Megawatt UV laser photolysis of disiloxanes: thermally stable polyoxocarbosilane powders

Megawatt ArF laser irradiation of gaseous disiloxanes [(CH₃)_nH_3−nSi]₂O, n=1,2,3 results in chemical vapour deposition of nano-sized polyoxocarbosilane powders that have large surface area, possess all possible SiC_xH_yO_z (x+y+z=4) configurations, contain SiH bonds and possess unpaired electron in orbital of Si atoms. The powders show superior thermal stability by losing only several weight per cent upon heating to 750 °C.     

Structures and bonding patterns of nanoannular carbon clusters (C4–C20) through AIM analyses

Structures, binding energies, harmonic frequencies, dipole moments, HOMO–LUMO energy gaps and particularly atoms in molecules (AIM) analyses of some nanoannular carbon clusters (C₄–C₂₀) are investigated at B3LYP/6-31+G(d) level of theory. No correlation is found by plotting the calculated binding energies as a functional number of carbon atoms of carbon clusters. The calculated binding energies sharply increase from C₄ to C₁₀ while slowly from C₁₀ to C₂₀. The binding energies of C_4n+2 clusters including C₆, C₁₀, C₁₄, and C₁₈ have a clear increase when compared with others indicating their aromatic characters which is confirmed by results of HOMO–LUMO energy gaps and geometrical parameters. AIM analyses show that most of our carbon clusters are topologically normal (non-conflict) with stable structures. Nevertheless, the topological networks of small antiaromatic rings, C₄ and C₈, at their equilibrium geometries may change via molecular vibrations. The existence of straight bond paths in 3D molecular graphs of carbon clusters with n > 10 implies that ring strains are decreased as the ring sizes grow. Except for C₄ and C₈, the ellipticity values for the remaining carbon clusters are small indicating that the C–C bond is conserved in these clusters. Dipole moments of even-numbered structures are negligible, whereas odd-numbered ones have μ values of 0.09−0.73 D.     

A novel boron-rich quaternary scandium borocarbosilicide Sc3.67−xB41.4−y−zC0.67+zSi0.33−w

A novel quaternary scandium borocarbosilicide Sc_3.67−xB_41.4−y−zC_0.67+zSi_0.33−w was found. Single crystallites were obtained as an intergrowth phase in the float-zoned single crystal of Sc_0.83−xB_10.0−yC_0.17+ySi_0.083−z that has a face-centered cubic crystal structure. Single crystal structure analysis revealed that the compound has a hexagonal structure with lattice constants a = b = 1.43055(8) nm and c = 2.37477(13) nm and space group (No. 187). The crystal composition calculated from the structure analysis for the crystal with x = 0.52, y = 1.42, z = 1.17, and w = 0.02 was ScB_12.3C_0.58Si_0.10 and that agreed rather well with the composition of ScB_11.5C_0.61Si_0.04 measured by EPMA. In the crystal structure that is a new structure type of boron-rich borides, there are 79 structurally independent atomic sites, 69 boron and/or carbon sites, two silicon sites and eight scandium sites. Boron and carbon form seven structurally independent B₁₂ icosahedra, one B₉ polyhedron, one B₁₀ polyhedron, one irregularly shaped B₁₆ polyhedron in which only 10.7 boron atoms are available because of partial occupancies and 10 bridging sites. All polyhedron units and bridging site atoms interconnect each other forming a three-dimensional boron framework structure. Sc atoms reside in the open spaces in the boron framework structure.     

A Novel Boron-Rich Scandium Borocarbide; Sc4.5−xB57−y+zC3.5−z(x=0.27, y=1.1, z=0.2)

A novel ternary boron-rich scandium borocarbide Sc_4.5−xB_57−y+zC_3.5−z (x=0.27, y=1.1, z=0.2) was found. Single crystals were obtained by the floating zone method by adding a small amount of Si. Single-crystal structure analysis revealed that the compound has an orthorhombic structure with lattice constants of a=1.73040(6), b=1.60738(6) and c=1.44829(6) nm and space group Pbam (No. 55). The crystal composition ScB_13.3C_0.78Si_0.008 calculated from the structure analysis agreed with the measured composition of ScB_12.9C_0.72Si_0.004. The orthorhombic crystal structure is a new structure type of boron-rich borides and there are six structurally independent B₁₂ icosahedra I1—I6, one B₈/B₉ polyhedron and nine bridging sites all which interconnect each other to form a three-dimensional boron framework. The main structural feature of the boron framework structure can be understood as a layer structure where two kinds of boron icosahedron network layer L1 and L2 stack each other along the c-axis. There are seven structurally independent Sc sites in the open spaces between the boron icosahedron network layers.     

