Chemisorption of O and O2 on Ag(110): An LCGTO-LSD cluster study

As part of a study aimed at better understanding of molecular and dissociative chemisorption of oxygen on Ag(110), linear combinations of Gaussian type orbitals-local spin density (LCGTO -LSD ) calculations have been performed for O, O^?, O₂, O^?₂, O^2?₂ and a variety of silver clusters interacting with O or O₂. For atomic O adsorption a very small cluster, Ag₄, chosen to model the long-bridge site already affords very good agreement with both recent EXAFS experiments and recent ab initio calculations. We calculate O to be 0.25 Å above the surface (exp. 0.2 Å). The Ag₄? O vibrational frequency is estimated to be 400 cm^?1, in reasonable accord with the experimental EELS value of 325 cm^?1. Determination of the geometry for O₂ (ads.) and, ultimately, of the dissociation path are far more difficult tasks. An extensive search for local minima in the vicinity of the LB site is being carried out. Results to date for small, Ag₂ and Ag₄, clusters have furnished insight into the factors influencing the structure. Overlap between the π* orbital of the O₂ moiety and Ag s functions is a key factor; that is, there is an important covalent component of the binding. For geometries with O₂ parallel to the surface, this is achieved by twisting the O₂ fragment with respect to the [11¯0] grooves (geometries either parallel or perpendicular to the grooves yield zero π‖*?s overlap by symmetry). The structure with O₂ perpendicular to the surface also achieves reasonable overlap and lies close in energy to the most stable ‘parallel’ geometry.     

Co‐Adsorption of O2 and CO on Au2−: Infrared Photodissociation Spectroscopy and Theoretical Study of [Au2O2(CO)n]− (n=2–6)

The co‐adsorption of O₂ and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au₂O₂(CO)_n]^? (n=2–6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au₂O₂(CO)_n]^? (n=2–6) complexes were characterized to have a core structure involving one CO and one O₂ molecule co‐adsorbed on Au₂^? with the other CO molecules physically tagged around. The CO stretching frequency of the [Au₂O₂(CO)]^? core ion is observed around =2032–2042 cm^?1, which is about 200 cm^?1 higher than that in [Au₂(CO)₂]^?. This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O₂ and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co‐adsorbed O₂ and CO on Au₂^? may have relevance to the elementary reaction steps on real gold catalysts.     

Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO2(H2O)n−, n=1–10, in the C−O Stretch Region

We investigate anionic [Co,CO₂,nH₂O]⁻ clusters as model systems for the electrochemical activation of CO₂ by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250–2234 cm⁻¹ using an FT-ICR mass spectrometer. We show that both CO₂ and H₂O are activated in a significant fraction of the [Co,CO₂,H₂O]⁻ clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C−O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C−O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)₂⁻. However, calculations find Co(HCOO)(OH)⁻ as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590–1730 cm⁻¹ are most likely due to higher lying isomers of the composition Co(HOCO)(OH)⁻. Upon additional hydration, all species [Co,CO₂,nH₂O]⁻, n≥2, undergo IRMPD through loss of H₂O molecules as a relatively weakly bound messenger. The main spectral features are the C−O stretching mode of the CO ligand around 1900 cm⁻¹, the water bending mode mixed with the antisymmetric C−O stretching mode of the HCOO⁻ ligand around 1580–1730 cm⁻¹, and the symmetric C−O stretching mode of the HCOO⁻ ligand around 1300 cm⁻¹. A weak feature above 2000 cm⁻¹ is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.     

Formation mechanism of nanostructured Ag films from Ag2O particles using a sonoprocess

Nanostructured Ag films composed of nanoparticles and nanorods can be formed by the ultrasonication of ethanol solutions containing Ag₂O particles. The present work examined the formation process of these films from ethanol solutions by two different agitation methods, including ultrasonication and mechanical stirring. The mass-transfer process from Ag₂O particles to ethanol solvent is accelerated by the mechanical effects of ultrasound. Ag⁺ ions and intermediately reduced Ag clusters were released into the ethanol. These Ag⁺ ions and Ag clusters provide absorption bands at 210, 275 and 300 nm in UV-vis spectra. These bands were assigned to the absorption of Ag⁺, Ag ₄ ²⁺ and Ag_n (n?≈?3). The Ag_n clusters that readily grow to become Ag nanoparticles were formed due to the surface reaction of Ag₂O particles with ethanol under ultrasonication. The reactions of Ag⁺ ions in ethanol to form Ag nanomaterials (through the formation of Ag ₄ ²⁺ clusters) were also accelerated by ultrasonication.     

