On the electronic and structural properties of aluminum diboride Al0.9B2

Single crystals of aluminum diboride (space group P6/mmm, No. 191) a=3.0050(1) Å, c=3.2537 (8)  Å; Z=1) were prepared by the aluminum flux method. Crystal structure refinement shows defects at the aluminum site and resulted in composition Al_0.894(9)B₂≈Al_0.9B₂. The defect structure model is confirmed by the measured mass density ρ_exp=2.9(1) g/cm³ in comparison with a calculated value ρ_x=3.17 g/cm³ for full occupancy of the aluminum position. The results of ¹¹B NMR measurements support the defect model and are in agreement with the structure obtained by X-ray diffraction methods. Electrical resistivity measured on a single crystal parallel to its hexagonal basal plane with ρ(300 K)−ρ(2 K)=2.35 μΩ cm shows temperature dependence like a typical metal. Charge is dominantly carried by holes (Hall-coefficient R=+2×10⁻¹¹ m/C). Respective, p-type conductivity is confirmed by theoretical calculations. Chemical bonding in aluminum diboride is discussed using the electron localization function.     

A thermochemical study of dissociative ionization of 4,4-dimethyl-1-thia-4-silacyclohexane and 2,3,3-trimethyl-1-thia-3-silacyclopentane. Decomposition pathways to produce a dimethylsilanethione ion-radical [Me2SiS]+•

The dissociative ionization of 4,4-dimethyl-1-thia-4-silacyclohexane (I) and 2,3,3-trimethyl-1-thia-3-silacyclopentane(II) has been studied by electron photoionization (PI) mass spectrometric methods. The molecular ion fragmentation is mainly related to the loss of ethylene and results in a [Me₂SiSC₂H₄]^+? (m/z 118) ion-radical (A). Further loss of ethylene from A produces a dimethylsilanethione [Me₂SiS]^+? (m/z 90) ion-radical (B). The latter is the most abundant ion in the mass spectra of I and II at 70 eV.The ionization energies (IE) of I (8.22 ± 0.07 eV) and II (8.06 ± 0.03 eV) and the appearance energies (AE) of ion-radicals A and B have been determined. Also, the following heats of formation were calculated (kJ/mol): ΔH_f⁰(I) = ?31.1; ΔH_f⁰(II) = ?65.8; ΔH_f⁰(M_I^+?) = 762.0; ΔH_f⁰(M_II^+?)= 712.1; ΔH_f⁰(A)_aver = 780.2; ΔH_f⁰(B)_aver = 847.7.     

Actinide(III) carbonate complexation

The following equations have been developed to estimate trivalent actinide and lanthanide carbonate stability constants: log B₁⁰ = −6.128+35.206R−21.557R²; log B₂⁰ = 14.797+7.945R−10.304R², where B₁⁰ = aMCO₃⁺/(aM³⁺aCO₃²⁻, B₂⁰ = aM(CO₃)₂⁻(aM³⁺(aCO₃²⁻), aX is the activity of ion X, M indicates a metal ion and R is the effective ionic radius of the metal ion in six-fold coordination. These equations describe carbonate stability constants for cerium, europium and ytterbium at zero ionic strength. Constants at zero ionic strength were estimated from experimental determinations made in 0.68 molal NaClO₄ by accounting for medium effects.     

Liquid-liquid phase equilibria for some polystyrene-methylcyclohexane mixtures

Liquid-liquid cloud point diagrams of solutions of nearly monodisperse samples of polystyrene (PS), and binary mixtures of nearly monodisperse PS’s, both in methylcyclohexane (MCH), were determined for several polymer molecular weights (M_w) at 0.1 MPa. The bimodal mixtures (PS[M_w(1),ρ(1)] + PS[M_w(2),ρ(2)], M_w(1)=90×10³ g/mol, M_w(2)=13×10³ g/mol, 5.78 × 10³ g/mol, and 2.2 × 10³ g/mol, ρ=1.06) were prepared constraining 〈M_w〉=38.6×10³ g/mol, ρ=M_w/M_n is the polydispersity index. In each case the cloud point curves (CPC’s) for the bimodal mixtures are strongly skewed, lying well above CPC for 〈M_w〉 when φ<φ_CRITICAL, and below CPC for 〈M_w〉 when φ>φ_CRITICAL; φ is volume fraction polymer in the polymer/solvent mixture. The experimental results are discussed in the context of empirical and mean-field representations.     

