Magic Number Clusters Research Articles

The electric field effect on the hydrogen storage properties of (MgO)9

(MgO)9 with a rocksalt structure, a magic number cluster of (MgO)n, exhibits high stability. In this study, the hydrogen storage properties of (MgO)9 under an external electric field are explored by DFT calculation. The results reveal that H2 can be adsorbed on single Mg/O atoms. Because the (MgO)9 and H2 are effectively polarized by the external electric field, the adsorption strength of H2 at certain adsorption sites is substantially enhanced. The adsorption energies of H2 on three-coordinated Mg/O atoms increase from −0.071/−0.056eV in the absence of an electric field to −0.186/−0.237 eV under a field intensity of 0.025a.u. Under the field, (MgO)9 can adsorb a maximum of 18 H2 molecules, and the corresponding mass density of hydrogen storage reaches 9.1wt%. Our results suggest that subjecting the (MgO)9 to an external electric field is a potential hydrogen storage method. In this paper, the interaction mechanism between H2 and (MgO)9 under the external electric field is also investigated through an electronic structure analysis.

Low-energy structures of benzene clusters with a novel accurate potential surface.

The benzene-benzene (Bz-Bz) interaction is present in several chemical systems and it is known to be crucial in understanding the specificity of important biological phenomena. In this work, we propose a novel Bz-Bz analytical potential energy surface which is fine-tuned on accurate ab initio calculations in order to improve its reliability. Once the Bz-Bz interaction is modeled, an analytical function for the energy of the Bzn clusters may be obtained by summing up over all pair potentials. We apply an evolutionary algorithm (EA) to discover the lowest-energy structures of Bzn clusters (for n=2-25), and the results are compared with previous global optimization studies where different potential functions were employed. Besides the global minimum, the EA also gives the structures of other low-lying isomers ranked by the corresponding energy. Additional ab initio calculations are carried out for the low-lying isomers of Bz3 and Bz4 clusters, and the global minimum is confirmed as the most stable structure for both sizes. Finally, a detailed analysis of the low-energy isomers of the n = 13 and 19 magic-number clusters is performed. The two lowest-energy Bz13 isomers show S6 and C3 symmetry, respectively, which is compatible with the experimental results available in the literature. The Bz19 structures reported here are all non-symmetric, showing two central Bz molecules surrounded by 12 nearest-neighbor monomers in the case of the five lowest-energy structures.

Atomic Cluster Generation with an Atmospheric Pressure Spark Discharge Generator

A new method for generating metal clusters in the gas phase is described and characterized in this work. The method is based on material evaporation by spark ablation at atmospheric pressure. The characterization of atomic clusters was done by measuring their electrical mobility. The measured mobilities were compared with values found in literature in order to identify the cluster species. We show that silver clusters consisting from one up to 25 atoms can be produced in helium at atmospheric pressure. In addition, the effect of oxygen concentration on the resulting cluster mobility distribution was investigated. Results show that at higher oxygen level, the mobility distribution is dominated by the abundance of stable clusters (i.e., magic number clusters). This can be attributed to an oxidation etching effect.Copyright 2015 American Association for Aerosol Research

Size Evolution Study of the Electronic and Magnetic Properties of MgO Nanoclusters

Magnesium oxide nanoclusters have attracted much attention due to their potential applications to catalysis and novel optoelectronic materials. In the present study, we have studied the electronic and magnetic properties of the stoichiometric magnesium oxide nanoclusters (MgO)n for n = 2-20. Although the binding energy increases with the size of the cluster, it reaches the asymptotic limit of about 66.0 eV per unit for relatively large n value. The static dipole polarizability also exhibits distinct size dependence, reflecting clearly the structural transition when the cluster grows. The polarizability and the binding energy of the clusters are found to be inversely related to each other and their correlation is rationalized by invoking the minimum polarizability principle. Moreover, principle of maximum hardness is also used to characterize the magic number clusters. A well-defined linear correlation is found between static dipole polarizability and the inverse of ionization potential. The most important feature of this work is the NMR study of MgO clusters which is reported for the first time. For each cluster size, the calculated NMR parameters at 17O nuclei together with electronic and structural data provide detailed insight into the properties of bulk and in particular of nanosized structures. Variation of 17O chemical shieldings demonstrates the electrostatic environment divisions around the oxygen nuclei which in turn originate from the cluster structure and its symmetry.