Altering Charges on Heterobimetallic Transition-Metal Carbonyl Clusters

The homoleptic group 5 carbonylates [M(CO)₆]⁻ (M=Nb, Ta) serve as ligands in carbonyl-terminated heterobimetallic Ag_mM_n clusters containing 3 to 11 metal atoms. Based on our serendipitous [Ag₆{Nb(CO)₆}₄]²⁺ ( 4 a ²⁺) precedent, we established access to such Ag_mM_n clusters of the composition [Ag_m{M(CO)₆}_n]^x (M=Nb, Ta; m=1, 2, 6; n=2, 3, 4, 5; x=1−, 1+, 2+). Salts of those molecular cluster ions were synthesized by the reaction of [NEt₄][M(CO)₆] and Ag[Al(OR^F)₄] (R^F=C(CF₃)₃) in the correct stoichiometry in 1,2,3,4-tetrafluorobenzene at −35 °C. The solid-state structures were determined by single-crystal X-ray diffraction methods and, owing to the thermal instability of the clusters, a limited scope of spectroscopic methods. In addition, DFT-based AIM calculations were performed to provide an understanding of the bonding within these clusters. Apparently, the clusters 3 ⁺ (m=6, n=5) and 4 ²⁺ (m=6, n=4) are superatom complexes with trigonal-prismatic or octahedral Ag₆ superatom cores. The [M(CO)₆]⁻ ions then bind through three CO units as tridentate chelate ligands to the superatom core, giving overall structures related to tetrahedral AX₄ ( 4 ²⁺) or trigonal bipyramidal AX₅ molecules ( 3 ⁺).     

Mechanism of silver nucleation upon the radiation-induced reduction of its ions in polyphosphate-containing aqueous solutions

Long-lived (hours to days) silver clusters, Ag ₄ ²⁺ , Ag ₄ ⁺ , Ag ₈ ²⁺ , etc., are formed upon the radiation-induced reduction of Ag⁺ ions in aqueous solutions containing sodium polyphosphate. The efficiency of the cluster formation decreases and the stability of the clusters increase with a rise in the concentration of the polymeric stabilizer. In the course of the aggregation of clusters, their sizes increase, quasi-metallic particles emerge, and the process terminates with the formation of silver nanoparticles. The mechanism of silver nucleation upon the radiation-induced reduction of silver ions in aqueous solutions is discussed.     

Redox properties of (RC2R')Co2(CO)6-nLn and reactivity of (RC2R')Co2(CO)6-nLn towards PPh3[LPPh3, P(OEt)3]

Cyclic voltammetric studies of clusters (C₅H₅-C₂C₆ H₄-R-p)Co₂(CO)6-n L_n[n=0,2; L=PPh₃, P(OEt)₃] and (RCH₂C)₂Co₂(CO₄) (PPh₃)₂ on Pt electrode are described. The primary reduction (0 / ?1) and oxidation (+ 1 / 0) steps are considered as a mono-electron process for all clusters. For the clusters (C₅H₅C₂C₆H₄-R-p)Co₂(CO)₆, a good linear relation between reduction potential E_p^red and Hammett constant σ_p of R in the clusters is found. For the clusters (RC₂R')Co₂(CO)₄L₂, their radical anions are extremely unstable at room temperature and fragment into a series of mononuclear species, one of which is (RC₂R')Co(CO)₂PPh₃. The reaction of radical anions of (RC₂R')Co₂(CO)_6–n (PPh₃)_n(n=0,2) with PPh₃ also produces mononuclear species (RC₂R')Co(CO)₂PPh₃ which has been detected by means of cyclic voltammetry and ESR. The influence of R on redox properties of clusters is discussed.     

