Relationships between cluster secondary ion mass intensities generated by different cluster primary ions

Measurements are described to evaluate the constitution of secondary ion mass spectra for both monatomic and cluster primary ions. Previous work shows that spectra for different primary ions may be accurately described as the product of three material-dependent component spectra, two being raised to increasing powers as the cluster size increases. That work was for an organic material and, here, this is extended to (SiO₂) _t OH⁻ clusters from silicon oxide sputtered by 25 keV Bi _n ⁺ cluster primary ions for n = 1, 3, and 5 and 1 ≤ t ≤ 15. These results are described to a standard deviation of 2.4% over 6 decades of intensity by the product of a constant with a spectrum, H _SiOH/*, and a power law spectrum in t. This evaluation is extended, using published data for Si _t ⁺ sputtered from Si by 9 and 18 keV Au⁻ and Au₃⁻, with confirmation that the spectra are closely described by the product of a constant with a spectrum, H _Si^*, and a simple spectrum that is an exponential dependence on t, both being raised to appropriate powers. This is confirmed with further published data for 6, 9, 12, and 18 keV Al⁻ and Al₂⁻ primary cluster ions. In all cases, the major effect of intensity is then related to the deposited energy of the primary ion at the surface. The constitution of SIMS spectra, for monatomic and cluster primary ion sources, is shown, in all cases, to be consistent with the product of a constant with two component spectra raised to given powers.     

X-ray photoelectron,X-ray emission and UV spectra of SiO2 calculated by the SCF Xα scattered wave method

Photoionization, X-ray emission and UV excitation energies are calculated for SiO₂ by the SCF Xα method and the transition-state procedure. In all cases agreement between calculation and experiment is good. The SiO^4?₄ cluster is found to be adequate for describing localized excitations in quartz. The valence orbitals of this cluster are found to be naturally separable into three sets (1) 3t₂ and 4a₁; O 2s, nonbonding: (2) 5a₁ and 4t₂; Si 3s, 3p and O 2p, sigma bonding: (3) 1e, 5t₂, 1t₁; O 2p nonbonding. The participation of Si 3d-type functions in the high-energy O 2p nonbonding orbitals is found to be very small.     

Die Bindungsverhältnisse in der Clusterverbindung CZr6I14

Summary The electronic properties of the cluster compound CZr₆I₁₄ are discussed on the basis of EHT results. In the model cluster CZr₆I ₁₈ ^4– the calculated Zr-Zr distance in the metal octahedron is enlarged by encapsulation of the interstitial C as well as by the surrounding ligands. The interstitial bond is realized by the two bond orbitals a_1g, t_1u, and, additional, by three t_1u orbitals of the 5 p(I) band. The Zr-Zr bonds are week. The cluster CZr₆I ₁₈ ^4– is held together by strong C-Zr and Zr-I bonds.

     

Hydrogen‐Atom Abstraction from Methane by Stoichiometric Vanadium–Silicon Heteronuclear Oxide Cluster Cations

Vanadium–silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O₂ background. Reactions of the heteronuclear oxide cations with methane in a fast‐flow reactor were studied with a time‐of‐flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V₂O₅)_n(SiO₂)_m]⁺ (n=1, m=1–4; n=2, m=1), and the estimated first‐order rate constants for the reactions were close to that of the homonuclear oxide cluster V₄O₁₀⁺ with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal‐oxygen‐centered radicals (O_t ^. ) were found in all of the stable isomers. These O_t ^. radicals are active sites of the clusters in reaction with CH₄. The O_t ^. radicals in [V₂O₅(SiO₂)_1–4]⁺ clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH₄ activation on silica‐supported vanadium oxide catalysts.     

The Silicon Carbonyls Revisited: On the Existence of a Planar Si(CO)4

Recently, some works have focused attention on the reactivity of silicon atom with closed-shell molecules. Silicon may form a few relatively stable compounds with CO, i.e. Si(CO), Si(CO)₂, Si[C₂O₂], while the existence of polycarbonyl (n>2) silicon complexes has been rejected by current literature. In this paper, the reaction of silicon with carbonyl has been reinvestigated by density functional calculations. It has been found that the tetracoordinated planar Si(CO)₄ complex is thermodynamically stable. In Si(CO), silicon carbonyl, and Si(CO)₂, silicon dicarbonyl, the CO are datively bonded to Si; Si(CO)₄, silicon tetracarbonyl, may be viewed as a resonance between the extreme configurations (CO)₂Si + 2CO and 2CO + Si(CO)₂; while Si[C₂O₂], c-silicodiketone, is similar to the compounds formed by silicon and ethylene. A detailed orbital analysis has shown that the Si bonding with two CO is consistent with the use of sp ²-hybridized orbitals on silicon, while the Si bonding with four CO is consistent with the use of sp ² d-hybridized orbitals on silicon, giving rise to a planar structure about Si.     

