Structure and energetics of equiatomic K-Cs and Rb-Cs binary clusters

The basin-hopping algorithm combined with the Gupta many-body potential is used to study the structural and energetic properties of (KCs)(n) and (RbCs)(n) bimetallic clusters with N=2n up to 50 atoms. Each binary structure is compared to those of the pure clusters of the same size. For the cluster size N=28 and for the size range of N=34-50, the introduction of K and Rb atoms in the Cs alkali metal cluster results in new ground state structures different from those of the pure elements. In the size range N>/=38 the binary and pure clusters show not only structural differences, but they also display different magic numbers. Most of the magic Rb-Cs and K-Cs clusters possess highly symmetric structures. They belong to a family of pIh structures, where a fivefold pancake is a dominant structural motif. Such geometries have not been reported for alkali binary clusters so far, but have been found for series of binary transition metal clusters with large size mismatch. Moreover, tendency to phase separation (shell-like segregation) is predicted for both K-Cs and Rb-Cs clusters with up to 1000 atoms. Our finding of a surface segregation in Rb-Cs clusters is different from that of theoretical and experimental studies on bulk Rb-Cs alloys where phase separation does not occur.     

Size-dependent reactions of ammonium bisulfate clusters with dimethylamine

The reaction kinetics of ammonium bisulfate clusters with dimethylamine (DMA) gas were investigated using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Clusters ranged in size from 1 to 10 bisulfate ions. Although displacement of the first several ammonium ions by DMA occurred with near unit efficiency, displacement of the final ammonium ion was cluster size dependent. For small clusters, all ammonium ions are exposed to incoming DMA molecules, allowing for facile exchange ("surface" exchange). However, with increasing cluster size, an ammonium ion can be trapped in an inaccessible region of the cluster ("core" exchange), thereby rendering exchange difficult. DMA was also observed to add onto existing dimethylaminium bisulfate clusters above a critical size, whereas ammonia did not add onto ammonium bisulfate clusters. The results suggest that as the cluster size increases, di-dimethylaminium sulfate formation becomes more favorable. The results of this study give further evidence to suggest that ambient sub-3 nm diameter particles are likely to contain aminium salts rather than ammonium salts.     

Adsorption of ammonia by sulfuric acid treated zirconium hydroxide

The adsorption of ammonia on Zr(OH)(4), as well as Zr(OH)(4) treated with sulfuric acid, were examined. The results show that treating Zr(OH)(4) with sulfuric acid leads to the formation of a sulfate on the surface of the material, and that the sulfate contributes to the ammonia adsorption capacity through the formation of an ammonium sulfates species. Calcination of Zr(OH)(4) decreases the ammonia adsorption capacity of the material and limits the formation of sulfate species. NMR and FTIR spectroscopy results are presented that show the presence of two distinct ammonium species on the surface of the material. The adsorption capacity of the materials is shown to be a complex phenomenon that is impacted by the surface area, the sulfur content, and the pH of the material. The results illustrate that Zr(OH)(4), which is known to adsorb acidic gases, can be modified and used to adsorb basic gases.     

Electron bubbles in helium clusters. I. Structure and energetics

In this paper we present a theoretical study of the structure, energetics, potential energy surfaces, and energetic stability of excess electron bubbles in ((4)He)(N) (N=6500-10(6)) clusters. The subsystem of the helium atoms was treated by the density functional method. The density profile was specified by a void (i.e., an empty bubble) at the cluster center, a rising profile towards a constant interior value (described by a power exponential), and a decreasing profile near the cluster surface (described in terms of a Gudermannian function). The cluster surface density profile width (approximately 6 A) weakly depends on the bubble radius R(b), while the interior surface profile widths (approximately 4-8 A) increase with increasing R(b). The cluster deformation energy E(d) accompanying the bubble formation originates from the bubble surface energy, the exterior cluster surface energy change, and the energy increase due to intracluster density changes, with the latter term providing the dominant contribution for N=6500-2 x 10(5). The excess electron energy E(e) was calculated at a fixed nuclear configuration using a pseudopotential method, with an effective (nonlocal) potential, which incorporates repulsion and polarization effects. Concurrently, the energy V(0) of the quasi-free-electron within the deformed cluster was calculated. The total electron bubble energies E(t)=E(e)+E(d), which represent the energetic configurational diagrams of E(t) vs R(b) (at fixed N), provide the equilibrium bubble radii R(b) (c) and the corresponding total equilibrium energies E(t) (e), with E(t) (e)(R(e)) decreasing (increasing) with increasing N (i.e., at N=6500, R(e)=13.5 A and E(t) (e)=0.86 eV, while at N=1.8 x 10(5), R(e)=16.6 A and E(t) (e)=0.39 eV). The cluster size dependence of the energy gap (V(0)-E(t) (e)) allows for the estimate of the minimal ((4)He)(N) cluster size of N approximately 5200 for which the electron bubble is energetically stable.     

