Gas-phase Clusters Research Articles

Theoretical evaluation on solubility of synthesized task specific ionic liquids in water

Task specific ionic liquids (TSILs) are synthesized and utilized for extraction of different metal atoms from their complexes. Since physico-chemical properties of ionic liquids (ILs) are modified by water (H2O) molecules, less hydrophilic ILs are suitable for the liquid–liquid metal extraction process. Recently, we have synthesized amino acid functionalized imidazolium based TSILs for extraction of rare earth metals. The synthesized ILs have a common functionalized cation [C20H28N3O3]+ and different anions namely, bromide-[Br]− and bis(trifluoromethylsulfonyl)imide-[NTF2]−. In this study, various first principles (Hatree–Fock and density functional theory) and molecular mechanics calculations are done to understand intermolecular interactions between H2O and the synthesized TSILs. The results of gas phase cluster calculations project the difference in solubility of these two TSILs in H2O.

Development of a ReaxFF reactive force field for tetrabutylphosphonium glycinate/CO2 mixtures.

Carbon dioxide interacts with the ionic liquid tetrabutylphosphonium glycinate, [P(C4)4][Gly], through both physical and chemical absorption. We present a parametrization of the ReaxFF force field for this system that accounts for both chemical and physical interactions. The parametrization was developed from an extensive training set including periodic density functional theory (DFT) calculations of reaction pathways between CO2 and the anion [Gly](-) in the condensed phase, condensed-phase molecular dynamics (MD) configurations, gas-phase CO2-anion and CO2-cation interactions, and gas-phase cluster calculations for intra-ion interactions. The optimized ReaxFF parameters capture the essential features of both physical and chemical interactions between CO2 and [P(C4)4][Gly] as compared with experiments, van der Waals-corrected DFT calculations, or, in the case of physical interactions, classical force field calculations. The probability distributions of the distance between C (from CO2) and N (from the anion) and the CO2 bend angles calculated from MD simulations with the optimized ReaxFF force field are in good general agreement with those from DFT-based MD simulations. We predict that the density of CO2/[P(C4)4][Gly] mixtures increases with increasing CO2 concentration up to at least 50 mol % CO2. We attribute the significant increase in density to the small effective volume occupied by chemically bound CO2 in the mixture. The predicted increase in density may be tested experimentally.

CO oxidation catalyzed by single gold atoms supported on aluminum oxide clusters.

The single gold atom doped aluminum oxide clusters AuAl3O3(+), AuAl3O4(+), and AuAl3O5(+) have been prepared and mass-selected to react with CO, O2, and mixtures of CO and O2 in an ion trap reactor under thermal collision conditions. The reactions have been characterized by mass spectrometry with isotopic substitution ((16)O2 → (18)O2) and density functional theory calculations. The AuAl3O5(+) cluster can oxidize two CO molecules consecutively to form AuAl3O4(+) and then AuAl3O3(+), the latter of which can react with one O2 molecule to regenerate AuAl3O5(+). The AuAl3(16)O3(+) ions interact with a mixture of C(16)O and (18)O2 to produce the fully substituted (18)O species AuAl3(18)O3-5(+), which firmly identifies a catalytic cycle for CO oxidation by O2. The oxidation catalysis is driven by electron cycling primarily through making and breaking a gold-aluminum chemical bond. To the best of our knowledge, this is the first identification of catalytic CO oxidation by O2 mediated with gas-phase cluster catalysts with single-noble-metal atoms, which serves as an important step to understand single-atom catalysis at strictly a molecular level.

New developments in first-principles excited-state dynamics simulations: unveiling the solvent specificity of excited anionic cluster relaxation and electron solvation

Charge-transfer-to-solvent excited iodide–polar solvent molecule clusters, [I− (Solv)n]*, have attracted substantial interest over the past 20 years as they can undergo intriguing relaxation processes leading ultimately to the formation of gas-phase molecular analogues of the solvated electron. In this review article, we present a comprehensive overview of the development and application of state-of-the-art first-principles molecular dynamics simulation approaches to understand and interpret the results of femtosecond photoelectron spectroscopy experiments on [I− (Solv)n]* relaxation, which point to a high degree of solvent specificity in the electron solvation dynamics. The intricate molecular details of the [I− (Solv)n]* relaxation process are presented, and by contrasting the relaxation mechanisms of clusters with several different solvents (water, methanol and acetonitrile), the molecular basis of the solvent specificity of electron solvation in [I− (Solv)n]* is uncovered, leading to a more refined view of the manifestation of electron solvation in small gas-phase clusters.

Structure and dynamics of the uranyl tricarbonate complex in aqueous solution: insights from quantum mechanical charge field molecular dynamics.

