Reactions of Laser-Ablated Copper Atoms with Dioxygen. Infrared Spectra of the Copper Oxides CuO, OCuO, CuOCuO, and OCuOCuO and Superoxide CuOO in Solid Argon

Enhancing the Electrocatalytic Hydrogenation of Furfural via Anion-Induced Molecular Activation and Adsorption

The electrocatalytic hydrogenation (ECH) of furfural (FF) to furfuryl alcohol, which does not require additional hydrogen or high pressure, is a green and promising production route. In this study, we explore the effects of anions on FF ECH in two buffer electrolytes (KHCO3 and phosphate-buffered saline [PBS]). Anions influence the yield of furfuryl alcohol through molecular activation and adsorption. Molecular dynamics simulations show that bicarbonate is present in the first shell layer of the FF molecule and induces strong hydrogen bonding interactions. In contrast, hydrogen phosphate is present only in the second shell layer, resulting in weak hydrogen bonding interactions. Owing to the interfacial anions and hydrogen bonding, FF molecules exhibit strong flat adsorption on the electrode surface in the KHCO3 solution, while weak adsorption is observed in the PBS solution, as confirmed by operando synchrotron-radiation Fourier-transform infrared spectroscopy and in situ Raman spectroscopy. Density-functional theory calculations reveal that the overall anionic hydrogen bonding network promotes the activation of the carbonyl group in the FF molecule in KHCO3, whereas electrophilic activity is inhibited in PBS. Consequently, FF ECH demonstrates much faster kinetics in KHCO3, while it exhibits sluggish ECH kinetics and a severe hydrogen evolution reaction in PBS. This work introduces a new strategy to optimize the catalytic process through the modulation of the microenvironment.

Photoelectron spectra of copper oxide cluster anions from first principles methods

We present results and analyses for the photoelectron spectra of small copper oxide cluster anions (CuO^-, Cu O2- , Cu O3- , and Cu₂O^-). The spectra are computed using various techniques, including density functional theory (DFT) with semi-local, global hybrid, and optimally tuned range-separated hybrid functionals, as well as many-body perturbation theory within the GW approximation based on various DFT starting points. The results are compared with each other and with the available experimental data. We conclude that as in many metal-organic systems, self-interaction errors are a major issue that is mitigated by hybrid functionals. However, these need to be balanced against a strong role of non-dynamical correlation-especially in smaller, more symmetric systems-where errors are alleviated by semi-local functionals. The relative importance of the two phenomena, including practical ways of balancing the two constraints, is discussed in detail.

Metal Cluster Models for Heterogeneous Catalysis: A Matrix-Isolation Perspective

Metal cluster models are of high relevance for establishing new mechanistic concepts for heterogeneous catalysis. The high reactivity and particular selectivity of metal clusters is caused by the wealth of low-lying electronically excited states that are often thermally populated. Thereby the metal clusters are flexible with regard to their electronic structure and can adjust their states to be appropriate for the reaction with a particular substrate. The matrix isolation technique is ideally suited for studying excited state reactivity. The low matrix temperatures (generally 4-40 K) of the noble gas matrix host guarantee that all clusters are in their electronic ground-state (with only a very few exceptions). Electronically excited states can then be selectively populated and their reactivity probed. Unfortunately, a systematic research in this direction has not been made up to date. The purpose of this review is to provide the grounds for a directed approach to understand cluster reactivity through matrix-isolation studies combined with quantum chemical calculations.

Identification of the chromophore in the apatite pigment [Sr10(PO4)6(Cu(x)OH(1-x-y))2]: linear OCuO- featuring a resonance Raman effect, an extreme magnetic anisotropy, and slow spin relaxation

