Neutral Clusters Research Articles

DFT Study of the Reaction Mechanism of N2O Decomposition on Au3+/0/− Clusters

AbstractIn this paper, the reaction mechanism of Au3+/0/− clusters with N2O was studied by density functional theory (DFT) calculations. The analysis of the potential energy surfaces showed that the Au3 neutral cluster exhibited highest catalytic activity on the decomposition of N2O, energy barrier is only 11.60 kcal/mol. The corresponding energy barriers for Au3− and Au3+ are 28.51 and 58.79 kcal/mol, respectively. The effects of Au3+/0/− clusters assistance analyzed using the activation strain model indicated that the dissociation of the N−O bond depends on the interaction energy of Au3 clusters and N2O. The electron transfer from the Au3+/0/− cluster to the N2O facilitates the dissociation of N−O bond. The analysis of frontier molecular orbitals (FMO) indicated that only the interaction of HOMO‐LUMO interactions is strongly enough and there is sufficient orbital overlap between the Au3+/0/− clusters and N2O, electron transfer can occur and activation of N2O can be achieved. The study of thermodynamic processes showed that there is evident correlation between the binding energy of the Au3O+/0/− cluster oxides and the barrier energy of the reaction. Therefore, the orbital interactions between Au3+/0/− and N2O and thermodynamic driving force have a great influence on the reaction. These results enrich our understanding of the catalytic dissociation of N2O by Au‐cluster‐based catalysts.

Experimental and calculated second harmonic generation of a guanidinium salt in the solid state

A new salt containing both dipolar and octupolar molecules has been synthesized and its structural features fully revealed, namely, its noncentrosymmetry. The microscopic optical properties of a neutral cluster were calculated probing several functionals; a better performance was observed for functionals with high percentage of Hartree-Fock exchange. The vector and septor irreducible components, for individual ions, were also calculated in order to evaluate their ratio and the dipolar/octupolar character. The optical properties of the solid state were computed, using the oriented gas model and two different local field factors. For the SHG generation, the calculated value compares well with the experimental value measured for the powder form, of the order of magnitude of the urea signal.

Neutral Sulfuric Acid-Water Clustering Rates: Bridging the Gap between Molecular Simulation and Experiment.

The role of sulfuric acid during atmospheric new particle formation is an ongoing topic of discussion. In this work, we provide quantitative experimental constraints for quantum chemically calculated evaporation rates for the smallest H2SO4-H2O clusters, characterizing the mechanism governing nucleation on a kinetic, single-molecule level. We compare experimental particle size distributions resulting from a highly supersaturated homogeneous H2SO4 gas phase with the results from kinetic simulations employing quantum chemically derived decomposition rates of electrically neutral H2SO4 molecular clusters up to the pentamer at a large range of relative humidities. By using high H2SO4 concentrations, we circumvent the uncertainties concerning contaminants and competing reactions present in studies at atmospheric conditions. We show good agreement between molecular simulation and experimental measurements and provide the first evaluation of theoretical predictions of the stabilization provided by water molecules.

Characterization of Magnetic Series of Iron–Carbon Clusters FenC0,±1 (n ≤ 13)

The evolution of structural and electronic properties of neutral and charged iron clusters doped with a single carbon atom, FenC0,±1 (n = 1-13) series, is studied in this work; which has been carri...

CO Oxidation Catalyzed by the Neutral Cluster IrAl2O8 with Iridium in a High Oxidation State of VI

Metals with high oxidation states (HOSs) (e.g., +V to +X) are of great interest to chemists, although it is challenging to trigger the reactivity of metals with HOSs, because they are usually highl...

Geometry and Stability of Small Gold Cluster Ions by Graph Theory and Hückel model

Structure and stability of the gold cluster ions of which skeleton are synthesized as a complex were analyzed using the Hückel method based on graph theory. Hückel Energy (HE) and Topological Resonance Energy (TRE) were determined for neutral, positive ion, and negative ion clusters, where all the isomers of the gold cluster up to octamer were considered. Since some graphically designed isomers include bonds that cannot be realized in three- dimensional space, the screening was carried out by a molecular force field calculation with LAMMPS (lammps.sandia.gov/.). Among the isomers thus obtained, both HE and TRE were most stable when the tetramer was Au4 2 + with a tetrahedral structure, and with the hexamer, Au6 2+ with two tetrahedrons sharing one side. The complexes with these structures have actually been synthesized. On the other hand, the synthesis example of the most stable cluster for octamer Au8 has not been reported.