Cluster cations ejected from liquid metal ion source: alkali metals (Li,Na) and group IV elements (Si,Ge, Sn,Pb)

Cluster abundance of Li _n ⁺ (n≤19), Na _n ⁺ (n≤25), Si _n ^z+ (n≤8 forz=1, 3≤n≤7 forz=2), Ge _n ^z+ (n≤11 forz=1, 3≤n≤9 forz=2,n=4 forz=3), Sn _n ^z+ (n≤7 forz=1, 3≤n≤9 forz=2,n=4 forz=3) and Pb _n ^z+ (n≤6 forz=1, 5≤n≤7 forz=2) ejected from a liquid metal ion source has been investigated by mass spectrometry. The abundance spectra of alkali metal clusters showed distinct maxima and steps atn=3, 7, 9, 13 and 19 for Li, and atn=3, 5, 11, 13 and 19 for Na. Mass spectra of Si, Ge and Sn clusters were very similar each other, showing intensity drops aftern=4 and 6 (and alson=10 for Ge) for singly charged clusters. The magic numbers observed are discussed in terms of stability of charged clusters.     

Density-Functional Theory Study on Neutral and Charged M n C2 (M = Fe, Co, Ni, Cu; n = 1–5) Clusters

The ground-state geometrical and electronic properties of neutral and charged M _n C₂ (M = Fe, Co, Ni, Cu; n = 1–5) clusters are systematically investigated by density-functional calculations. The growth evolution trends of neutral and charged Fe _n C₂, Co _n C₂, Ni _n C₂ and Cu _n C₂ (n = 1–5) clusters are all from lower to higher dimensionality, while it is special for Cu _n C ₂ ^± (n = 1–5) clusters which favor planer growth model. The space directional distributions of Co and Ni indicate stronger magnetic anisotropy than that in Cu atoms. Compare with experimental data (photoelectron spectroscopy), our results are in good agreement. The interaction strengths between metal and carbon atoms in TM–C (TM = Fe, Co, Ni) clusters are comparable and are obviously larger than that in Cu–C clusters, and this interaction strengths also decrease through the sequence: cation > neutral > anion, which may be crucial in exploring the differences in the growth mechanisms of metal–carbon nano-materials.     

Ternary rare earth and actinoid transition metal carbides viewed as carbometalates

Ternary carbides A_xT_yC_z (A=rare earth metals and actinoids; T=transition metals) with monoatomic species C⁴⁻ as structural entities are classified according to the criteria (i) metal to carbon ratio, (ii) coordination number of the transition metal by carbon atoms, and (iii) the dimensionality of the anionic network [T_yC_z]ⁿ⁻. Two groups are clearly distinguishable, depending on the metal to carbon ratio. Those where this ratio is equal to or smaller than 2 may be viewed as carbometalates, thus extending the sequence of complex anions from fluoro-, oxo-, and nitridometalates to carbometalates. The second group, metal-rich carbides with metal to carbon ratios equal to or larger than 4 is better viewed as typical intermetallics (“interstitial carbides”). The chemical bonding properties have been investigated by analyzing the Crystal Orbital Hamilton Population (COHP). The chemical bonding situation with respect to individual T-C bonds is similar in both classes. The main difference is the larger number of metal-metal bonds in the crystal structures of the metal-rich carbides.     

Normal pressure synthesis of (Tl1−yCy)Ba2Ca3Cu4O12−δ superconductor

Single phase (Tl_1−yC_y)Ba₂Ca₃Cu₄O_12−δ (Tl_1−yC_y-1234) (y = 0, 0.25, 0.5 and 0.75) superconductor samples have been prepared by solid state reaction method. The FTIR absorption measurements have confirmed the substitution of carbon at thallium site in the charge reservoir layer, (Tl_1−yC_y)Ba₂O_4−δ. The electron micrographs of these samples have shown that the carbon substitution has improved the grain morphology of Tl_0.75C_0.25-1234 sample. The y = 0.25 was found to be the optimum carbon concentration to achieve higher superconducting transition temperature T_c[0] and improved grain morphology. The superconducting transition temperature of Tl_0.75C_0.25-1234 sample has been increased to 100 K whereas a decrease in the superconducting transition temperature of Tl_1−yC_y-1234 (y = 0.5 and 0.75) samples was observed. However, the magnitude of diamagnetism has been decreased in all the carbon substituted samples.     