Electronic structure of UH,UF, and their ions

A relativistic effective core potential (REP) has been generated for the uranium atom and used in self-consistent-field calculations of the A states of UH, UF, and their ions. Energy curves were calculated at the base configuration level which ensures the dissociating atoms are described by Hartree–Fock wavefunctions. The electronic bonding of these molecules is found to be similar to that of comparable alkaline–earth hydrides and fluorides. The uranium 6p, 6d, and 5f orbitals retain their atomic character but the orbitals extend into the bonding region and are distorted by overlap repulsion and electrostatic effects. Nonetheless, the atomic energetic coupling determines that low energy states will have the maximum spin multiplicity and maximum orbital angular momentum projection consonant with the charge-transfer bonding.     

Theoretical Characterisation of Phosphinyl Radicals and Their Magnetic Properties: g Matrix

The g matrices (g tensors) of various phosphinyl radicals (R₂P^.) were calculated using the DFT and multireference configuration interaction (MRCI) methods. The g matrices were distinctly dependent on the molecular structure of the radical. To thoroughly examine this dependence, the contributions from individual atoms and excited states were calculated. The former revealed the gain from the phosphorus atom to be preeminent unless P?O or P?S bonds are present in the radical molecule. The contributions owing to excited states arising from electronic transitions between doubly occupied molecular orbitals and the SOMO were clearly positive, as in the case of semiquinone and niroxide radicals. The transitions from the phosphorus lone pair were of paramount importance. Surprisingly, unlike for semiquinones and nitroxides, a significant negative contribution was observed from excitations from the SOMO to unoccupied molecular orbitals. For radicals with P?O bonds, this contribution to the g₂ component was dominant.     

The Family of Ferrocene‐Stabilized Silylium Ions: Synthesis, 29Si NMR Characterization,Lewis Acidity,Substituent Scrambling,and Quantum‐Chemical Analyses

The purpose of this systematic experimental and theoretical study is to deeply understand the unique bonding situation in ferrocene‐stabilized silylium ions as a function of the substituents at the silicon atom and to learn about the structure parameters that determine the ²⁹Si NMR chemical shift and electrophilicity of these strong Lewis acids. For this, ten new members of the family of ferrocene‐stabilized silicon cations were prepared by a hydride abstraction reaction from silanes with the trityl cation and characterized by multinuclear ¹H and ²⁹Si NMR spectroscopy. A closer look at the NMR spectra revealed that additional minor sets of signals were not impurities but silylium ions with substitution patterns different from that of the initially formed cation. Careful assignment of these signals furnished experimental proof that sterically less hindered silylium ions are capable of exchanging substituents with unreacted silane precursors. Density functional theory calculations provided mechanistic insight into that substituent transfer in which the migrating group is exchanged between two silicon fragments in a concerted process involving a ferrocene‐bridged intermediate. Moreover, the quantum‐chemical analysis of the ²⁹Si NMR chemical shifts revealed a linear relationship between δ(²⁹Si) values and the Fe???Si distance for subsets of silicon cations. An electron localization function and electron localizability indicator analysis shows a three‐center two‐electron bonding attractor between the iron, silicon, and C′_ipso atoms, clearly distinguishing the silicon cations from the corresponding carbenium ions and boranes. Correlations between ²⁹Si NMR chemical shifts and Lewis acidity, evaluated in terms of fluoride ion affinities, are seen only for subsets of silylium ions, sometimes with non‐intuitive trends, indicating a complicated interplay of steric and electronic effects on the degree of the Fe???Si interaction.     