Binding energies and isomer distribution of neutral acetonitrile clusters

AbstractStructures and relative stabilities of neutral acetonitrile clusters up to decamer have been investigated. We used the ABCluster code to thoroughly explore the potential energy surfaces (PESs) of the acetonitrile clusters and to generate initial geometries, which are further fully optimized at the MN15/6‐31++G(d,p) level of theory. Our exploration yielded several new local and global minima structures of the acetonitrile clusters. We located new global minimum structures of the acetonitrile pentamer to decamer. The results show that the PESs of the acetonitrile clusters are very flat, yielding several isoenergetic structures. Our investigations also revealed that the stability of the acetonitrile clusters is due both to CH⋯N and dipole–dipole interactions. Structures stabilized by the latter are found to be more stable than those stabilized by the former at low temperatures. Furthermore, we examined the effect of temperature on the stability of the structures of the acetonitrile clusters for temperatures in the range 20 to 400 K. We found that several structures contribute to the population of the clusters at high temperatures, and also that a significant contribution to the population comes from isomers that lie within 2.0 kcal/mol from the most stable one. In addition, we used the most stable structures to compute the binding energies of the acetonitrile clusters and to assess the performance of some DFT functionals (MN15, ωB97XD, PW6B95D3, and B2PLYP) in comparison with the MP2 method associated to the aug‐cc‐pVTZ basis set in calculating the binding energies. We found that the MN15 functional performs better than ωB97XD and PW6B95D3, and that MN15 is less sensitive to the basis set change. We also used an extrapolation scheme to calculate the binding energies of the acetonitrile clusters at the highly accurate W1BD, CBS‐QB3, and G4MP2 levels of theory. The resulting binding energies are recommended for future benchmarking of the acetonitrile clusters.

Raman spectroscopic investigations on UV irradiated phosphate glasses with high content of silver or sodium

The influence of UV radiation on structural properties of phosphate glasses, with addition of high concentration of silver or sodium was studied by Raman spectroscopy. It is shown that glass synthesis conditions (reducing or oxidizing) influences considerably on glass structure reaction on UV irradiation. The role of additions (silver or sodium) is also studied. It is shown that structural changes in sodium-containing glass after UV irradiation are more manifested, than in silver-containing glasses. The destruction of subnanosized neutral silver molecular clusters under UV irradiation is also observed. The results, obtained for all types of glasses are compared and discussed. For the definition of the influence of silver and sodium atoms on chemical bonds in glass structural units, computer simulation, using density functional theory, was performed.

Infrared + vacuum ultraviolet two-color ionization spectroscopy of neutral metal complexes based on a tunable vacuum ultraviolet free-electron laser.

This paper describes an experimental technique for studying neutral metal complexes using infrared + vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy based on a tunable VUV free-electron laser (VUV-FEL). The preliminary IR spectroscopy results of mass-selected nickel tetracarbonyl are reported in this work. The results demonstrate that the tunable VUV-FEL light allows the selective ionization of a given neutral cluster free of confinement along with the recording of well-resolved IR spectra. As the ionization energies of many neutral clusters are accessible by a broadly tunable VUV-FEL (50-150 nm) and near-threshold ionization can be readily achieved, the proposed experimental method offers unique possibilities for the size-specific study of a wide variety of confinement-free neutral clusters.

Soft cluster-induced desorption/ionization mass spectrometry: How soft is soft?

Desorption/ionization induced by neutral clusters (DINeC) is used as an ultrasoft desorption/ionization method for the analysis of fragile biomolecules by means of mass spectrometry (MS). As a test molecule, the glycopeptide vancomycin was measured with DINeC-MS, and resulting mass spectra were compared to the results obtained with electrospray ionization (ESI), matrix assisted laser desorption ionization, and time-of-flight secondary ion MS. Of the desorption-based techniques, DINeC spectra show the lowest abundance of fragments comparable to ESI spectra. The soft desorption nature of DINeC was further demonstrated when applied to MS analysis of teicoplanin.