Studying (n, 0) and (m,m) GaP nanotubes (n = 3–10 and m = 2–6) through DFT calculations of Ga-69 quadrupole coupling constants

We investigated properties of representative zigzag and armchair gallium phosphide (GaP) nanotubes by performing density functional theory (DFT) calculations. To achieve our purpose, eight models of (n,0) zigzag GaP nanotubes with n = 3–10 and five models of (m,m) armchair GaP nanotubes m = 2–6 were considered. Each model was firstly optimized and quadrupole coupling constants (C_Q) were subsequently calculated for gallium-69 atoms of the optimized structures. The results indicated that the optimized properties including dipole moments, energy gaps, binding energies, and bond lengths could be mainly dependent on the diameters of GaP nanotubes, which are directly determined by n or m indices. Moreover, comparing the values of C_Q parameters indicated that the narrower GaP nanotubes could be considered as more reactive materials than the wider nanotubes, in which the reactivities are very important in determining the applications of nanotubes. And finally, the atoms at the sidewalls of nanotubes could be divided into atomic layers based on the similarities of properties for atoms of each layer, in which the properties of Ga atoms at the edges of nanotubes are significantly different from other layers only for wider nanotubes.     

Size effect of fullerene cages and encaged clusters in M<Subscript>3</Subscript>N@C<Subscript>2<Emphasis Type="Italic">n</Emphasis></Subscript>

Based on the calculated findings that the sizes of encaged clusters determine the structures and the stability of C₈₀-based trimetallic nitride fullerenes (TNFs), more extensive density functional theory calculations were performed on M₃N@C₆₈, M₃N@C₇₈ and M₃N@C₈₀ (M=Sc, Y and La). The calculated results demonstrated that the structures and stability undergo a transition with the increasing of the sizes of the cages and clusters. Sc₃N is planar inside the three considered cages, Y₃N is slightly pyramidal inside C₆₈-6140 and C₇₈-5 and planar inside I_h C₈₀-7, however, La₃N is pyramidal inside all the three cages. Those cages with pyramidal clusters inside deformed considerably, compared with their parent cages. In these cases, the bonding of metallic atoms toward the cages does not play an important role, and the encaged cluster tends to be located inside the cages with the largest M-M and M-C distances so that the strain energy can be released mostly. These calculations revealed the size effect of fullerene cages and encaged clusters, and can explain the position priority of M3N inside fullerene cages and the differences in yield of M₃N@C_2n . Supported by the Southwest University, China (Grant No. SWNUB2005002)     

Type A-B carbonate chlorapatite synthesized at high pressure

Sodium-bearing type A-B carbonate chlorapatites {CCLAP; Ca_10−(y+z)Na_y□_z[(PO₄)_6−(y+2z)(CO₃)_y+2z][Cl_2−2x(CO₃)_x], with x≈y≈4z≈0.4} have been synthesized from carbonate-rich melts at 1350-1000 °C and 1.0 GPa, and investigated by single-crystal X-ray structure and FTIR spectroscopy. Typical crystal and compositional data are: a=9.5321(4) Å, c=6.8448(3) Å, space group P6₃/m, R=0.027, R_w=0.025, x=0.37(3), y=0.57(2). Crystal-chemical features and FTIR spectra are similar to Na-bearing type A-B carbonate hydroxyapatites (CHAP) and fluorapatites (CFAP) reported recently. The molar amounts of Na and channel (type A) carbonate maintain a near 1:1 ratio in all three composition series, confirming that the Na cation and A and B carbonate ion substituents exist as a defect cluster within the apatite matrix, to facilitate charge compensation and spatial accommodation. Uptake of carbonate is significantly lower in CCLAP than in CHAP for similar conditions of crystal synthesis.     

Theoretical Study of the Structures,Properties and Spectroscopies on Fullerene Hydrides C<Subscript>26</Subscript>H<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 2, 4, 6, 8)

The structures, stability patterns of C₂₆H _n (n = 2) formed from the initial D _3h C₂₆ fullerene were investigated by use of second-order-Moller–Plesset perturbation theory. The study of the stability patterns of hydrogenation reaction on C₂₆ cage revealed that type (β) carbons were the active site and the analyses of π-orbital axis vector indicated that the reactivity of C₂₆ was the result of the high strain and the hydrogenation reaction on C₂₆ cage was highly exothermic. The calculated ¹³C NMR spectra of C₂₆H _n (n = 2) predicted that the two sp ³ hybridization carbons in C₂₆H _n (n = 2) obviously moved to high field compare with that in D _3h C₂₆. Hence, the C₂₆H₂ should be obtained and detected experimentally. Similarly, the structures and reaction energies of C₂₆H _n (n = 4, 6, 8) were further studied at HF/6-31G*, B3LPY/6-31G* and MP2/6-31G* level. The results suggested the hydrogenation products of C₂₆, C₂₆H _n (n = 4, 6, 8), were more stable than the C₂₆ cage.     

