Alkali Halide Clusters Research Articles

Isomerism of Cyclodextrin Tetramer Induced by Alkali Halide Cluster Ions Observed by Ion Mobility Spectrometry-Mass Spectrometry.

Cyclodextrins (CDs) exhibit versatile self-assembly properties due to their hydrophilic and hydrophobic components, with applications such as drug delivery and selective binding. While research on CD self-assembly is extensive, limited studies have explored their aggregation behavior, particularly in interactions with small ionic guests. The present work investigates the structure of β-CD tetramers aggregated with alkali metal chloride clusters using ion mobility spectrometry-mass spectrometry (IMS-MS). The results revealed that diverse structures emerge in the tetramer depending on the alkali metal cluster size. Notably, the doubly charged tetramer exhibits distinct aggregation trends with specific numbers of MCl clusters for Na+ and K+ ions. After initially adopting a bucket-wheel structure with two internal cations, the structure transforms into a new isomer with a tetrahedral configuration upon cluster addition. The formation of the new isomer structure is closely linked to filling the cavity volume with MCl clusters and ionic interactions, which possibly compensate for the weakened hydrogen bonds between CDs. Theoretical calculations further support the structures, showing well-matched collision cross-section (CCS) values compared with the experimental CCS values. This study highlights the role of alkali metal chloride clusters as potential templates, leading to the formation of novel CD assemblies.

Trapping Alkali Halide Cluster Ions Inside the Cucurbit[7]uril Cavity.

In this study, the distinctive behavior of cucurbit[n]uril (CB[n]), which captures a variety of alkali halide clusters inside the cavity during the droplet evaporation, has been investigated by using ion mobility spectrometry-mass spectrometry. Complexes of CB[7] with various alkali chloride cluster cations or anions generated during the electrospray ionization were studied, and their collision cross-section (CCS) values were obtained to determine whether these clusters were trapped inside the cavity or not. It was found that the clusters smaller than a specific critical size were trapped inside the CB[7] cavity in the gas phase, although trapping of alkali halide clusters at the given concentration is supposed to be unfavorable in solution. We suggest that the rapid solvent evaporation rapidly increases ion concentrations and subsequently forms alkali-chloride contact ion pairs; therefore, it may provide a specific environment to enable the formation of the inclusion complexes.

Geometrical Structures and Molecular Adsorption Reactions of Alkali-halide Cluster Ions Investigated by Ion Mobility-mass Spectrometry

Geometrical Structures and Molecular Adsorption Reactions of Alkali-halide Cluster Ions Investigated by Ion Mobility-mass Spectrometry

Surface site coordination dependent responses resolved in free clusters: applications for neutral sub-nanometer cluster studies.

In this paper we demonstrate how surface site specific experimental information can be obtained from free low nanometer scale clusters using photoelectron spectroscopy utilising synchrotron radiation. In addition, we show how it can be used to gain insight into the geometry and surface structure of the clusters. The present experiments were conducted on alkali metal halides, RbCl and CsCl, which were chosen as advantageous test cases due to their simple electronic and geometric structures. These heavy alkali metal salts provide additional clarity since the surface and bulk responses can be separated, which is not the case for clusters of lighter alkali metal salts. Computational chemical shift calculations and simple alkali halide cluster size modelling were used to interpret the experimental results.

On the origin of atomistic mechanism of rapid diffusion in alkali halide nanoclusters

To elucidate the atomistic diffusion mechanism responsible for the rapid diffusion in alkali halide nano particles, called Spontaneous Mixing, we execute molecular dynamics simulations with empirical models for KCl-KBr, NaCl-NaBr, RbCl-RbBr and KBr-KI. We successfully reproduce essential features of the rapid diffusion phenomenon. It is numerically confirmed that the rate of the diffusion clearly depends on the size and temperature of the clusters, which is consistent with experiments. A quite conspicuous feature is that the surface melting and collective motions of ions are inhibited in alkali halide clusters. This result indicates that the Surface Peeling Mechanism, which is responsible for the spontaneous alloying of binary metals, does not play a dominant role for the spontaneous mixing in alkali halide nanoclusters. Detailed analysis of atomic motion inside the clusters reveals that the Vacancy Mechanism is the most important mechanism for the rapid diffusion in alkali halide clusters. This is also confirmed by evaluation of the vacancy formation energy: the formation energy notably decreases with the cluster size, which makes vacancy formation easier and diffusion more rapid in small alkali halide clusters.

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.

Solvation at nanoscale: Alkali-halides in water clusters

The solvation of alkali-halides in water clusters at nanoscale is studied by photoelectron spectroscopy using synchrotron radiation. The Na 2p, K 3p, Cl 2p, Br 3d, and I 4d core level binding energies have been measured for salt-containing water clusters. The results have been compared to those of alkali halide clusters and the dilute aqueous salt solutions. It is found that the alkali halides dissolve in small water clusters as ions.