On a proposed radiation-induced polaronic hole in silicon dioxide

Ab-initio self-consistent-field molecular-orbital (SCF MO) Hartree–Fock (HF) calculations using the STO-3G, 6-31G, and 6-31G* basis sets, were performed to model quasi-tetrahedral silicon species in silicon dioxide. Mostly nine-atom clusters, [Si(OH)₄]^qt, with charge number q_t = 0 or + 1, were studied. The positions of the Si and O atoms were varied to achieve minimum total energies, while the protons were held fixed in the O-(neighboring)Si direction to simulate the rigid crystal surroundings. The α-quartz-type local symmetry C₂ was found to be retained for the neutral cluster, but not for the ionic one. The unrestricted HF calculations indicate that the latter paramagnetic centre, (q_t = +1), has its spin population almost entirely on one short-bonded oxygen ion bonded weakly to its neighboring Si, and is quite high in energy (9.55 eV with 6-31G) compared to the diamagnetic centre (q_t = 0). The ionization energy is much higher than the self-trapping potential of the polaronic hole, a fact which may account for the failure so far to observe a [SiO₄]⁺¹ center in quartz by means of continuous-wave electron paramagnetic resonance spectroscopy. Calculations on the [SiO₄]⁺¹ center agree well with ultraviolet spectra, and with the [hole portion of a] proposed radiation-induced exciton in quartz. The hole in [Si(OH)₄]⁺¹ can be shifted from a short-bonded to a long-bonded oxygen to give the excited state [Si(OH)₄]_es⁺¹. Conclusions reached with the nine-atom clusters were confirmed by a series of calculations on the extended model [Si(OSiH₃)₄]^qt. Comparisons with the known isoelectronic species [AlO₄]⁰ were carried out.     

Synthesis and Hydrolysis of Hybridized Silicon Alkoxide: Si(OEt)x(OBut)4-x. Part I: Synthesis and Identification of the Si(OEt)x(OBut)4-x

Using silicon tetrachloride (SiCl₄), tert-butanol (t-BuOH), ethanol (EtOH) and NH₃, the hybridized silicon ethoxide 3-tert-butoxide (Si(OEt)_x(OBu^t)_4-x) was synthesized and the configuration of the material was investigated by FT-IR,¹ H and ¹³C NMR and gas chromatogram/mass spectrum (GC/MS) techniques. The results confirm that both the ethoxy and the tert-butoxy groups have been attached to silicon atoms. Furthermore, the alkoxy group types and their relative amounts in the alkoxide were also determined by ¹H and ¹³C NMR and GC/MS.     

Ein neuer Aluminium/Nickel/Oxo‐Cluster: [Ni(acac)OAl(OtBu)2]4

A New Aluminum/Nickel/Oxo‐Cluster: [Ni(acac)OAl(OtBu)₂]₄ When bis(tert‐butoxy)alane (tBu‐O)₂AlH is allowed to react with nickeldiacetylacetonate at elevated temperature a new nickel/aluminum/oxo cluster [Ni(acac)OAl(OtBu)₂]₄ is formed together with aluminum acetylacetonate Al(acac)₃ and some other products. The metal/oxo cluster is isolated by crystallization and structurally fully characterized by X‐ray diffraction analysis. The molecule [Ni(acac)OAl(OtBu)₂]₄ contains an eight membered Al₄O₄ cycle, to which eight mutually edge sharing NiO₂Al cycles are fused. The overall point symmetry of the metal/oxo cluster is almost S₄. While the aluminum atoms are tetrahedrally surrounded by oxygen ligands (mean distances Al‐O in‐between 1, 730(6) and 1, 789(6) Å)), the nickel atoms are in a square pyramidal coordination sphere of oxygen atoms (Ni‐O_axial = 1.938(6) Å, Ni‐O_basal = 2.056(9) Å; all polyhedra are distorted). The nickel atoms have a d⁸ high spin electron configuration (μ_eff = 3.32 B.M.).     

Ab initio calculations on CO4 centers in silicon dioxide

Ab initio SCF-MO Hartree–Fock calculations were performed using the STO-3G, 6-31G, and 6-31G* basis sets to model hypothetical substitutional carbon impurities in silicon dioxide. We utilized nine-atom clusters, [C(OH)₄]^qt, with charge number q_t = 0 and + 1. The positions of the C and O atoms were varied to achieve minimum total energies, while the fixed protons served to simulate the rigid crystal surroundings. In the optimized configuration of the neutral cluster, the C? O bond lengths are appreciably longer than typical C? O bonds, indicating relatively weak bonds for a carbon impurity at a silicon site. For comparison, the relative positions of all nine atoms in the [C(OH)₄]⁰ model were allowed to vary. This unconstrained model yielded more normal bond lengths and was lower in energy than the fixed-proton model by 6.80 eV with the 6-31G* basis set. The free-H model compared favorably with the x-ray diffraction data for an analogous orthocarbonate. Our results are in concert with the lack of reports of any substitutional carbon impurity in α-quartz. In the fixed-H models, the twofold local symmetry was found to be retained when q_t is 0 but not when q_t is + 1. For the latter ion, the unrestricted H-F calculations indicate that this paramagnetic center has its spin population almost entirely on one oxygen ion and is high in energy (5.31 eV with 6-31G) compared to the diamagnetic neutral one. Conclusions reached with the nine-atom clusters were confirmed by a series of calculations on the extended model [C(OSiH₃)₄]⁰.     