Para-hydrogen and helium cluster size distributions in free jet expansions based on Smoluchowski theory with kernel scaling

The size distribution of para-H2 (pH2) clusters produced in free jet expansions at a source temperature of T0 = 29.5 K and pressures of P0 = 0.9-1.96 bars is reported and analyzed according to a cluster growth model based on the Smoluchowski theory with kernel scaling. Good overall agreement is found between the measured and predicted, Nk = A k(a) e(-bk), shape of the distribution. The fit yields values for A and b for values of a derived from simple collision models. The small remaining deviations between measured abundances and theory imply a (pH2)k magic number cluster of k = 13 as has been observed previously by Raman spectroscopy. The predicted linear dependence of b(-(a+1)) on source gas pressure was verified and used to determine the value of the basic effective agglomeration reaction rate constant. A comparison of the corresponding effective growth cross sections σ11 with results from a similar analysis of He cluster size distributions indicates that the latter are much larger by a factor 6-10. An analysis of the three body recombination rates, the geometric sizes and the fact that the He clusters are liquid independent of their size can explain the larger cross sections found for He.

Lithium formate ion clusters formation during electrospray ionization: Evidence of magic number clusters by mass spectrometry and ab initio calculations.

Small cationic and anionic clusters of lithium formate were generated by electrospray ionization and their fragmentations were studied by tandem mass spectrometry (collision-induced dissociation with N2). Singly as well as multiply charged clusters were formed in both positive and negative ion modes with the general formulae, (HCOOLi)nLi(+), (HCOOLi)nLim (m+), (HCOOLi)nHCOO(-), and (HCOOLi)n(HCOO)m (m-). Several magic number cluster (MNC) ions were observed in both the positive and negative ion modes although more predominant in the positive ion mode with (HCOOLi)3Li(+) being the most abundant and stable cluster ion. Fragmentations of singly charged positive clusters proceed first by the loss of a dimer unit ((HCOOLi)2) followed by the loss of monomer units (HCOOLi) although the former remains the dominant dissociation process. In the case of positive cluster ions, all fragmentations lead to the magic cluster (HCOOLi)3Li(+) as the most abundant fragment ion at higher collision energies which then fragments further to dimer and monomer ions at lower abundances. In the negative ion mode, however, singly charged clusters dissociated via sequential loss of monomer units. Multiply charged clusters in both positive and negative ion modes dissociated mainly via Coulomb repulsion. Quantum chemical calculations performed for smaller cluster ions showed that the trimer ion has a closed ring structure similar to the phenalenylium structure with three closed rings connected to the central lithium ion. Further additions of monomer units result in similar symmetric structures for hexamer and nonamer cluster ions. Thermochemical calculations show that trimer cluster ion is relatively more stable than neighboring cluster ions, supporting the experimental observation of a magic number cluster with enhanced stability.

The electric field effect on the hydrogen storage of (MgO)12 by ab intio calculations

(MgO)12 in a tube structure is one of the magic number clusters of (MgO)n and exhibits particular stability. To study the electric field effect on the hydrogen storage properties of (MgO)12, the H2 adsorption behavior on the surface of the tube (MgO)12 in an external electric field is explored at the level of B3LY/6-31G**. In the external electric field, the (MgO)12 keeps the frame of tube structure but with little distortion, implying that the (MgO)12 cluster can sustain the strong electric field for hydrogen storage. The NBO analysis also indicates that (MgO)12 is polarized by the external electric field; and its dipole momenta increase to 9.21 and 19.39 Debye at the field intensities of 0.01 and 0.02 a.u., respectively. Without the external electric field, H2 can be adsorbed on Mg atoms in the end on modes, while on O atoms in the top on modes. When the external electric field is applied, whether H2 is adsorbed on Mg or O atoms, the stable adsorption structures are all top on modes and the molecular orientation of H2 is turned to the direction of the external electric field. Because (MgO)12 and H2 are effectively polarized by the external electric field, the adsorption strength of H2 on some adsorption sites are enhanced remarkably. The adsorption energies of H2 on the three-coordinated Mg/O are promoted from 0.08/0.06 eV in free field to 0.12/0.11 eV and 0.20/0.26 eV at field intensities of 0.01 a.u. and 0.02 a.u., respectively. Electronic structure analysis reveals that when H2 is adsorbed on Mg atoms, this process denotes charges moving to the cluster, and the improvement of adsorption interaction of H2 on Mg atoms is mainly due to the polarization effect. While the adsorption on O atoms, on the one hand implies the polarization effect of O anion is stronger than that of Mg cation, on the other hand, H2 receives charges from (MgO)12 and its valence orbitals also take part in the bonding with the valence orbitals of the cluster. Thus the structures of H2 adsorbed on O atoms are more stable. In an external electric field, (MgO)12 can adsorb sixteen H2 molecules at most, and the corresponding mass density of hydrogen storage reaches 6.25wt%.