Ag[Fe(CO)5]2+: A Bare Silver Complex with Fe(CO)5 as a Ligand

Attempts to prepare Fe(CO)₅⁺ from Ag[Al(OR^F)₄] (R^F=C(CF₃)₃) and Fe(CO)₅ in CH₂Cl₂ yielded the first complex of a neutral metal carbonyl bound to a simple metal cation. The Ag[Fe(CO)₅]₂⁺ cation consists of two Fe(CO)₅ molecules coordinating Ag⁺ in an almost linear fashion. The ν(CO) modes are blue‐shifted compared to Fe(CO)₅, with one band above 2143 cm^?1 indicating that back‐bonding is heavily decreased in the Ag[Fe(CO)₅]₂⁺ cation.     

Multiple ionization and Coulomb explosion of mercury clusters in femtosecond laser fields

We report on studies of multiple ionization and fragmentation of free Hg_n (n ≤ 80) clusters in the femtosecond time domain at wavelengths ranging from 255 nm to 800 nm. After excitation by single laser pulses of an intensity of 5 * 10¹¹ W/cm² we observe prompt formation of multiply charged Hg_n clusters. The Hg_n cluster size distribution observed up to n ≈ 80 shows in additon to singly charged also doubly and triply charged clusters with a surprisingly high amount of doubly charged clusters. The measured cluster size distribution is nearly independent of laser wavelengths. For higher laser intensities (2 * 10¹² W/cm²) we observe multiply charged mercury atoms up to Hg⁵⁺. At 10¹³ W/cm² molecules and clusters eventually disappear due to Coulomb explosion and complete Fragmentation. Only atomic ions, singly and multiply charged, with high kinetic energies are then observed.     

Infrared Photodissociation Spectroscopic and Theoretical Study of the HC2nO+ (n=3-6) Cations

The carbon chain cations, HC_2nO⁺ (n=3-6) are produced via a pulsed laser vaporization supersonic expansion ion source in the gas phase. Their infrared spectra are measured via mass-selected infrared photodissociation spectroscopy of the CO “tagged” [HC_2nO·CO]⁺ cation complexes in 1600-3500 cm^-1 frequency range. The geometric and electronic structures of the [HC_2nO·CO]⁺ complexes and the core HC_2nO⁺ (n=3-6) cations are determined with the aid of density functional theory calculations. These HC_2nO⁺(n=3-6) ions are identified to be linear carbon chain derivatives terminally capped by hydrogen and oxygen. The triplet ground states are 10-15 kcal/mol lower in energy than the singlet states, indicating cumulene-like carbon chain structures.     

Stability of multiply charged silver clusters

Using the gasaggregation technique it is possible to generate metal clusters in narrow size distributions and to vary their mean size by adjusting the cell parameters. The high intensity of this source allows to detect besides singly charged clusters also multiply charged ones. Ag _n ²⁺ and Ag _n ³⁺ are observed forn≧9 andn≧31, respectively; i.e. at values well below the critical sizes reported for spheres.     

Synthesis and X-Ray Crystal Structures of Fe2Ru(CO)12−n (CNBu t ) n and FeRu2(CO)12−n (CNBu t ) n (n=1, 2)

The clusters Fe₂Ru(CO)_12–n (CNBu ^t ) _n (3, n=1; 4, n=2), FeRu₂(CO)_12–n (CNBu ^t ) _n (5, n=1, 6, n=2) and FeRu₂(CO)₁₁(CNCy) (5a) have been prepared by direct substitution from the parent carbonyl precursors Fe₂Ru(CO)₁₂ (1) and FeRu₂(CO)₁₂ (2). All compounds have been characterized spectroscopically and clusters 3, 4, 5, and 6 by single crystal X-ray determinations. In all cases, the isonitrile ligands adopt axial or pseudo-axial positions on a ruthenium atom. The structures of 3–5 are very similar to their parent clusters, but the extent of metal framework disorder is significantly less. Cluster 6 adopts the same C _2v Fe₃(CO)₁₂ type structure as 4, and thus differs markedly from the parent compound 2, which has a D ₃ structure .     