Structures and vertical detachment energies of water cluster anions (H2O)−n with n = 6–11

The structures and vertical detachment energies (VDEs) of water cluster anions (H2O)− with n = 6–11 are examined by an unbiased global search algorithm, namely comprehensive genetic algorithm (CGA) combined with density functional theory. Benchmark evaluation shows that the B3LYP-D3/6-31(+,3+)G* level of theory could give comparable accuracy of MP2/6-31(+,3 +)G* about the geometric property of water cluster anions. Meanwhile, the energies simulated at the MP2/6-31(+,3 +)G* level of theory converge to the results of CCSD(T)/6-31(+,3 +)G* level. Therefore, the relative energies and VDE of water cluster anions are calculated at the MP2/6-31(+,3 +)G*//B3LYP-D3/6-31(+,3 +)G* level of theory. The structures of (H2O)6–11− clusters obtained from CGA represent that the excess electron destroys the hydrogen bond network and forms an electron hole in most structures. The water cluster anions prefer to form three-membered rings and four-membered rings. As the cluster size increases, the VDE of the water cluster anions increases because the excess electron becomes less diffuse in the larger-sized clusters. Our work gives a comprehensive study about the excess electron disturbing the small-sized neutral water clusters.

Analysis of Complex Molecules and Their Reactions on Surfaces by Means of Cluster-Induced Desorption/Ionization Mass Spectrometry.

Desorption/Ionization Induced by Neutral SO2 Clusters (DINeC) is employed as a very soft and efficient desorption/ionization technique for mass spectrometry (MS) of complex molecules and their reactions on surfaces. DINeC is based on a beam of SO2 clusters impacting on the sample surface at low cluster energy. During cluster-surface impact, some of the surface molecules are desorbed and ionized via dissolvation in the impacting cluster; as a result of this dissolvation-mediated desorption mechanism, low cluster energy is sufficient and the desorption process is extremely soft. Both surface adsorbates and molecules of which the surface is composed of can be analyzed. Clear and fragmentation-free spectra from complex molecules such as peptides and proteins are obtained. DINeC does not require any special sample preparation, in particular no matrix has to be applied. The method yields quantitative information on the composition of the samples; molecules at a surface coverage as low as 0.1 % of a monolayer can be detected. Surface reactions such as H/D exchange or thermal decomposition can be observed in real-time and the kinetics of the reactions can be deduced. Using a pulsed nozzle for cluster beam generation, DINeC can be efficiently combined with ion trap mass spectrometry. The matrix-free and soft nature of the DINeC process in combination with the MSn capabilities of the ion trap allows for very detailed and unambiguous analysis of the chemical composition of complex organic samples and organic adsorbates on surfaces.

Density functional study of structures, stabilities and electronic properties of AgAunλ(λ=0,±1;n=1-12){\rm{AgAu}}_n^\lambda \left( {\lambda = 0, \pm 1;n = 1 - 12} \right) clusters: comparison with pure gold clusters

Abstract Geometrical structures, relative stabilities and electronic properties of neutral, cationic and anionic pure gold A u n + 1 λ {\rm{A}}u_{n + 1}^\lambda and Ag-doped bimetallic AgAu n λ ( λ = 0 , ± 1 ; n = 1 - 12 ) {\rm{AgAu}}_n^\lambda \left( {\lambda = 0, \pm 1;n = 1 - 12} \right) clusters have been systematically investigated by using density functional theory methodology. The optimized structures show that planar to three-dimensional structural transition occurs at n = 5 for cationic clusters. Due to strong relativistic effect of Au clusters, the ground state configurations of neutral and anionic bimetallic clusters favor planar geometry till n = 12. Silver atoms tend to occupy the most highly coordinated position and form the maximum number of bonds with Au atoms. The computed HOMO-LUMO energy gaps, fragmentation energies and second-order difference of energies show interesting odd-even oscillation behavior. The result indicates that AgAu5, AgAu 2 + {\rm{AgAu}}_2^ + and AgAu 2 - {\rm{AgAu}}_2^ - are the most stable clusters in this molecular system. The DFT based descriptors of bimetallic clusters are also discussed and compared with pure gold clusters. The high value of correlation coefficient between HOMO-LUMO energy gaps and DFT based descriptors supports our analysis. A good agreement between experimental and theoretical data has been obtained in this study.