Formation and evolution of nano-clusters in a large-scale system of Cu–Zr alloy during rapid solidification process

To investigate the formation and evolution mechanisms of nano-clusters formed during the rapid solidification process, a molecular dynamics (MD) simulation study has been performed for a system consisting of 106atoms of Cu64.5Zr35.5 alloy. Adopting a new cluster-type index method (CTIM-3) to analyse the local atomic clusters, it is found that the dominant microstructural configurations in the supercooled liquid and amorphous are the icosahedrons and their combinations, especially the intercross-sharing (IS) clusters linked of two icosahedrons (IS-ICO-clusters) can be used to build some larger IS-ICO nano-clusters with different sizes. Highly interesting, in the system the number of atoms in various IS-ICO nano-clusters demonstrates a magic number sequence of 19, 23, 25, 27, 29, 31, 33, 35, 37, 41, 43, and so on. The magic number clusters can be classified into three main types: chain-like, triangle-tailed and quadrilateral-tailed. The degree of tightness for IS-ICO nano-cluster can be expressed by formula nI/nIS. And some IS-ICO nano-structures have stronger mechanical stability and can bear higher load.

Protonated primary amines induced thymine quintets studied by electrospray ionization mass spectrometry and density functional theory calculations

As the novel magic number clusters of nucleobases, the thymine quintets induced by ammonium ion (NH4(+)), and particularly by its derivatives such as protonated alkyl amines and protonated aryl amines, have been studied by electrospray ionization mass spectrometry (ESI-MS) and density functional theory (DFT) calculations. The DFT-optimized geometry of NH4(+) induced thymine quintet ([T5 + NH4](+)) reveals some new features including three additional hydrogen bonds between NH4(+) and its surrounding thymine molecules when compared with that of the alkali metal ions induced thymine quintets. In addition, the fourth hydrogen atom of NH4(+) is sticking out the assembly, and, thus, it might be replaced by an organic group R to form the protonated primary amine induced thymine quintet ([T5 + R - NH3](+)), a hypothesis that has been confirmed by both DFT calculations and ESI-MS experiments. Furthermore, the relative abilities of the different protonated primary amines for inducing the thymine quintets are investigated by ESI-MS competition experiments, and the results have shown a clear trend of stronger ability as the alkyl chain gets longer or as the aryl ring gets larger for the alkyl amines or the aryl amines. Two basic influence factors are consequently identified: one is the ability of the alkyl amine to accept proton, another is the π-π stacking interaction between the aryl ring and the π-surface of the thymine molecule(s), whose explanations are strongly supported by multiple types of thermochemical data, various control experiments and DFT calculations.

Melting of “non-magic” argon clusters and extrapolation to the bulk limit

The melting of argon clusters ArN is investigated by applying a parallel-tempering Monte Carlo algorithm for all cluster sizes in the range from 55 to 309 atoms. Extrapolation to the bulk gives a melting temperature of 85.9 K in good agreement with the previous value of 88.9 K using only Mackay icosahedral clusters for the extrapolation [E. Pahl, F. Calvo, L. Koči, and P. Schwerdtfeger, "Accurate melting temperatures for neon and argon from ab initio Monte Carlo simulations," Angew. Chem., Int. Ed. 47, 8207 (2008)]. Our results for argon demonstrate that for the extrapolation to the bulk one does not have to restrict to magic number cluster sizes in order to obtain good estimates for the bulk melting temperature. However, the extrapolation to the bulk remains a problem, especially for the systematic selection of suitable cluster sizes.

The enigma of Au21(SC2H4Ph)14 nanocluster: a synthetic challenge

Recent investigations into atomically precise gold clusters show that not all magic-numbered clusters can be readily obtained through conventional synthetic routes. For example, Au21(SR)14, a magic-numbered cluster, was only obtained as a minor product from a mixture of clusters but was never synthesized in pure form using a single-step synthetic approach. We have made several attempts, albeit without any success, using a variety of approaches to synthesize Au21(SR)14 clusters in a single step. We show in this communication that synthetic failure is not likely due to electronic instabilities based on computational investigation of its electronic structure. Our DFT calculations show that the optimized cluster consists of a center Au13 core cluster capped by two Au2(SCH2CH3)3 fragments and four Au(SCH2CH3)2 fragments and is energetically stable. The findings reported here should give more confidence for the synthetic chemists to successfully synthesize Au21(SR)14 in the near future.