This investigation presents the characterization of structural and dynamical properties of uranyl tricarbonate in aqueous solution employing an extended hybrid quantum mechanical/molecular mechanical (QM/MM) approach. It is shown that the inclusion of explicit solvent molecules in the quantum chemical treatment is essential to mimic the complex interaction occurring in an aqueous environment. Thus, in contrast to gas phase cluster calculations on a quantum chemical level proposing a 6-fold coordination of the three carbonates, the QMCF MD simulation proposes a 5-fold coordination. An extensive comparison of the simulation results to structural and dynamical data available in the literature was found to be in excellent agreement. Furthermore, this work is the first theoretical study on a quantum chemical level of theory able to observe the conversion of carbonate (CO₃²⁻) to bicarbonate (HCO₃⁻) in the equatorial coordination sphere of the uranyl ion. From a comparison of the free energy ΔG values for the unprotonated educt [UO₂(CO₃)₃]⁴⁻ and the protonated [UO₂(CO₃)₂(HCO₃)]³⁻, it could be concluded that the reaction equilibrium is strongly shifted toward the product state confirming the benignity for the observed protonation reaction. Structural properties and the three-dimensional arrangement of carbonate ligands were analyzed via pair-, three-body, and angular distributions, the dynamical properties were evaluated by hydrogen-bond correlation functions and vibrational power spectra.

40 Atomlu Pd-Co İkili Metal Atom Yığınlarının Yapısal Özelliklerinin İncelenmesi

A detailed theoretical study has been made of 40 atom Pd-Co nanoalloy clusters in gas phase, with the interatomic interactions modelled by Gupta many-body potential. For all compositions of Pd n Co m (n+m=40) clusters, a genetic algortihm has been used to find the lowest energy structures. A number of different structure motifs have been identified based on polyicosahedron. We’ve calculated the energy difference between the Global Minimum and first isomer structures in order to analyse the stability. The other tools we’ve used for analysing stability are excess energy and second difference energy calculations.

Tautomerism and proton transfer in photoionized acetaldehyde and acetaldehyde-water clusters.

Understanding the gas-phase chemistry of acetaldehyde can be challenging because the molecule can assume several tautomeric forms, namely keto, enol and carbene. The two last forms are the most stable ionic forms. Here, insight into the gas-phase cluster ion chemistry of homogeneous acetaldehyde and mixed water-acetaldehyde clusters is provided by mass spectrometry/vacuum ultraviolet photoionization combined with density functional theory calculations. (AA)nH(+) clusters (AA = acetaldehyde) and mixed (AA)nH3O(+) clusters were detected using tunable vacuum ultraviolet photoionization. Barrierless proton transfers were observed during the geometry optimization of the most stable dimer structures and helped to explain the cluster ion chemistry induced by photoionization, namely the formation of deprotonated tautomers and protonated keto tautomers. Water was found to catalyze the keto-enol and keto-carbene isomerizations and facilitate the proton transfer from the ionized enol or carbene part of the cluster to the neutral keto part, resulting in protonated keto structures. The production of protonated keto structures was identified to be the main fragmentation channel following ionization of the homogeneous acetaldehyde cluster and a channel for ionized mixed clusters as well. These findings are significant for a broad range of fields, including current atmospheric models, because acetaldehyde is one of the most prominent organic species in the troposphere and ions play a crucial role in aerosol formation.

Boronyl chemistry: the BO group as a new ligand in gas-phase clusters and synthetic compounds.