Chiral Chalcogenyl-Substituted Naphthyl- and Acenaphthyl-Silanes and Their Cations

Cyclic silylated chalconium borates 13 [B(C₆F₅)₄] and 14 [B(C₆F₅)₄] with peri-acenaphthyl and peri-naphthyl skeletons were synthesized from unsymmetrically substituted silanes 3 , 4 , 6 , 7 , 9 and 10 using the standard Corey protocol (Chalcogen Ch=O, S, Se, Te). The configuration at the chalcogen atom is trigonal pyramidal for Ch=S, Se, Te, leading to the formation of cis- and trans-isomers in the case of phenylmethylsilyl cations. With the bulkier tert-butyl group at silicon, the configuration at the chalcogen atoms is predetermined to give almost exclusively the trans-configurated cyclic silylchalconium ions. The barriers for the inversion of the configuration at the sulfur atoms of sulfonium ions 13 c and 14 a are substantial (72–74 kJ mol⁻¹) as shown by variable temperature NMR spectroscopy. The neighboring group effect of the thiophenyl substituent is sufficiently strong to preserve chiral information at the silicon atom at low temperatures.     

Theoretical Study of Pd11Si6 Nanosheet Compounds Including Seven‐Coordinated Si Species and Its Ge Analogues

Nanosheet compounds Pd₁₁(SiiPr)₂(SiiPr₂)₄(CNtBu)₁₀ ( 1 ) and Pd₁₁(SiiPr)₂(SiiPr₂)₄(CNMes)₁₀ ( 2 ), containing two Pd₇(SiiPr)(SiiPr₂)₂(CNR)₄ plates (R=tBu or Mes) connected with three common Pd atoms, were investigated with DFT method. All Pd atoms are somewhat positively charged and the electron density is accumulated between the Pd and Si atoms, indicating that a charge transfer (CT) occurs from the Pd to the Si atoms of the SiMe₂ and SiMe groups. Negative regions of the Laplacian of the electron density were found between the Pd and Si atoms. A model of a seven‐coordinated Si species, that is, Pd₅(Pd?SiMe), is predicted to be a stable pentagonal bipyramidal molecule. Five Pd atoms in the equatorial plane form bonding overlaps with two 3p orbitals of the Si atom. This is a new type of hypervalency. The Ge analogues have geometry and an electronic structure similar to those of the Si compounds. But their formation energies are smaller than those of the Si analogues. The use of the element Si is crucial to synthesize these nanoplate compounds.     

Mixed silicon carbide clusters studied by laser ablation Fourier transform ICR mass spectrometry

Ions produced by the 1064 nm Nd:YAG laser ablation of various Si–C samples include mixed silicon carbon (Si_xC_y) cluster ions of low molecular mass. Unique Si_xC_y cluster ion distributions are observed. A charge state effect for Si_xC_y clusters is evident from differences between the respectively charged distributions. This may be accounted for by either differing energetics and/or differing formation mechanisms for positive and negative Si_xC_y cluster ions. Several specific Si_xC_y clusters of probable high stability consistently dominate the mass spectra. The chemistry of the more abundant Si_xC_y ions was probed by collision-induced dissociation and ion–molecule reactivity experiments. Similar Si_xC_y⁺˙ dissociation results in the common loss of an Si ion. Parent Si_xC_y ion and small neutral reagent reactants give rise to oxidation, addition and exchange reactions.     

Ca14Si19 – a Zintl Phase with a Novel Twodimensional Silicon Framework

Ca₁₄Si₁₉ is an overlooked binary phase in the Ca/Si system with a novel type of twodimensional silicon framework (R3 c, a = 867.85(6), c = 6852.8(8) pm, Z = 6). The basic building units are 3,3,3-barrelanes Si₁₁ which are linked by Si₃ bridges to form a twodimensional silicon framework leaving space for interstitial calcium atoms. The thickness of the silicon layers is about 800 pm. The compound is a semiconductor with a band gap of about E_G = 0.1 eV and a diamagnetic moment of χ_mole = ?9 · 10^?4 cm³mol^?1. According to the relatively high linking of silicon atoms the reaction with air and moisture is fairly slow.     