A new chromophore has been identified in copper-doped apatite pigments having the general composition [Sr(10)(PO(4))(6)(Cu(x)OH(1-x-y))(2)], in which x=0.1, 0.3 and y=0.01-0.42. By using X-ray absorption spectroscopy, low-temperature magnetization measurements, and synchrotron X-ray powder structure refinement, it has been shown that the oxygenated compounds contain simultaneously diamagnetic Cu(1+) and paramagnetic Cu(3+) with S=1. Cu(3+) is located at the same crystallographic position as Cu(1+), being linearly coordinated by two oxygen atoms and forming the OCuO(-) anion. The Raman spectroscopy study of [A(10)(PO(4))(6)(Cu(x)OH(1-x-y))(2)], in which A=Ca, Sr, Ba, reveals resonance bands at 651-656 cm(-1) assigned to the symmetric stretching vibration (ν(1)) of OCuO(-). The strontium apatite pigment exhibits a strong paramagnetic anisotropy with an unprecedentedly large negative zero-field splitting parameter (D) of ≈-400 cm(-1). The extreme magnetic anisotropy causes slow magnetization relaxation with relaxation times (τ) up to 0.3 s at T=2 K, which relates the compounds to single-ion magnets. At low temperature, τ is limited by a spin quantum-tunneling, whereas at high temperature a thermally activated relaxation prevails with U(eff)≈48 cm(-1). Strong dependence of τ on the paramagnetic center concentration at low temperature suggests that the spin-spin relaxation dominates in the spin quantum-tunneling process. The compound is the first example of a d-metal-based single-ion magnet with S=1, the smallest spin at which an energy barrier arises for the spin flipping.

Structure characterization of metal oxide clusters by vibrational spectroscopy: possibilities and prospects

This article summarizes the methodological progress that has been made in the vibrational spectroscopy of isolated polynuclear metal oxide clusters, with particular emphasis on free electron laser-based infrared action spectroscopy of gas phase clusters, over the last decade. The possibilities, limitations and prospects of the various experimental approaches are discussed using representative examples from pivotal studies in the field.

Matrix infrared spectroscopic and theoretical of the difluoroamino metal fluoride molecules: F2NMF (M = Cu, Ag, Au)

The difluoroamino coinage metal fluoride molecules F(2)NMF (M = Cu, Ag, Au) have been made via spontaneous reactions of coinage metals and NF(3) in solid argon and neon matrixes during sample annealing without formation of the M(NF(3)) complexes. Comparisons between the matrix infrared spectra and the density functional frequency calculations provide strong support for identification of the F(2)NMF molecules, which are found to have doublet ground states with C(2v) or near C(2v) geometries. The F(2)NCuF molecule can isomerize to the less stable FNCuF(2) isomer upon UV-visible irradiation, while no similar reactions were observed for the silver and gold species. The M-N bonds in the F(2)NMF molecules are stronger than those in the FNMF(2) isomers with the Ag-N bond being longest and weakest in both cases.

Study of cluster anions generated by laser ablation of titanium oxides: a high resolution approach based on Fourier transform ion cyclotron resonance mass spectrometry

Laser ablation of titanium oxides at 355 nm and ion-molecule reactions between [(TiO(2))(x)](-•) cluster anions and H(2)O or O(2) were investigated by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) with an external ion source. The detected anions correspond to [(TiO(2))(x)(H(2)O)(y)OH](-) and [(TiO(2))(x)(H(2)O)(y)O(2)](-•) oxy-hydroxide species with x=1 to 25 and y=1, 2, or 3 and were formed by a two step process: (1) laser ablation, which leads to the formation of [(TiO(2))(x)](-•) cluster anions as was previously reported, and (2) ion-molecule reactions during ion storage. Reactions of some [(TiO(2))(x)](-•) cluster anions with water and dioxygen conducted in the FTICR cell confirm this assessment. Tandem mass spectrometry experiments were also performed in sustained off-resonance irradiation collision-induced dissociation (SORI-CID) mode. Three fragmentation pathways were observed: (1) elimination of water molecules, (2) O(2) loss for radical anions, and (3) fission of the cluster. Density functional theory (DFT) calculations were performed to explain the experimental data.

Density functional calculation of the structure and electronic properties of Cu(n)O(n) (n = 1-8) clusters

Ab initio simulations and calculations were used to study the structures and stabilities of copper oxide clusters, Cu(n)O(n) (n = 1-8). The lowest energy structures of neutral and charged copper oxide clusters were determined using primarily the B3LYP/LANL2DZ model chemistry. For n ≥ 4, the clusters are nonplanar. Selected electronic properties including atomization energies, ionization energies, electron affinities, and Bader charges were calculated and examined as a function of n.

