucial model system for understanding the diverse catalytic activity of gold. While the Au₂⁺ ground state (X²Σ_g⁺) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A²Σ_u⁺). Herein, we present the vibrationally resolved electronic spectrum of this state for cold Ar‐tagged Au₂⁺ cations. This exceptionally low‐lying and well isolated A²Σ_(u)⁺←X²Σ_(g)⁺ transition occurs in the near‐infrared range. The observed band origin (5738 cm⁻¹, 1742.9 nm, 0.711 eV) and harmonic Au−Au and Au−Ar stretch frequencies (201 and 133 cm⁻¹) agree surprisingly well with those predicted by standard time‐dependent density functional theory calculations. The linearly bonded Ar tag has little impact on either the geometric or electronic structure of Au₂⁺, because the Au₂⁺⋅⋅⋅Ar bond (∼0.4 eV) is much weaker than the Au−Au bond (∼2 eV). As a result of 6 s←5d excitation of an electron from the antibonding σ_u^* orbital (HOMO‐1) into the bonding σ_g orbital (SOMO), the Au−Au bond contracts substantially (by 0.1 Å).

The Optical Spectrum of Au2. Angew Chem Int Ed Engl 2020;59:21403-21408. [PMID: 32888257 PMCID: PMC7756737 DOI: 10.1002/anie.202011337]

The electronic structure of the Au₂ ⁺ cation is essential for understanding its catalytic activity. We present the optical spectrum of mass-selected Au₂ ⁺ measured via photodissociation spectroscopy. Two vibrationally resolved band systems are observed in the 290-450 nm range (at ca. 440 and ca. 325 nm), which both exhibit rather irregular structure indicative of strong vibronic and spin-orbit coupling. The experimental spectra are compared to high-level quantum-chemical calculations at the CASSCF-MRCI level including spin-orbit coupling. The results demonstrate that the understanding of the electronic structure of this simple, seemingly H₂ ⁺ -like diatomic molecular ion strictly requires multireference and relativistic treatment including spin-orbit effects. The calculations reveal that multiple electronic states contribute to each respective band system. It is shown that popular DFT methods completely fail to describe the complex vibronic pattern of this fundamental diatomic cation.

On the Crucial Role of Isolated Electronic States in the Thermal Reaction of ReC+ with Dihydrogen

Presented here is that isolated, long‐lived electronic states of ReC⁺ serve as the root cause for distinctly different reactivities of this diatomic ion in the thermal activation of dihydrogen. Detailed high‐level quantum chemical calculations support the experimental findings obtained in the highly diluted gas phase using FT‐ICR mass spectrometry. The origin for the existence of these long‐lived excited electronic states and the resulting implications for the varying mechanisms of dihydrogen splitting are addressed.