The Binary Silicides Eu5Si3 and Yb3Si5 – Synthesis,Crystal Structure,and Chemical Bonding

The binary silicides Eu₅Si₃ and Yb₃Si₅ were prepared from the elements in sealed tantalum tubes and their crystal structures were determined from single crystal X-ray data: I4/mcm, a = 791.88(7) pm, c = 1532.2(2) pm, Z = 4, wR2 = 0.0545, 600 F² values, 16 variables for Eu₅Si₃ (Cr₅B₃-type) and P62m, a = 650.8(2) pm, c = 409.2(1) pm, Z = 1, wR2 = 0.0427, 375 F² values, 12 variables for Yb₃Si₅ (Th₃Pd₅ type). The new silicide Eu₅Si₃ contains isolated silicon atoms and silicon pairs with a Si–Si distance of 242.4 pm. This silicide may be described as a Zintl phase with the formula [5 Eu²⁺]¹⁰⁺[Si]^4–[Si₂]^6–. The silicon atoms in Yb₃Si₅ form a two-dimensional planar network with two-connected and three-connected silicon atoms. According to the Zintl-Klemm concept the formula of homogeneous mixed-valent Yb₃Si₅ may to a first approximation be written as [3 Yb]⁸⁺[2 Si^–]^2–[3 Si^2–]^6–. Magnetic susceptibility investigations of Eu₅Si₃ show Curie-Weiss behaviour above 100 K with a magnetic moment of 7.85(5) μ_B which is close to the free ion value of 7.94 μ_B for Eu²⁺. Chemical bonding in Eu₅Si₃ and Yb₃Si₅ was investigated by semi-empirical band structure calculations using an extended Hückel hamiltonian. The strongest bonding interactions are found for the Si–Si contacts followed by Eu–Si and Yb–Si, respectively. The main bonding characteristics in Eu₅Si₃ are antibonding Si1₂-π* and bonding Eu–Si1 states at the Fermi level. The same holds true for the silicon polyanion in Yb₃Si₅.     

Analysis of thin films and molecular orientation using cluster SIMS

A study of phenylalanine films of different thicknesses from submonolayer to 55 nm on Si wafers has been made using Bi_n⁺ and C₆₀⁺ cluster primary ions in static SIMS. This shows that the effect of film thickness on ion yield is very similar for all primary ions, with an enhanced molecular yield at approximately 1 monolayer attributed to substrate backscattering. The static SIMS ion yields of phenylalanine at different thicknesses are, in principle, the equivalent of a static SIMS depth profile, without the complication of ion beam damage and roughness resulting from sputtering to the relevant thickness. Analyzing thin films of phenylalanine of different thicknesses allows an interpretation of molecular bonding to, and orientation on, the silicon substrate that is confirmed by XPS. The large crater size for cluster ions has interesting effects on the secondary ion intensities of both the overlayer and the substrate for monolayer and submonolayer quantities. This study expands the capability of SIMS for identification of the chemical structure of molecules at surfaces. © Crown copyright 2010.     

The interface effect on the lithiation of silicon/graphene composites: The first principles study

To reveal the interaction mechanism between lithium (Li) and silicon/graphene (Si/Gra) interface at the atomic scale, it was calculated that the energy band structure, density of states, charge transfer, radial distribution function and Li diffusion coefficient based on the first principles. The results indicated that the volume expansion of Si was effectively limited by the Si/Gra interface during Li insertion. There appeared the interface effect of Si/Gra on the combination of Li and Si atoms, according to the longer Li-C (2.9 Å) and the larger electron cloud near the Li atom at the Si/Gra interface. The better diffusion channel for Li atoms was constructed at the Si/Gra interface, due to the lower diffusion energy barrier (0.42–0.44 eV) and higher diffusion coefficient (D_Li = 0.784 × 10⁻⁴ cm²/s) for Li⁺ diffusion.     

Synthesis and Crystal Structure of BaLaSi2H0.80

The polyanionic compound BaLaSi₂ featuring cis-trans silicon chains takes up hydrogen to form a hydride BaLaSi₂H_0.80. The crystal structure of the parent intermetallic compound is largely retained upon hydrogenation with the same space group type, a unit cell volume increase of 3.29 % and very similar atomic positions in the hydride. Hydrogen could be located in the crystal structure by neutron diffraction on the deuteride. Deuterium atoms occupy a tetrahedral Ba₃La interstitial with 40.6(2) % occupation (Cmcm, a = 464.43(4) pm, b = 1526.7(1) pm, c = 676.30(6) pm). BaLaSi₂H_0.80 is thus an interstitial Zintl phase hydride like LaSiH_1–x, but unlike BaSiH_2–x does not feature any covalent Si–H bonds. Si–Si distances within the polyanion increase upon hydrogenation from 240.1(6) and 242.9(5) pm to 244.7(2) pm and 245.5(2) pm. This is probably due to oxidation of the polyanion by hydrogen, which leads to the formation of hydride ions and the depopulation of the polyanion's antibonding π* states. Interatomic Ba–D [260.9(4) pm, 295.7(5) pm] and La–D distances [241.2(7) pm] are in the typical range of ionic hydrides.