Interaction of soft x-ray photons with free alkali-halide molecular clusters

Alkali-halide molecular clusters were formed from the vaporized molecules in collisions with an intense beam of argon clusters. Photoionization of the alkali-halide clusters as the result of interaction with x-ray radiation has been utilized in a photoelectron spectroscopy study of cluster bonding mechanisms. The changes in the energy structure due to the transformation from molecules to the solid salt were observed both in the inner and outer shells of constituent elements (metal and halogen) and were used to deduce the geometric structure of the clusters in question.

The Growth of Ionic Crystals Based on the Halogenation of Copper Cluster Anions

We investigated the halogenation reactivity of copper cluster anions produced via a magnetron-sputter source after introduction into a fast-flow tube reaction apparatus simultaneously with chlorine gas. Interesting cluster products corresponding to [Cu(n)Cl(n+1)](-) (n = 1-6) were observed with notable stability, and the mass distribution of these clusters exhibits an exponential decay with increasing values of n. Reaction kinetics analysis is provided on the gas-phase reactivity of copper cluster anions with chlorine. First-principle calculations suggest a series of cubic-like structures for these species similar to the structure of alkali halide clusters due to their similar electronic configurations. These structures act as a starting point in the formation of ionic crystals.

Alkali halide clusters produced by fast ion impact

The most abundant geometries and relative stabilities of alkali halide clusters with a ( XY ) n 0 configuration (e.g., LiF, NaCl, KBr) are described. Five main series were obtained: linear, cyclic, cubic, arc strips and nanotubes. The stability analysis shows that higher members are likely to be formed from the lower member of the same series and/or from two building blocks ( n = 1, 2). The energy analysis ( D-plot) indicates that the most compact ones (e.g., cubic and nanotubes) present higher stability when compared to the linear, cyclic and arc strip structures; moreover, relative stability between the cubic and nanotube series varies with the cluster size.

A numerical study upon the atomistic mechanisms of rapid diffusion in nanoclusters

An isoenergetic molecular dynamics simulation is done to investigate two fundamental atomistic mechanisms responsible for rapid diffusion in nanoclusters. Using a semi-empirical potential we successfully reproduce the so-called Spontaneous Alloying and Spontaneous Mixing phenomena, which have been found in experiments for bimetallic and ternary alkali halide clusters, respectively. We found that a novel mechanism, named here the Surface Peeling Mechanism, is of primary importance for Spontaneous Alloying, while the Vacancy Mechanism is dominant in Spontaneous Mixing. We also show that the dominant mechanism responsible for rapid diffusion is related to the size dependence of vacancy formation energy.

Correlation-bound anions of NaCl clusters

In the past a variety of electron binding motifs has been identified for sodium chloride cluster anions. As for all of these clusters the excess electron is predicted to be bound in self-consistent-field calculations, the different binding mechanisms can be understood in terms of a one-electron potential largely due to the permanent multipole moments of the neutral cluster. Here we investigate a new class of (NaCl)(N)(-) anion that is predicted to be bound only after electron correlation has been taken into account. Correlation-bound states of the trimer, (NaCl)(3)(-), and tetramer, (NaCl)(4)(-), are characterized using Green's function and an equation-of-motion coupled-cluster method, and the computed electron binding energies as well as the distributions of the excess electrons as inferred from natural orbitals of the coupled-cluster calculations are compared with that of dipole-bound, quadrupole-bound, and defectlike (NaCl)(N)(-) anions. For the (NaCl)(4)(-) tetramer anion the correlation-bound state is predicted to represent the most stable isomer. Our results provide a sensitive test case for the development of improved one-electron model potentials for excess electrons bound to alkali halide clusters, and suggest that cluster abundance as inferred from peak intensities of photoelectron spectra is not directly related to the relative stability of the clusters.

Very high critical energy fragmentations observed in CID

High energy CID fragmentations of triatomic alkali halide cluster ions were examined to estimate the energy transfer during the CID process. A detailed CID study using keV collisions revealed that M 2X + clusters yield not only the expected M + ions but practically all possible combinations of mono- and biatomic ions. In the case of Cs 2I +, for example Cs +, Cs 2 +, CsI +, and even I + fragment ions are observed. The results suggest that collisional energy transfer in keV collisions has a monotonously decreasing shape (full width at half height is ∼2 eV) and has a long high energy tail extending well over 10 eV (with at least 5% probability), the average collisional energy transfer being about 5 eV.

Different approaches for the calculation of electronic excited states of nonstoichiometric alkali halide clusters: The example of Na3F

The electronic structure and excited states of the Na(3)F cluster are investigated using different approximate, but numerically efficient, computational schemes, such as a 2e hybrid quantum/classical pseudopotential model with full-configuration interaction or time-dependent density-functional theory. Various quantities such as geometries and transition energies are compared with results previously obtained by multireference configuration interaction calculations, taken as reference data. The potential energy surfaces of the lowest excited states are investigated and the finite-temperature absorption spectra are calculated. The good agreement with recent beam experiments [J.-M. L'Hermite, V. Blanchet, A. Le Padellec, B. Lamory, and P. Labastie, Eur. Phys. J. D 28, 361 (2004)] leads to the conclusion that the absorption spectrum observed experimentally corresponds to the lowest energy isomer which has a C(2v) planar rhombic geometry.