Spectral characteristics of aqueous dispersions enriched in oxygen

The IR absorption and reflection spectra of aqueous dispersions consisting of (H₂O)_n, O₂(H₂O)_n, and (O₂)₂(H₂O)_n clusters (10 ≤ n ≤ 50) were calculated by the method of molecular dynamics using a flexible model of molecules. The frequency distribution of the power scattered by the cluster systems was obtained in the range 0 ≤ ω ≤ 3000 cm^?1. The capture of one oxygen molecule by the clusters is accompanied by a decrease in the absorption of the low-frequency IR radiation and by a peak of the absorption intensity in the vicinity of ω 2704 cm^?1. This is also accompanied by a decrease in the reflection coefficient throughout the frequency range and a decrease in the emission power at ω < 1030 and ω > 1700 cm^?1. Addition of two oxygen molecules to the clusters decreases the capability of the dispersions for the absorption, reflection, and scattering of IR radiation.     

An STM Study of Chemically Deposited Silver Nanoclusters on Mixed Self-Assembled Monolayers

Small silver clusters Ag _n (primarily probably Ag₄ clusters which aggregate to Ag_n (400<n<2000)) are generated in the immediate vicinity of a four-electron reducing agent (based on hydroquinone) which is incorporated in a monolayer of long-chain alkanethiols. The hydroquinone derivative is oxidized to quinone (see the picture). Molecularly resolved scanning tunneling microscopy (STM) images were obtained of self-assembled monolayers with and without silver clusters.     

Mass-spectrometric study of the formation of silver nanoclusters in polyethers: I. Laser desorption/ionization

Within the framework of a problem of the synthesis of silver nanoparticles and Ag _n nanoclusters in polyethers, the systems containing silver nitrate AgNO₃ and the low-molecular-weight polyethers poly(ethylene glycol) PEG-400 and oxyethylated glycerol OEG-5, in which silver ions were reduced, were studied by laser desorption/ionization mass spectrometry. The occurrence of Ag _n silver nanoclusters with n up to 35 in the systems was detected. For n > 2, the presence of ??magic numbers?? was observed; that is, positively charged Ag _n ⁺ clusters with predominantly odd values of n were detected. Negatively charged Ag _n clusters with n = 1?C3 were also detected. It was shown that one of the expected processes, namely, the formation of the stable clusters of polyether oligomers (M _m ) with the silver cation M _m · Ag⁺, took place in the test systems.     

In situ infrared and EXAFS studies of an Ag cluster in zeolite X

The in situ measurements of infrared spectra and the Ag K-edge EXAFS spectra of the fully Ag⁺ exchanged zeolite X (Ag₈₆–X) were carried out from room temperature to 300 °C under vacuum. By evacuation at room temperature the O–H stretch vibration (ν(O–H)) mode around 3 μm disappears and the coordination number of oxygen around Ag, N_Ag–O, decreases due to removal of water molecules. The T–O asymmetric stretch (ν_as(T–O)) mode associated with zeolite framework oxygen appears around 10 μm. These infrared spectra are fitted by summing up Gaussian peaks. The positions of the main two peaks are 1000 and 1100 cm⁻¹ at room temperature. At 100 °C, a third infrared peak appears at around 955 cm⁻¹, the total N_Ag–O becomes small and the coordination number of Ag around Ag, N_Ag–Ag, is 0.5. These results suggest that Ag atoms change sites in the zeolite and play an important role as a precursor of the Ag clusters. At 300 °C, the peaks around 1000 and 1100 cm⁻¹ shift to 1050 and 1140 cm⁻¹, respectively, and N_Ag–Ag becomes 2.9, which indicates that the Ag clusters attached to the zeolite framework are stabilized at high temperature. When the zeolite with Ag clusters is exposed to atmosphere, it is found that: (1) the ν(O–H) mode around 3 μm appears again, (2) there are two main peaks (1000 and 1100 cm⁻¹) and a small peak around 856 cm⁻¹ and (3) the local structure of the Ag clusters formed at 300 °C never reverses.     