Boronyl (BO) is a monovalent σ radical with a robust B≡O triple bond. Although BO/BO(-) are isovalent to CN/CN(-) and CO, the chemistry of boronyl has remained relatively unknown until recently, whereas CN/CN(-) and CO are well-known inorganic ligands. Further analogy may be established for BO versus H or Au ligands, which are all monovalent σ radicals. This Account intends to provide an overview of research activities over the past few years that are relevant to the development of boronyl chemistry, in particular, in size-selected gaseous clusters containing BO. The systems covered herein include transition metal boronyl clusters, carbon boronyl clusters, boron oxide clusters and boron boronyl complexes, the boronyl boroxine, and the first synthetic Pt-BO bulk compound. In these boronyl clusters and compounds, the BO groups show remarkable structural and chemical integrity as a ligand. Among transition metal boronyls, gold monoboronyl clusters Aun(BO)(-) and Aun(BO) (n = 1-3) have been characterized, and they are shown to possess electronic and structural properties similar to the corresponding Au(n+1)(-) and Au(n+1) bare clusters, demonstrating the BO/Au analogy. The Au-B bonding in the Au-BO clusters is highly covalent. A recent advance in boronyl chemistry is the successful synthesis and isolation of the first boronyl compound, trans-[(Cy3P)2BrPt(BO)]. This unique Pt-BO compound and other potential transition metal boronyl compounds may find applications in catalysis and as chemical building blocks. Carbon boronyl clusters versus boron carbonyl clusters is a topic of interest in designing new aromatic complexes. Experimental and theoretical data obtained to date show that carbon boronyl clusters are generally far more stable than their boron carbonyl counterparts, highlighting the potency of boronyl as a ligand in aromatic compounds. Notably, in light of the BO/H analogy, the perfectly hexagonal (CBO)6 cluster is a carbon boronyl analogue of benzene. The BO groups also dominate the structures and bonding of boron oxide clusters and boron boronyl complexes, in which BO groups occupy terminal, bridging, or face-capping positions. The bridging η(2)-BO groups feature three-center two-electron bonds, akin to the BHB τ bonds in boranes. A close isolobal analogy is thus established between boron oxide clusters and boranes, offering vast opportunities for the rational design of novel boron oxide clusters and compounds. Boron boronyl clusters may also serve as molecular models for mechanistic understanding of the combustion of boron and boranes. An effort to tune the B versus O composition in boron oxide clusters leads to the discovery of boronyl boroxine, D3h B3O3(BO)3, an analogue of boroxine and borazine and a new member of the "inorganic benzene" family. Furthermore, a unique concept of π and σ double conjugation is proposed for the first time to elucidate the structures and bonding in the double-chain nanoribbon boron diboronyl clusters, which appear to be inorganic analogues of polyenes, cumulenes, and polyynes. This Account concludes with a brief outlook for the future directions in this emerging and expanding research field.

Self-assembled silver nanoparticles: correlation between structural and surface plasmon resonance properties

We report a facile method for controllable fabrication of high-density silver nanoparticle films with a widely adjustable surface plasmon resonance (SPR) frequency, based on the gas phase cluster beam deposition. On the one hand, we can control the particle size by depositing clusters on silica substrate. Light extinction spectra of the self-assembled Ag nanoparticles with various particle sizes are characterized and show two SPRs, in which a SPR exhibits a redshift from less 400 nm to more than 570 nm with an increase in the particle size, whereas the other shows a slight position shifting. On the other hand, the inter-particle distance of the self-assembled Ag nanoparticles can also be controlled by depositing clusters on silica glass coated with Formvar film, and the SPR wavelength shows a redshift from <400 nm to more than 560 nm, which can be attributed to the increase of the fraction of closely spaced nanoparticle pairs that are near-field coupled with the deposition mass. The size and coverage-dependent SPR properties are also compared with the results from the discrete dipole approximation calculations. The present method of tailoring metallic microstructures could find important applications in plasmonics.

Persistent ion pairing in aqueous hydrochloric acid.

For strong acids, like hydrochloric acid, the complete dissociation into an excess proton and conjugated base as well as the formation of independent solvated charged fragments is assumed. The existence of chloride-hydronium (Cl(-)···H3O(+)) contact ion pairs even in moderate concentration hydrochloric acid (2.5 m) demonstrates that the counterions do not behave merely as spectators. Through comparison of recent extended X-ray absorption fine structure (EXAFS) measurements to state-of-the-art density functional theory (DFT) simulations, we are able to obtain a unique view into the molecular structure of medium-to-high concentrated electrolytes. Here we report that the Cl(-)···H3O(+) contact ion pair structure persists throughout the entire concentration range studied and that these structures differ significantly from moieties studied in microsolvated hydrochloric acid gas phase clusters. Characterizing distinct populations of these ion pairs gives rise to a novel molecular level description of how to view the reaction network for acid dissociation and how it relates to our picture of acid-base equilibria.

Anomalous properties of water predicted by the BK3 model

Recently, we proposed a new model for water [P. T. Kiss and A. Baranyai, J. Chem. Phys. 138, 204507 (2013)]. We presented a detailed description of the development of this classical, polarizable model, and a large number of calculated properties. The model provided excellent estimates for ambient liquid properties and reasonably good results from high-pressure solids to gas-phase clusters. In this paper we present results of extensive calculations for temperature-dependent water anomalies in terms of the pressure. The calculated isobars of the temperature-density and the self-diffusion diagrams provide excellent estimates of the experimental values. The estimated compressibility isobars perfectly match the experimental ones if we shift our numbers by ∼10 K upwards. The calculated pressure-dependent viscosity values are excellent at higher temperatures and qualitatively correct at lower temperatures.