Infrared spectroscopy of large ammonia clusters as a function of size

We have measured the vibrational spectra of large ammonia (NH3)n clusters by photofragment spectroscopy in the spectral range from 3150 to 3450 cm(-1) for the average sizes n = 29, 80, 212, 447, and 989 and by depletion spectroscopy for n=8. The spectra are dominated by peaks around 3385 cm(-1) which are attributed to the asymmetric nu3 NH-stretch mode. Two further peaks between 3200 and 3260 cm(-1) have about equal intensity for n = 8 and 29, but only about 0.40 of the intensity of the nu3 peak for the larger sizes. The spectra for the smallest and largest size agree with those obtained by Fourier transform infrared spectroscopy in slit jet expansion and collision cells, respectively. By accompanying calculation we demonstrate that the energetic order of the spectral features originating from the bending overtone 2nu4 and the symmetric NH-stretch nu1 in the range from 3150 to 3450 cm(-1) is changed between n = 10 and 100, while the asymmetric NH-stretch nu3 only exhibits a moderate redshift. The reason is the coupling of the ground state modes to the overtones.     

Adsorption of carbon on Pd clusters of nanometer size: a first-principles theoretical study

Adsorbed atomic C species can be formed in the course of surface reactions and commonly decorate metal catalysts. We studied computationally C adsorption on Pd nanoclusters using an all-electron scalar relativistic density functional method. The metal particles under investigation, Pd(55), Pd(79), Pd(85), Pd(116), Pd(140), and Pd(146), were chosen as fragments of bulk Pd in the form of three-dimensional octahedral or cuboctahedral crystallites, exposing (111) and (100) facets as well as edge sites. These cluster models are shown to yield size-converged adsorption energies. We examined which surface sites of these clusters are preferentially occupied by adsorbed C. According to calculations, surface C atoms form strongly adsorbed carbide species (with adsorption energies of more than 600 kJ mol(-1)) bearing a significant negative charge. Surface sites allowing high, fourfold coordination of carbon are overall favored. To avoid effects of adsorbate-adsorbate interaction in the cluster models for carbon species in the vicinity of cluster edges, we reduced the local symmetry of selected adsorption complexes on the nanoclusters by lowering the global symmetry of the nanocluster models from point group O(h) to D(4h). On (111) facets, threefold hollow sites in the center are energetically preferred; adsorbed C is calculated to be slightly less stable when displaced to the facet borders.     

Reaction of Aln clusters with oxygen and ammonia

Aluminum clusters were generated by a laser vaporization method. Ionization potential of Al₂ was found to be lower than that of Al. In the reaction with oxygen at high concentration, the mixed cluster Al₉O₇ survives predominantly as a stable cluster. Ammonia was found to be adsorbed weakly on the Al_n surface.     

Structures and energetics of hydrated oxygen anion clusters

Hydration of the atomic oxygen radical anion is studied with computational electronic structure methods, considering (O(-))(H(2)O)(n) clusters and related proton-transferred (OH(-))(OH)(H(2)O)(n)(-)(1) clusters having n = 1-5. A total of 67 distinct local-minimum structures having various interesting hydrogen bonding motifs are obtained and analyzed. On the basis of the most stable form of each type, (O(-))(H(2)O)(n)) clusters are energetically favored, although for n > or = 3, there is considerable overlap in energy between other members of the (O(-))(H(2)O)(n) family and various members of the (OH(-))(OH)(H(2)O)(n)(-)(1) family. In the lower-energy (O(-))(H(2)O)(n) clusters, the hydrogen bonding arrangement about the oxygen anion center tends to be planar, leaving the oxygen anion p-like orbital containing the unpaired electron uninvolved in hydrogen bonding with any water molecule. In (OH(-))(OH)(H(2)O)(n)(-)(1) clusters, on the other hand, nonplanar arrangements are the rule about the anionic oxygen center that accepts hydrogen bonds. No instances are found of OH(-) acting as a hydrogen bond donor. Those OH bonds that form hydrogen bonds to an anionic O(-) or OH(-) center are significantly stretched from their equilibrium value in isolated water or hydroxyl. A quantitative inverse correlation is established for all hydrogen bonds between the amount of the OH bond stretch and the distance to the other oxygen involved in the hydrogen bond.     

Extraction of sulfuric acid with TOPO

A study on solvent extraction of sulfuric acid by tri-octylphosphine oxide (TOPO) in n-heptane has been made. Extraction coefficients of H₂SO₄ as a function of H₂SO₄ concentration in aqueous phase, and extractant concentrations in organic phase have been studied. The composition of extracted species, equilibrium constants of extraction reaction have been evaluated. These results are important for interpreting extraction equilibrium data of uranium(VI) or other metal ions with TOPO in sulfuric acid media.     