The pure rotational raman spectrum of 1,3-5-trideuterobenzene,C61H32H3

The pure rotational Raman spectrum of C₆¹H₃²H₃ has been recorded photographically at 288 K and analysed to yield values of the rotational constants B₀ and D_J. In combination with the B₀ values for C₆¹H₆ and C₆²H₆ the bond lengths r(C—H) and r₀(C—;C) were calculated; r₀(C—H) = 108.60 ± 0.04 pm; r₀(C—C) = 139.660 ± 0.008 pm.     

The line profiles of single rotational lines of the H 2 B - X(3, 0) band in an intense infrared light field

The first investigation of the lineshape of single rotational lines of the H ₂ B - X(3, 0) band in an intense infrared light field (λ = 1064 nm) at intensities between 10¹⁰ W/cm² and 10¹² W/cm² is reported. The B - X(3, 0) band is excited with low intensity vacuum ultraviolet radiation (λ ≈ 106 nm). B-state excitation is observed by multiphoton ionization and dissociation of H ₂ with H ⁺ as final product. The lineshapes of the B-X(3, 0) band behave differently in both decay channels. This indicates that they branch before the molecule is photoionized. The intensity dependence of the lineshapes seems to show that at certain intensities in the focused infrared light beam the AC-Stark effect induces transient resonances into the multiphoton excitation process starting at the B(v = 3, J) rotational levels. A qualitative analysis of the states which may influence the B(v = 3, J) multiphoton excitation rate is given.     

Fluorescence line narrowing spectroscopy of Eu in zinc-thallium-tellurite glass

The environment of Eu³⁺ in zinc-thallium-tellurite glass of the molar composition 60TeO₂-30TlO_0.5-9.9ZnO-0.1Eu₂O₃ was investigated by laser-induced fluorescence line narrowing (FLN) techniques using Eu³⁺ as a local site probe. From the site selective luminescence spectra of Eu³⁺ at 7 K, the energies of the Stark components of the ⁷F₁ and ⁷F₂ states were recorded and then the crystal field parameters B_nm were calculated assuming a C_2v site symmetry. The ratios B₂₂/B₂₀ and B₄₄/B₄₀ for each excitation energy within ⁷F₀-⁵D₀ transition were obtained and compared with the values calculated for Eu³⁺ in other types of glasses.     

Electronic absorption spectrum of Fe3+ doped in ammonium chloride single crystal

The electronic absorption spectrum of the Fe²⁺ ion doped in ammonium chloride has been studied at room and liquid air temperatures. The observed bands have been assigned transitions from the ground ⁶A_1g(S) state to the excited ⁴A_1g(⁴E_g), ⁴T_1g(G) and ⁴T_2g(G) states. The cubic field approximation with D_q = 675 cm^?1, B = 645 cm^?1 and C = 4.4 B is found to give a good fit to the observed band positions.It is further concluded that the site symmetry of the Fe³⁺ ion in the crystal is lowered from O_h to C_4v symmetry at liquid air temperature.     

Numerical evaluation of second and third virial coefficients of some inert gases via classical cluster expansion

In this project we evaluate second virial coefficient of some inert gases via classical cluster expansion, assuming each atomic pair interaction is of Lennard-Jones type. We also try to numerically evaluate the third virial coefficient of Argon gas based on bipolar-coordinate integration (Mas et?al. in J Chem Phys 10:6694, 1999), assuming the same Lennard-Jones potential as before. The second virial coefficient (Vega et?al. in Phys Chem Chem Phys 4:3000–3007, 2002) calculated from our model are compatible to the experimental data [19] The temperature at which B ₂(T) → 0 is called the Boyle’s temperature T _B (Vega et?al. in Phys Chem Chem Phys 4:3000–3007, 2002) for the Lennard-Jines (12-6) potential. For the second virial coefficient of He, we obtain the Boyle’s temperature as follow: T _B ?=?34.9312438964844 (K) B ₂(T) = 9.82958 × 10^?6 (cm³/mol).     

Evolution structurale de monoxydes non-stoechiométriques du type wüstite: Simulations et relations entre paramètres structuraux