Carbon Monoxide Bonding With BeO and BeCO3: Surprisingly High CO Stretching Frequency of OCBeCO3

The complexes OCBeCO₃ and COBeCO₃ have been isolated in a low‐temperature neon matrix. The more stable isomer OCBeCO₃ has a very high C? O stretching mode of 2263 cm^?1, which is blue‐shifted by 122 cm^?1 with respect to free CO and 79 cm^?1 higher than in OCBeO. Bonding analysis of the complexes shows that OCBeO has a stronger OC? BeY bond than OCBeCO₃ because it encounters stronger π backdonation. The isomers COBeCO₃ and COBeO exhibit red‐shifted C? O stretching modes with respect to free CO. The inverse change of C? O stretching frequency in OC? BeY and CO? BeY is explained with the reversed polarization of the σ and π bonds in CO.     

Controlling the Microstructure of Conjugated Polymers in High‐Mobility Monolayer Transistors via the Dissolution Temperature

It remains a challenge to precisely tailor the morphology of polymer monolayers to control charge transport. Herein, the effect of the dissolution temperature (T_dis) is investigated as a powerful strategy for morphology control. Low T_dis values cause extended polymer aggregation in solution and induce larger nanofibrils in a monolayer network with more pronounced π–π stacking. The field‐effect mobility of the corresponding monolayer transistors is significantly enhanced by a factor of four compared to devices obtained from high T_dis with a value approaching 1 cm² V^?1 s^?1. Besides that, the solution kinetics reveal a higher growth rate of aggregates at low T_dis, and filtration experiments further confirm that the dependence of the fibril width in monolayers on T_dis is consistent with the aggregate size in solution. The generalizability of the T_dis effect on polymer aggregation is demonstrated using three other conjugated polymer systems. These results open new avenues for the precise control of polymer aggregation for high‐mobility monolayer transistors.     

IR-photodissociation of size selected molecular clusters and their structures

Infrared photodissociation spectra of (CH₃NH₂) _n clusters were measured fromn=2 ton=6 near the monomer absorption of the C-N stretching mode at 1044 cm^?1 using a cw-CO₂ laser. The clusters were size-selected by scattering from a helium beam. The spectrum of cold dimers shows a red (1038 cm^?1) and a blue (1048 cm^?1) shifted peak which is attributed to the non-equivalent position of the C-N in the open dimer structure. The larger clusters exhibit only one peak between 1045.4 cm^?1 and 1046.0 cm^?1 caused by the equivalent position of the C-N in the cyclic structures of the larger clusters. Structure calculations confirm these results. Secondly, the mixed complexes C₂H₄-CH₃COCH₃ and C₂H₄-(CH₃COCH₃)₂ were investigated. The dimer spectrum, measured around the monomer frequency of the out-of-plane bending mode of C₂H₄ at 949 cm^?1, shows two peaks at 946.2 cm^?1 and 961.3 cm^?1. This splitting is attributed to two different isomers that are found in configuration calculations. A similar behaviour is found for the trimer.     

Controlling the Microstructure of Conjugated Polymers in High-Mobility Monolayer Transistors via the Dissolution Temperature

It remains a challenge to precisely tailor the morphology of polymer monolayers to control charge transport. Herein, the effect of the dissolution temperature (T_dis) is investigated as a powerful strategy for morphology control. Low T_dis values cause extended polymer aggregation in solution and induce larger nanofibrils in a monolayer network with more pronounced π–π stacking. The field-effect mobility of the corresponding monolayer transistors is significantly enhanced by a factor of four compared to devices obtained from high T_dis with a value approaching 1 cm² V⁻¹ s⁻¹. Besides that, the solution kinetics reveal a higher growth rate of aggregates at low T_dis, and filtration experiments further confirm that the dependence of the fibril width in monolayers on T_dis is consistent with the aggregate size in solution. The generalizability of the T_dis effect on polymer aggregation is demonstrated using three other conjugated polymer systems. These results open new avenues for the precise control of polymer aggregation for high-mobility monolayer transistors.