Application of Ion Mobility-Mass Spectrometry to the Study of Ionic Clusters: Investigation of Cluster Ions with Stable Sizes and Compositions.

Stable cluster sizes and compositions have been investigated for cations and anions of ionic bond clusters such as alkali halides and transition metal oxides by ion mobility-mass spectrometry (IM-MS). Usually structural information of ions can be obtained from collision cross sections determined in IM-MS. In addition, we have found that stable ion sizes or compositions were predominantly produced in a total ion mass spectrum, which was constructed from the IM-MS measurement. These stable species were produced as a result of collision induced dissociations of the ions in a drift cell. We have confirmed this result in the sodium fluoride cluster ions, in which cuboid magic number cluster ions were predominantly observed. Next the stable compositions, which were obtained for the oxide systems of the first row transition metals, Ti, Fe, and Co, are characteristic for each of the metal oxide cluster ions.

Highly sensitive and specific detection of histamine via the formation of a self-assembled magic number cluster with thymine by mass spectrometry.

A novel method for the detection of histamine (HIM) via the formation of a self-assembled magic number cluster with thymine (T) by electrospray ionization tandem mass spectrometry (ESI-MS/MS) is described. The formation of the magic number cluster [T17 + HIM + 2H](2+) shifts the MS signal of histamine to the interference-free higher mass range and the signal intensity is increased by four orders of magnitude. In addition, the formation of [T17 + HIM + 2H](2+) is highly specific to histamine compared with its metabolite and other similar biogenic amines, which may be attributed to both of its amino and imidazole groups. The linear dynamic range of the method is in the range of 1 nM-20 μM, and the limit of detection can be as low as 0.1 nM. The feasibility of this method is further demonstrated by the quantitative analysis of histamine in a red wine sample. Since little sample preparation or separation is required before the analysis, this method provides a rapid new way for the sensitive and specific detection of histamine by MS.

Structure and stability of Ti2Bn (n=1-10) clusters: an ab initio investigation

Structures and stabilities of Ti2Bn (n=1-10) clusters have been systematically investigated by using the density-functional theory B3LYP method and ab initio CCSD(T) method. It is found that the ground state structures of the Bn clusters are substantially modified by doping two Ti atoms. Ti2Bn clusters have very clear growth patterns, namely to form bipyramid. All the most stable Ti2Bn can be visualized as bipyramids with the two Ti atoms located at the two apexes. Ti2B6, Ti2B7 and Ti2B8 are confirmed to be the magic number clusters based on the analysis of the second-order difference of energies. The dissociation energies, vertical ionization potentials and vertical electron affinities of Ti2Bn isomers are discussed. Ti2B6 cluster is found to be stable both kinetically and thermodynamically.

Density functional theory investigation on the structure and stability of Sc2Bn (n = 1–10) clusters

The geometries, stabilities, and electronic properties of Sc2Bn clusters, up to n=10, have been systematically investigated by using the density-functional B3LYP and CCSD(T) method. It was found that the ground state structures of the Bn clusters are substantially modified by the doping of Sc atoms. Sc2Bn clusters have very regular growth pattern, namely to form bipyramid. All the most stable Sc2Bn can be viewed as bipyramid (uncompleted when n⩽5) with two Sc atoms at the apexes. Sc2B7 and Sc2B8 are confirmed as the magic number clusters based on the analysis of the second-order difference of energies. The dissociation energies, vertical ionization potentials (VIP) and electron affinities (VEA) of Sc2Bn isomers are discussed. In most of the case, the Sc2Bn clusters have smaller HOMO–LUMO gap, VIP, VEA than the corresponding Bn clusters. There was no explicit correlation between the energetic stability and HOMO–LUMO gap, VIP or VEA.

Locating Protonated Amines in Clathrates

The structures and inherent stabilities of hydrated, protonated ammonia, select protonated primary, secondary, and tertiary amines as well as tetramethylammonium with 19-21 water molecules were investigated using infrared photodissociation (IRPD) spectroscopy and blackbody infrared radiative dissociation (BIRD) at 133 K. Magic number clusters (MNCs) with 20 water molecules were observed for all ions except tetramethylammonium, and the BIRD results indicate that these clusters have stable structures, which are relatively unaffected by addition of one water molecule but are disrupted in clusters with one less water molecule. IRPD spectra in the water free O-H stretch region are consistent with clathrate structures for the MNCs with 20 water molecules, whereas nonclathrate structures are indicated for tetramethylammonium as well as ions at the other cluster sizes. The locations of protonated ammonia and the protonated primary amines either in the interior or at the surface of a clathrate were determined by comparing IRPD spectra of these ions to those of reference ions; Rb(+) and protonated tert-butylammonia with 20 water molecules were used as references for an ion in the interior and at the surface of a clathrate, respectively. These results indicate that protonated ammonia is in the interior of the clathrate, whereas protonated methyl- and n-heptylamine are at the surface. Calculations suggest that the number of hydrogen bonds in these clusters does not directly correlate with structural stability, indicating that both the number and orientation of the hydrogen bonds are important. These experimental results should serve as benchmarks for computational studies aimed at elucidating ion effects on the hydrogen-bonding network of water molecules and the surface activity of ions.