Reactivity of cationic decamethylmetallocene complexes towards ketones

Reaction of decamethylmetallocene cations [Cp∗₂M]⁺ (M = Sc, Ti, V) with acetone and benzophenone resulted in the formation of the corresponding acetone adducts [Cp∗₂M(OCMe₂)_n]⁺ (M = Sc, n = 2; M = Ti, n = 1; M = V, n = 1) and benzophenone adducts [Cp∗₂M(OCPh)]⁺. The stoichiometry of these adducts is determined by both the electronic configuration of the metal center as well as steric pressure imparted by the large Cp∗-ligands. In addition, the M-O-C angle is controlled by the number of free valence orbitals of the Cp∗₂M unit.     

Structural,mechanical and electronic properties of transition metal hydrides MH2 (M = Ti,Zr, Hf,Sc, Y,La, V and Cr)

First-principles calculations have been carried out to investigate the structural, mechanic and electronic of transition metal hydrides MH₂ (M = Ti, Zr, Hf, Sc, Y, La, V and Cr). It is found that TiH₂ is mechanically unstable because of a negative C₄₄ = −21.31 GPa and C₁₁–C₁₂ < 0, the same behavior can be found in MH₂ (M = Zr, Hf, and Y) compounds. Also there is a strong interaction between M (Ti, Zr, Hf, Sc, Y, La, V and Cr) and H. On the other hand, the H–H bond orders are always negative or nil reason of brittleness.     

Theoretical Study of Hydrogen Adsorption on Ruthenium Clusters

The geometries, stabilities, electronic, and magnetic properties of hydrogen adsorption on Ru _n clusters have been systematically investigated by using density functional theory with generalized gradient approximation. The result indicates the absorbed species does not lead to a rearrangement of the basic cluster. For n > 2, three different adsorption patterns are found for the Ru _n H₂ complexes: One H atom binds to the Ru top site, and another H binds to the bridge site for n = 3, 5, 6, 8; bridge site adsorption for n = 4; hollow site and top site adsorption for n = 7. The adsorption energies display oscillation and reach the peak at n = 2, 4, 7, implying their high chemical reactivity. The small electron transferred number between H atoms and Ru _n clusters indicates that the interaction between H atoms and Ru _n clusters is small. When H₂ is absorbed on the Ru _n clusters, the chemical activity of corresponding clusters is dramatically increased. The absorbed H₂ can lead to an oscillatory behavior of the magnetic moments, and this behavior is rooted in the electronic structure of the preceding cluster and the changes in the magnetic moment are indicative of the relative ordering of the majority and minority LUMO’s. The second order difference indicates 5 is magic number in Ru _n H₂ and Ru _n clusters.     

Combined DFT with NBO and QTAIM studies on the hydrogen bonds in (CH<Subscript>3</Subscript>OH)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 2–8) clusters

The (CH₃OH) _n (n = 2–8) clusters formed via hydrogen bond (H-bonds) interactions have been studied systemically by density functional theory (DFT). The relevant geometries, energies, and IR characteristics of the intermolecular OH···O H-bonds have been investigated. The quantum theory of atoms in molecule (QTAIM) and natural bond orbital (NBO) analysis have also been applied to understand the nature of the hydrogen bonding interactions in clusters. The results show that both the strength of H-bonds and the deformation are important factors for the stability of (CH₃OH) _n clusters. The weakest H-bond was found in the dimer. The strengths of H-bonds in clusters increase from n = 2 to 8, moreover, the strengths of H-bonds in (CH₃OH) _n (n = 4–8) clusters are remarkably stronger than those in (CH₃OH) _n (n = 2, 3) clusters. The small differences of the strengths of H-bonds among (CH₃OH) _n (n = 6–8) clusters indicate that a partial covalent character is attributed to the H-bonds in these clusters. The linear relationships between the electron density of BCP (ρ_b) and the H···O bond length of H-bonds as well as the second-perturbation energies E(2) have also been investigated and used to study the nature of H-bonds, respectively.