Laser ablation mass spectrometry of inorganic transition metal compounds. Additional knowledge for the understanding of ion formation

Laser ablation of transition-metal oxides have been investigated to better understand the formation processes of inorganic cluster ions. The study of binary oxide mixtures and the relative distribution of the ions produced suggest three salient mechanisms that occur after laser/matter interaction, that function to produce the observed ensemble of ionic species. Molecular recombination reactions, unimolecular dissociation processes, emission of small neutrals, including molecular oxygen from transition-metal oxide samples, or from species expelled in gas phase appear to be a significant mechanism, especially under high laser irradiance conditions. These processes are used to propose a set of pathways to rationalize the envelope of ionic clusters formed under photon bombardment.

Infrared Spectra and Structures of the Coinage Metal Dihydroxide Molecules

Laser-ablated Cu, Ag, and Au atoms react with H2O2 and with H2 + O2 molecules during condensation in excess argon to give four new IR absorptions in each system (O-H stretch, M-O-H bend, O-M-O stretch, and M-O-H deformation modes) that are due to the coinage metal M(OH)2 dihydroxide molecules. Isotopic substitution (D2O2, 18O2, 16O18O, D2, and HD) and comparison with frequencies computed by DFT verify these assignments. The calculations converge to 2B(g) ground electronic state structures with C2h symmetry, 111-117 degrees M-O-H bond angles, and substantial covalent character for these new metal dihydroxide molecules, particularly for Au(OH)2. This is probably due to the high electron affinity of gold owing to the effect of relativity.

A theoretical study of small copper oxide clusters: Cu2Ox (x=1–4)

Density functional theory (DFT) calculations are performed to study Cu2Ox (x = 1 - 4) clusters in their neutral, anionic and cationic states. The ground state structures are obtained and found to exhibit linear or near linear structures, which are different from the two- or three-dimensional ones suggested by the previous theoretical calculations. The calculated electron affinities of the clusters are in good agreement with the experimental ones. The low-lying excited states for the clusters are calculated using time-dependent DFT and used to assign the features in the photoelectron spectra. Our results compare well with the available experimental data.

A density functional study of oxygen activation by unsaturated complexes [M(bipy)(2)](2+), M = Cr and Fe

Density functional theory is used to probe the reaction of O(2) with the unsaturated transition-metal fragments [M(bipy)(2)](2+), M = Cr, Fe. In both cases, calculations indicate that the O(2) molecule is initially trapped as an eta(2)-bound superoxide ion, where the unpaired electron in the out-of-plane pi orbital of O(2) is weakly coupled to those on the trivalent metal ion. In the chromium case, a cis-dioxo Cr(VI) complex is found to be significantly more stable than the superoxo species. The two minima are, however, separated by a large barrier, along with a change in spin state. For the iron analogue, the relative energies of the two minima are reversed, the superoxo complex being the global minimum. The energetics of the O(2) activation processes are consistent with previously reported mass spectrometric experiments, where an adduct, [M(bipy)(2)(O(2))](2+), was detected only for chromium.

Formation of copper oxychloride and reactive oxygen species as causes of uterine injury during copper oxidation of Cu-IUD

The lining of the uterus and cervix might be injured by a variety of oxidation products of Cu in a Cu-IUD, including cuprous ions, dissolved and precipitated cupric ions, and reactive oxygen species such as superoxide radicals, hydrogen peroxide, and hydroxyl radicals. In this study, the human amnious WISH cell line was employed as a model of uterine cells in the presence of copper. The cell viability was decreased by elemental copper, which was alleviated up to 70% by the addition of catalase. The addition of copper oxychloride caused cell death in a dose-dependent manner. Hydroxyl radicals in the presence of copper were determined by the formation of malondialdehyde. Soluble cuprous chloride complexes are formed in the uterus by slowly entering oxygen. The complexes are partly oxidized to insoluble copper oxychloride. which damages the endometrium. Unoxidized cuprous ions migrate to the oxygen-rich cervix and are oxidized to copper oxychloride, causing cervix damage.