Particle Number Dependence of Size and Energy Quantities in Sodium Clusters

The effective radial electronic potentials for neutral sodium clusters, which were determined by Ekardt on the basis of the local density approximation and the jellium model, are parametrized by means of the (symmetrized) Woods-Saxon and "Wine-Bottle" symmetrized Woods-Saxon potentials with the aim of investigating the dependence of size and energy quantities on the cluster particle number. The potential parameters are determined by vari­ous least-squares fitting procedures. It is found that for the radius R of the above potentials, complex expressions are more appropriate than the stan­dard one R = r0N^{1/3} for relatively small values of N. Furthermore, N-power expansions are derived for those complex expressions of R, as well as for the r.m.s. radius of the potential. It is also found that improved results in these cases are obtained with an expression of the form R = r0N^{1/3}+b, which is still very simple. There is also investigated the variation of energy quan­tities, such as the single particle energies of the 1s and 1p states, the level spacing |E1p-E1s| and the average energy level spacing, with respect to the particle number N. Expressions for the first three of these quantities with N-dependent terms of the form aN^{2/3} + βΝ^{-1} give good results.

Intramolecular Reactions in Ionized Ammonia Clusters: A Direct Ab Initio Molecular Dynamics Study

Ammonia cluster cations are a chemical species that has recently attracted considerable research attention as ion-molecule reaction species in the planetary atmosphere, surface reaction species in materials chemistry, and super-alkali species. Reactions of the radical cation of an ammonia cluster, [(NH3)n]+ (n = 2-6), following the ionization of the parent neutral cluster, were investigated using direct ab initio molecular dynamics (AIMD) to elucidate the reactions of ammonia cluster cation under astrochemical conditions. The calculations showed that two competing reaction channels-proton transfer (PT) channel and complex formation channel-operate after the ionization of neutral clusters. In the PT channel, a proton of NH3+ was transferred to a neighboring ammonia molecule. The PT channel was found in all clusters (n = 2-6). Reaction via the PT channel became faster with increasing cluster size, and saturated around n = 5-6. In the complex formation channel, a face-to-face complex having a H3N--NH3+ structure (with a N-N bond) was formed. This channel was found only in larger clusters (n = 5-6). Time scales of PT and complex formation channels were calculated to be 20-30 and 40-50 fs, respectively. The reaction mechanism was discussed based on the results of theoretical calculations.

Pattern recognition of amino acids based on highly fluorescent SDS modified pyridyl thiazole derivative

Selective and sensitive detection of amino acids is extremely significant as they play a key role in many biological processes. Herein, a pyridyl-thiazole based fluorophore (PTCN), modified with sodium dodecyl sulphate (SDS) assemblies, has been employed for the sensing of AA in aqueous solution. PTCN molecule exhibited excellent pH response due to the protonation of pyridyl nitrogen followed by the intramolecular charge transfer. PTCN showed dual emission along with enhanced quantum yield in SDS micellar assemblies. This improved the sensitivity of the PTCN –SDS system (PS), not only by lowering the limit of detection but also by detecting basic AA along with acidic AA. A fluorescence turn on response of PS system with acidic AA i.e. Asp and Glu has been observed. However, the PS system exhibited a ratiometric sensing pattern for basic AA i.e. Arg, Lys and Hys. Protonation and deprotonation of the fluorophore in presence of different types of AA are responsible for the changes observed in fluorescence spectra. The detection limits for Asp and Glu were found to be 0.47 μM and 0.64 μM respectively. Arg, Lys and Hys showed a detection limit of 8.36 μM, 26.6 μM and 51.9 μM respectively. Principal component analysis (PCA) effectively demonstrated discrimination of acidic, basic and neutral AA clusters. This study demonstrates the efficiency of the PS ensemble for selective, sensitive and discriminative sensing of AA.

The striking influence of oxophilicity differences in heterometallic Mo-Mn oxide cluster reactions with water.

Mixed-metal oxides have proven to be effective catalysts for the hydrogen evolution reaction, often outperforming either of the binary metal oxides. The reactivity of MnxMoOy - (x = 1, 2; y = 3, 4) clusters toward H2O was investigated via time-of-flight mass spectrometry with clear evidence of cluster oxidation and corresponding H2 production, specifically for MnxMoO3 - (x = 1, 2) clusters. Unlike previously studied MoxOy - clusters, which assumed a broad distribution of stoichiometries (typically x ≤ y ≤ 3x), both MnMoOy - and Mn2MoOy - preferentially formed y = 3 and 4 compositions in significant quantities under our source conditions. The electronic and molecular structures of the MnxMoOy (x = 1, 2; y = 3, 4) anion and neutral clusters were probed with anion photoelectron spectroscopy and analyzed with supporting density functional theory calculations. Our studies suggest that both metal centers are involved in initial cluster-water complex formation, while Mo is the center that undergoes oxidation; hence, reactivity terminates when Mo is saturated in its highest oxidation state of +6. Across these four clusters, Mn remains relatively reduced and is stable in a high-spin electronic configuration. The preferential reactivity of water molecules toward the Mo center rather than Mn is rationalized by the much lower relative oxophilicity of Mn.