Magic Number Pt13 and Misshapen Pt12 Clusters: Which One is the Better Catalyst?

A relationship between the size of metal particles and their catalytic activity has been established over a nanometer scale (2-10 nm). However, application on a subnanometer scale (0.5-2 nm) is difficult, a possible reason being that the activity no longer relies on the size but rather the geometric structure as a cluster (or superatomic) compound. We now report that the catalytic activity for the oxygen reduction reaction (ORR) significantly increased when only one atom was removed from a magic number cluster composed of 13-platinum atoms (Pt13). The synthesis with an atomic-level precision was successfully achieved by using a dendrimer ligand as the macromolecular template strictly defining the number of metal atoms. It was quite surprising that the Pt12 cluster exhibited more than 2-fold catalytic activity compared with that of the Pt13 cluster. ESI-TOF-mass and EXAFS analyses provided information about the structures. These analyses suggested that the Pt12 has a deformed coordination, while the Pt13 has a well-known icosahedral atomic coordination as part of the stable cluster series. Theoretical analyses based on density functional theory (DFT) also supported this idea. The present results suggest potential activity of the metastable clusters although they have been "missing" species in conventional statistical synthesis.

Magic number effect on cluster formation of polyhydroxylated fullerenes in water–alcohol binary solution

Due to the spherical shape with a diameter of ca. 1 nm, the aggregation behaviour of fullerene C60 is very interesting in view of the possible formation of magic number particle in a similar manner as metal cluster in gas phase. Herein, we report for the first time the magic number aggregation behaviours of polyhydroxylated fullerenol C60(OH)36 in water–alcohol (methanol, ethanol and 1-propanol) binary solution with increasing alcohol component. The diameters of particle were ca. 6–8 nm depending on the alcohol used. The particle sizes were precisely measured by the novel-induced grating method which is superior for the particle-size measurement in single-nano region (1–10 nm). The magic number cluster was also detected by scanning probe microscopy observation. However, such aggregation behaviours were not found in DMSO–alcohol system or for the use of less hydroxylated C60(OH)10.

Large mixed complexes involving uracil, cytosine, thymine and/or 1‐methyl uracil around Ca2+ ions: an electrospray ionization/MS study

We investigated the possible formation of mixed B(n)B'(n')Ca(2+) complexes where B and B' are two different nucleobases. Electrospray ionization (ESI) mass spectrometric experiments from solutions containing two different kinds of nucleobases and calcium ions were carried out to investigate the formation of magic number clusters that may be relevant in a biological point of view. The results presented here clearly show that mixed complexes can be formed and are stable in the gas phase. This represents an important step toward more complex solutions in which several nucleobases are present simultaneously and may compete in the formation of cationized clusters. We believe that thorough investigations on such systems may help understanding biological processes that may effect the tridimensional structure of the DNA macromolecule. The formation of mixed hexamers, decamers, dodecamers and tetradecamers are clearly favored from solution containing uracil (Ura), thymine (Thy) and Ca(2+), whereas mixed octamers are preferred from 1-methyl uracil (MeU), uracil and Ca(2+) mixtures. Cytosine (Cyto) can form mixed complexes with either uracil or 1-methyl uracil or thymine. On the other hand, the main species formed in these latter cases are mixed tetramers.

Laser desorption dual spray post‐ionization mass spectrometry for direct analysis of samples via two informative channels

A laser desorption dual spray post-ionization mass spectrometry method is described, and its usefulness is demonstrated with the examples of selective detection of food components, manipulation of protein charge state distribution and investigation on the formation of magic number clusters. The method is carried out by adopting two spray emitters for post-ionization of analytes desorbed by a pulsed infrared laser. Various components in a complex sample or distinct behavior of an analyte in two different spray reagents can be rapidly probed by the method quasi-simultaneously, highlighting the potential applications of this method for protein characterization, reaction study and food analysis.