Gas Phase Synthesis of Vanadium Oxide Nanoparticle Films with Temperature Controlled Surface Plasmon Resonance Properties

Vanadium oxide nanoparticles were synthesized with controlled size and dispersity by gas phase cluster beam deposition. The composition of the nanoparticle film is dominated with VO2 nanoparticles. The VO2 nanoparticles undergo a phase transition between the room temperature monoclinic insulator phase and the higher temperature rutile metal phase. In the metallic phase, the VO2 nanoparticles exhibit a strong surface plasmon resonance in the near-IR region from 900nm to the 2000nm, which generates a large enhancement on the extinction coefficient. This plasmon resonance is thermally controlled by the VO2 MIT and can be used to improve the optical switching characteristics of VO2 based devices in the near-IR region.

Theoretical Investigation on Structures, Stabilities, and Hydrolysis Reactions of Small RuO2 Nanoclusters

AbstractThe geometric structures and relative stabilities of both neutral and ionic (RuO2)n (n=1–5) nanoclusters have been systematically investigated using density functional theory calculations. Differently from the crystal structure, ruthenium tends to adopt a 4‐fold coordination with either terminal RuO or bridging RuO bonds in gas phase clusters. Our calculations show that three‐dimensional compact structures are more preferred with the increase of the cluster size. For the low‐lying (RuO2)n clusters, the cluster formation energy per RuO2 unit and the electron affinity are found to increase whereas the ionization potential is computed to decrease with the cluster size. Based on the optimized lower‐energy structures of neutral and cationic (RuO2)n (n=1–5), the hydrolysis reactions of these clusters with water molecules have been further explored. The calculated results indicate that the adsorption of water molecules to the cationic species is much easier than the neutral ones. The overall hydrolysis reaction energies are more exothermic for the cationic clusters than for the neutral clusters. Our calculations indicate that thermodynamically, H2O can be more easily split on the cationic ruthenium oxide clusters than on the neutral clusters.

Dissociation of Gas-Phase Bimetallic Clusters as a Probe of Charge Densities: The Effective Charge of Uranyl

Complementary experimental and computational methods for evaluating relative charge densities of metal cations in gas-phase clusters are presented. Collision-induced dissociation (CID) and/or density functional theory computations were performed on anion clusters of composition MM'A(m+n+1)(-), where the two metal ions have formal charge states M(m+) and M'(n+) and A is an anion, NO3(-), Cl(-), or F(-) in this work. Results for alkaline earth and lanthanide metal ions reveal that cluster CID generally preferentially produces MA(m+1)(-) and neutral M'An if the surface charge density of M is greater than that of M': the metal ion with the higher charge density takes the extra anion. Computed dissociation energies corroborate that dissociation occurs via the lowest energy process. CID of clusters in which one of the two metal ions is uranyl, UO2(2+), shows that the effective charge density of U in uranyl is greater than that of alkaline earths and comparable to that of the late trivalent lanthanides; this is in accord with previous solution results for uranyl, from which an effective charge of 3.2+ was derived.

Photodissociation of (SO2⋯XH) Van der Waals complexes and clusters (XH = C2H2, C2H4, C2H6) excited at 32 040–32 090 cm−1 with formation of HSO2 and X

We studied photodecomposition dynamics of (SO2⋯XH) Van der Waals' (VdW) complexes and clusters in gas phase, with X = C2H, C2H3, and C2H5. SO2 was excited by frequency-doubled radiation of a tunable dye laser and resonance-enhanced multiphoton ionization was used to detect the C2H (m/z 25), C2H3 (m/z 27), and C2H5 (m/z 29) ions by time-of-flight mass spectroscopy. Spectra obtained at higher nozzle pressures (P0 > 2.5 atm) indicate formation of clusters. Detailed studies of the VdW complex structure were carried out by analyzing the rotational structure of the respective action spectra. We also performed ab initio theoretical analysis of structures of the VdW complexes and transitional states leading to photodecomposition. We find that the structure of the transition state is significantly different as compared to the equilibrium ground-state structure of the respective complex. The photodecomposition mechanism depends on the hydrocarbon molecule bound to SO2.

Extraction of light trapped due to total internal reflection using porous high refractive index nanoparticle films.