IR dissociation of ammonia clusters

Dissociation spectra of NH₃ clusters have been recorded using a cw CO₂ laser. For the dimer two absorption bands have been found at 979 and 1004 cm⁻¹, which originate from the excitation of two non-equivalent NH₃ molecules. A tunneling motion is held responsible for the observed structure on one of these bands. The symmetry group of the NH₃ dimer is presented considering the tunneling motion solely. Heavier NH₃ clusters dissociate at frequencies between 1020 and 1100 cm⁻¹. The dissociation spectrum of the SiH₄-NH₃ complex shows one peak centered at 972.3 cm⁻¹.     

A molecular beam electric deflection study of clusters: acid and salt molecules in microscopic aqueous and ammonia clusters

Results of a molecular beam electric deflection study of the polarity of gas phase clusters comprised of HNO₃H₂O, HNO₃H₂ONH₃, and NH₄HSNH₃ are reported. No refocusing was observed for any species containing more than two molecular subunits. These results are contrasted with previous studies made in our laboratory which showed focusing for higher polymers of acetic acid; possible reasons are given for observed defocusing in the present systems where ion-pair formation is expected to occur.     

Tritium isotope fractionation between ammonia,methylamine, dimethylamine and n-butane thiol

An automatic gamma-ray spectrometer equipped with a micro-robot for sample changing has been developed and constructed. The facility is comprised of a commercially available micro-robot, sample changer for up to 36 samples, personal computer programmed in BASIC language, input/output devices, detector and multichannel analyzer. This paper describes the components, software and effective uses of the above facility.     

Reactions of methylcyclohexanones with concentrated sulfuric acid

Conclusions In reaction with concentrated sulfuric acid 2-, 3-, and 4-methylcyclohexanones undergo autocondensation and simultaneous intramolecular cyclization with formation of furan compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 1958–1963, November, 1966.     

Reaction of cycloheptanone with concentrated sulfuric acid

Treatment of cycloheptanone or 2-cycloheptylidenecycloheptanone with concentrated sulfuric acid, leads to self-condensation accompanied by intramolecular rearrangement and cyclization to give substituted tetrahydrofurans, viz. 2-methyl-2, 3-cyclohexano-4, 5-cyclohepteno-(⁵)-2, 3, 4, 5-tetrahydrofuran and 2-methyl-2, 3-cyclohexano-5-hydroxy-4, 5-cycloheptano-2, 3, 4, 5-tetrahydrofuran. Self-condensation of ketones with 6-membered rings under the action of concentrated sulfuric acid, proceeding via carbonium ions, and accompanied by contraction to a 5-membered ring, is also characteristic of cycloheptanone, the 7-membered ring of which isomerizes to a 6-membered one.     

Structures and dynamics of protonated ammonia clusters

The structures and infrared spectra of protonated ammonia clusters NH(4+)(NH3)n, for n < or = 8, are investigated using density functional-theory (DFT) calculations and semiempirical DFT/molecular dynamics simulations. For n < 5 the clusters are found to be mostly stable up to 100 K, while the larger clusters (n > or = 5) isomerize. Temperature effects are taken into account by performing ab initio molecular dynamics simulations with the computationally tractable self-consistent charges density functional tight-binding method. The infrared spectra at 10 K for the most stable isomers for n = 3-8 compare qualitatively with predissociation experiments, and using a common scaling factor almost quantitative agreement is found. For n > or = 6 the notion of multiple isomers present under the experimental conditions is supported. Of the 13 stable structures for n = 8 only three are found to survive at 100 K. All other clusters isomerize. Cluster structures are inferred from the analysis of the cumulative radial distribution function of the ammonia molecules surrounding the NH(4+) core. The infrared spectra are found to be typical for the structure of the clusters, which should help to relate the experimentally measured infrared spectra to the number and identity of the contributing isomers. For clusters that reorganize to a more stable isomer during the dynamics, the infrared spectrum is generally similar to that of the stable isomer itself. The clusters are found to preferably form globular structures, although chain-like arrangements are also among the low-energy configurations.     

Reaction of sphene with sulfuric acid solutions

The kinetics of titanium(IV) extraction from sphene with boiling 4–6 M sulfuric acid was studied. The rate constants of the sphene dissolution were determined. The influence of the particle size of the starting mineral on the rate of its dissolution was established. The feasibility of performing sphene dissolution by a liquid-phase procedure yielding stable titanium-containing sulfate solutions suitable for the synthesis of multifunctional composite materials was examined.     

Bonding and energetics in small clusters of gallium and arsenic

Density functional theory is used to study the electronic, geometric, and bonding properties of small Ga_n, As_m, and Ga_nAs_m clusters with up to eight atoms. New ground state structures for many of these clusters are reported. Electronic states, stabilities, and charges are also reported and discussed in terms of the bonding in the clusters. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:189–196, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10127