Thiolato Protected Copper Sulfide Cluster with the Tentative Composition Cu74S15(2-PET)45.

Ligand protected copper nanoclusters with precise compositions have attracted considerable attention due to their unique photoluminescent properties. However, the acquisition of structural information, knowledge of the factors affecting the stability, and high quantum yields are prerequisites for assessing their applications in biomedicine as fluorescent contrast agents, biosensors, and probes for cells. Despite all the effort, only finite examples of single crystal structures of CuNCs are reported. Herein, we report the phosphine-free synthesis and structure determination of 2-PET protected CuNCs. The structure analysis established by single crystal X-ray diffraction reveals the formation of binary Cu74S15(2-PET)45 sulfide cluster. A similar phenomenon has been observed for several other chalcogenide-bridged copper clusters. The synthesized cluster possesses a rod-like structure, protected with 45 thiol ligands on the surface. Fifteen independent bridged-sulfur atoms couple to the copper atoms inside the core. Calculations for both a neutral and negatively charged cluster showed no major differences in their geometrical structures. Further analysis of frontier MO levels of the closed-shell anion predicts the HOMO-LUMO transition to be intramolecular L7 → L1 charge transfer, where "L7" and "L1" abbreviations refer to the corresponding sulfur layer in the structure. For the neutral cluster, the calculated spin density is delocalized over the two moieties. On the basis of TDDFT+TB calculations, the onset of the measured absorption spectrum could be satisfactorily reproduced.

Structural Evolution and Stability Trend of Small-Sized Gold Clusters Aun (n = 20-30).

Structural evolution and stability pattern of pure neutral gold clusters Aun in the small size range of n = 20-30 are examined using density functional theory (DFT) calculations. The equilibrium geometries are either confirmed or determined, and some new ground state structures are identified. The most stable configurations of Au21-Au23 sizes are formed by adding extra gold atoms to the highly stable pyramidal structure of Au20, while flat-cage shapes are the best candidates for the global minima of both Au24 and Au25. For larger sizes of n = 26-30, pyramidal motifs tend to dominate the lower-lying population rather than tubular conformations as previously reported. The energy gaps, excitation energies, and exciton binding energies are also computed to test out the performance of the computational methods employed. Accordingly, a density functional with long-range exchange effects is highly recommended to quantitatively investigate both the ground and excited states of pure gold clusters.

Complications in the interpretation of the charge-asymmetry-dependent π flow for the chiral magnetic wave

The charge asymmetry ($A_{\rm ch}$) dependence of the $\pi^{-}$ and $\pi^{+}$ elliptic flow difference, $\Delta v_{2}(A_{\rm ch})$, has been regarded as a sensitive observable for the possible chiral magnetic wave (CMW) in relativistic heavy ion collisions. In this work, we first demonstrate that, due to non-flow backgrounds, the flow measurements by the Q-cumulant method using all charged particles as reference introduce a trivial linear term to $\Delta v_{2}(A_{\rm ch})$. The trivial slope can be negative in the triangle flow difference $\Delta v_{3}(A_{\rm ch})$ if the non-flow is dominated by back-to-back pairs. After eliminating the trivial term, we find that the non-flow between like-sign pairs gives rise to an additional positive slope to $\Delta v_{2}(A_{\rm ch})$ because of the larger dilution effect to $\pi^{+}$ ($\pi^{-}$) at positive (negative) $A_{\rm ch}$. We further find that the competition between different $\pi$ sources can introduce another non-trivial linear-$A_{\rm ch}$ term due to their different multiplicity fluctuations and anisotropic flows. We then study the effect of neutral cluster (resonance) decays as a mechanism for local charge conservation on the slope parameter of $\Delta v_{2}(A_{\rm ch})$. We find that the slope parameter is sensitive to the kinematics of those neutral clusters. Light resonances give positive slopes while heavy resonances give negative slopes. Local charge conservation from continuum cluster mass distribution can give a positive slope parameter comparable to experimental data. Our studies indicate that many non-CMW physics mechanisms can give rise to a $A_{\rm ch}$-dependent $\Delta v_{2}(A_{\rm ch})$ and the interpretation of $\Delta v_{2}(A_{\rm ch})$ in terms of the CMW is delicate.