TiO₂ nanoparticle layers composed of columnar TiO₂ nanoparticle piles separated with nanoscale pores were fabricated on the bottom surface of the hemispherical glass prism by performing gas phase cluster beam deposition at glancing incidence. The porosity as well as the refractive index of the nanoparticle layer was precisely tuned by the incident angle. Effective extraction of the light trapped in the substrate due to total internal reflection with the TiO₂ nanoparticle layers was demonstrated and the extraction efficiency was found to increase with the porosity. An enhanced Rayleigh scattering mechanism, which results from the columnar aggregation of the nanoparticles as well as the strong contrast in the refractive index between pores and TiO₂ nanoparticles in the nanoporous structures, was proposed. The porous TiO₂ nanoparticle coatings were fabricated on the surface of GaN LEDs to enhance their light output. A nearly 92% PL enhancement as well as a 30% EL enhancement was observed. For LED applications, the enhanced light extraction with the TiO₂ nanoparticle porous layers can be a supplement to the microscale texturing process for light extraction enhancement.

Strain sensors based on chromium nanoparticle arrays

High sensitivity strain sensors have been fabricated by depositing chromium (Cr) nanoparticles between silver interdigital electrodes coated on the substrate of a flexible plastic material like polyethylene terephthalate. A gas phase cluster beam deposition system was used for the preparation of the nanoparticle arrays with a well-defined conductance. We demonstrate that the conductance of the nanoparticle arrays is dominated by the quantum transport between the closely spaced nanoparticles. Compared to the traditional metallic foil or semiconductor strain sensors, this kind of strain sensor demonstrates both a higher gauge factor, which can be as large as the order of 100, and a wider dynamic range, with a workable maximum applied strain beyond 3%.

Gd doped Au nanoclusters: Molecular magnets with novel properties

The structural, magnetic, and optical properties of subnanometer AuN and AuN−1Gd1 gas phase clusters (N=2 to 8) are systematically investigated in the framework of (time-dependent) density functional theory, using the B3LYP hybrid exchange correlation functional. The size dependent evolution of the gap between the highest occupied and lowest unoccupied molecular orbitals, the magnetism, and the absorption spectra are studied. The simultaneous appearance of large magnetic moments, significant band gaps, and plasmon resonances in the visible spectral region leads to novel multi-functional nanomaterials for applications in drug delivery, magnetic resonance imaging, and photo-responsive agents.

Sequential Observation of Alkali-halide Gas Phase Clusters in High Resolution TOF-MS and Prediction of Their Structures

Alkali halide clusters are interesting model systems that can provide information about how crystal properties evolve. To study these properties, a high-resolution atmospheric pressure inlet time-of-flight mass spectrometry (APi-TOF-MS) study of the sequential sodium halides series, Cl− (NaCl)n and Br− (NaBr)m, has been reported, and the viability of the APi-TOF-MS equipped with an electrospray ionization source in determining cluster compositions has been demonstrated. The isotopic patterns were well resolved, as n=4 and 7 were determined to be the magic numbers for Cl− (NaCl)n clusters, which were particularly abundant in the mass spectra. A global minimum search based on density functional theory enabled basin hopping yield the most stable structures for the mentioned series. The structures exhibit several distinct motifs which can be roughly categorized as linear chain, rock salt, and hexagonal ring. This work provides an effective way to discover and elucidate the nonstoichiometry sodium halide clusters. These clusters possess very high vertical detachment energies and are generally called as superhalogens, which play important roles in chemistry because they are widely used in the synthesis of new classes of charge-transfer salts.

The Influence of Functionals on Density Functional Theory Calculations of the Properties of Reducible Transition Metal Oxide Catalysts

Reducible transition metal oxides (RTMOs) comprise an important class of catalytic materials that are used for the selective oxidation and electro- and photochemical splitting of water, and as supports for metal nanoparticles. It is, therefore, highly desirable to model the properties of these materials accurately using density functional theory (DFT) in order to understand how oxide structure and performance are related and to guide the search for materials exhibiting superior performance. Unfortunately, accurate description of the structural and electronic properties of RTMOs using DFT has proven particularly challenging. The M06-L density functional, which has been shown to be broadly accurate for calculations of gas phase clusters, has recently become available to researchers carrying out calculations in the solid state, but its performance in determining the properties RTMOs has been little investigated. The aim of this work was to assess the performance of the M06-L functional for describing the structural and electronic properties of a family of RTMOs: MoO2, MoO3, and Bi2Mo3O12. Lattice constants, band gaps, and densities of states calculated using the M06-L functional are compared to those obtained from DFT+U. We have also used the M06-L functional to determine the reaction barrier for propene activation over Bi2Mo3O12, the rate-limiting step in the oxidation of propene to acrolein. We find that while DFT calculations carried out with the M06-L functional are roughly five times more expensive computationally than those performed with DFT+U, the results obtained using the M06-L functional provide sensible results for all properties investigated, while avoiding the necessary trade-off between accurate electronic structure and accurate thermochemistry that occurs in DFT+U.