Photodesorption of alkali anions from alkali-halide cluster anions.

Using photoelectron spectroscopy, we have observed alkali anion photodesorption from alkali-halide cluster anions that contain two weakly bound electrons. In the alkali iodides, we have found this type of desorption in almost every (MI)(n)M- cluster we have studied (M=Na, K, Cs; n<9), although it depends on the probe laser frequency and cluster temperature. Using pump-probe techniques, we have shown that the process occurs on a picosecond time scale by way of an electronic excitation of the cluster's spin-paired electrons.

Electron detachment energies in high-symmetry alkali halide solvated-electron anions

We decompose the vertical electron detachment energies (VDEs) in solvated-electron clusters of alkali halides in terms of (i) an electrostatic contribution that correlates with the dipole moment (μ) of the individual alkali halide molecule and (ii) a relaxation component that is related to the polarizability (α) of the alkali halide molecule. Detailed numerical ab initio results for twelve species (MX)n− (M=Li,Na; X=F,Cl,Br; n=2,3) are used to construct an interpolation model that relates the clusters’ VDEs to their μ and α values as well as a cluster size parameter r that we show is closely related to the alkali cation’s ionic radius. The interpolation formula is then tested by applying it to predict the VDEs of four systems [i.e., (KF)2−, (KF)3−, (KCl)2−, and (KCl)3−] that were not used in determining the parameters of the model. The average difference between the model’s predicted VDEs and the ab initio calculated electron binding energies is less than 4% (for the twelve species studied). It is concluded that one can easily estimate the VDE of a given high-symmetry solvated electron system by employing the model put forth here if the α, μ and cation ionic radii are known. Alternatively, if VDEs are measured for an alkali halide cluster and the α and μ values are known, one can estimate the r parameter, which, in turn, determines the “size” of the cluster anion.

Structural electric dipole in small ionic nanocrystals

We present the results of gas phase electric dipole measurements for a series of M 18X 17 alkali–halide clusters. This cluster has a large structural dipole, which is due to the periodic arrangement of the ions in the nanocrystal and is directly related to the size of the particle. Experimental values are compared to the prediction of a Born Mayer model and to ab initio calculations. The results for the various salts are reproduced by our calculations.

Molecular dynamics study of the solidification process in alkali halide cluster

Molecular dynamics (MD) simulations of the solidification process of an NaCl cluster are carried out. Voronoi analysis is employed to distinguish a crystal nucleus from molten NaCl. In the early stage of simulation, some small solid clusters of size smaller than the critical nucleus size are repeatedly formed and broken. Under the low-temperature condition ( T=700 K), in the later stage of the simulation, a polycrystal NaCl solid, in which two or three large solid grains survive, appears. Under the high-temperature condition ( T=740 K), one large single crystal cluster is formed. All simulations of this work are performed in a special-purpose computer for MD simulation, called MDGRAPE-2.

Structure of nano-objects through polarizability and dipole measurements

The electric polarizability and the electric permanent dipole are important quantities for understanding the electronic properties of a cluster. Experimental techniques, the simulations necessary to interpret the experimental results, and a review of measurements on atomic and mixed clusters are presented. For atomic clusters, the polarizability is related to the type of bonding. In simple metal clusters such as alkali clusters, the results are well interpreted by the electron delocalization characteristic of the metallic bonding. In other metal clusters, the polarizability reflects the difficulty of establishing a clear and regular picture of the size evolution of electronic properties. The size evolution observed for covalent and semiconductor clusters is different from the evolution for metal clusters, and the influence of the geometry is preponderant, as demonstrated in the case of fullerenes. For mixed clusters, the measurements of the electric dipole allows one to deduce the charge transfers and the geometric arrangement. This is illustrated in the case of the metal-fullerene system and alkali halide clusters. To cite this article: M. Broyer et al., C. R. Physique 3 (2002) 301–317.

Spontaneous mixing of binary alkali halide crystal by successive evaporation

Experimental studies on the mixing of alkali halide clusters of 200 nm sizes were performed systematically by the successive evaporation of different alkali halides on carbon substrates. The alkali halides used were NaCl, NaBr, KCl, KBr, KI, RbCl, RbBr, CsCl and CsBr. Intermixing took place independent of the crystal structure. It became evident that the limit of mixing was governed by the ratio of ionic radii. If the ratio of ionic radii between the cation and/or anion was more than 68%, intermixing occurred. It was also found that the crystal structure formed by intermixing between NaCl-type and CsCl-type structures becomes the NaCl type. If four different elements of cations and anions are involved, two mixture phases grew. The key elements of mixing were the nearest ionic radii.