For nonstoichiometric monoxides M_1?zO (or MO_x) of the wüstite type, it is possible to forecast the trend of experimental graphs representing the parameters a of the cubic cell or the temperature factor B vs temperature θ or composition z (or x). A criterion for the experimental accuracy allows to justify the existence or not of a curvature on the graphs. The parameter a is calculated a priori for wüstite (M = Fe) vs z. A model gives the law of variation of avs some ionic species and the ratio $\text{? =}\text{(z + t)}\text{t}$; t is the rate of intertitials. The law is $\text{a = a}{}_0\text{[1 ?}\text{1}\text{3}\text{β1z]}$. The calculated value obtained for β1 agrees well with the experimental mean value 〈β〉 = 0.28. This model applies to the monoxide Mn_1?zO for which the calculated value of β1 is close to 〈β〉 = 0.29. In a second model, the cell volume is defined as being the weighted mean of the volumes of distorted and undistorted cells. With knowledge of β1 and ?, it is possible to evaluate the mean radius of a vacancy for each oxide. The factor B is the sum of two contributions B_Th(θ) and B_St(z). This latter varies linearly with z. The coefficient $\text{p = (}\text{?B}\text{?z}\text{)}{}_{\mathrm{\theta }}$ can be calculated a priori from simulations of the shifts resulting from clusters of point defects. Knowing β1, p may be located between 3 and 10 Ų according to the assumptions. The experimental value for the wüstite under equilibrium conditions is p = 4.2 Ų. An empirical relation between B_Th(θ) and a(θ) is discussed from the point of view of Grüneisen's law. When the molar heat C_p is known, it is possible to evaluate the mean force constant D = 0.78 mdyne/Å for the bonds in Fe_1?zO. The compressibility coefficient χ₀ is then obtained. It can be compared with the measured value from the literature, at 25°C under zero pressure.     

Cluster bonding and energetics of the borane anions,BnHn2− (n = 5–12): A comparative study using bond length—bond enthal

Cluster bond enthalpies, E_L(BB), and orders, n?(BB), for the structurally characterised closo anions, B_nH_n^2? (n = 6 and 8–12), have been estimated using the logarithmic length—enthalpy and enthalpy—order relationships E_L(BB) (kJ mol^?1) = 1.766 × 10¹¹ [L(BB)]^?4.0 and E_L(BB) (kJ mol^?1) = 318.8[n?(BB)]^0.697, respectively. In a parallel study, the molecular-orbital bond index CNDO-based calculation method has been used to give BB and BH bond indices, I(BB) and I(BH), from which bond index based bond enthalpies, E_I, have been calculated using the relationships E_I(BB) = 297.9 I(BB) and E_I(BH) = 374.8I(BH) (enthalpies in kJ mol^?1; lengths in pm). From these, total skeletal bond enthalpies Σ E(BB), and total bond enthalpies, Σ E(BB) + Σ E(BH), have been calculated. Although calculated values of E_L and Σ E_L generally exceed those of E_I and Σ E_I by some 8% and calculated values of I generally exceed those of n? by a greater amount, the trends in these parameters for the series of B_nH_n^2? anions are very similar, showing the greater efficiency with which the n + 1 skeletal electron pairs are used as n increases. However, the two approaches differ in that, whereas the Σ E_I values suggest that the anions are all of comparable stability, the ΣE_L values clearly show B₆H₆^2?, B₁₀H₁₀^2? and B₁₂H₁₂^2? to be more stable than B₈H₈^2?, B₉H₉^2? and B₁₁H₁₁^2?. The interatomic distances in B₇H₇^2? and in the unknown B₅ H₅^2? are estimated and used to assess their relative stabilities. The E_L values suggest that B₇ H₇^2? is of comparable stability to B₈H₈^2? etc., but show B₅H₅^2? as relatively unstable. The E_I values suggest that both of these anions should be relatively stable members of the series of closo anions.     

Synthesis and crystal structure of new uranyl tungstates M2(UO2)(W2O8) (M=Na, K), M2(UO2)2(WO5)O (M=K, Rb), and Na10(UO2)8(W5O20)O8

The solid-state reactions of UO₃ and WO₃ with M₂CO₃ (M=Na, K, Rb) at 650°C for 5 days result, accordingly the starting stoichiometry, in the formation of M₂(UO₂)(W₂O₈) (M=Na (1), K (2)), M₂(UO₂)₂(WO₅)O (M=K (3), Rb (4)), and Na₁₀(UO₂)₈(W₅O₂₀)O₈ (5). The crystal structures of compounds 2, 3, 4, and 5 have been determined by single-crystal X-ray diffraction using Mo(Kα) radiation and a charge-coupled device detector. The crystal structures were solved by direct methods and Fourier difference techniques, and refined by a least-squares method on the basis of F² for all unique reflections. For (1), unit-cell parameters were determined from powder X-ray diffraction data. Crystallographic data: 1, monoclinic, a=12.736(4) Å, b=7.531(3) Å, c=8.493(3) Å, β=93.96(2)°, ρ_cal=6.62(2) g/cm³, ρ_mes=6.64(1) g/cm³, Z=4; 2, orthorhombic, space group Pmcn, a=7.5884(16) Å, b=8.6157(18) Å, c=13.946(3) Å, ρ_cal=6.15(2) g/cm³, ρ_mes=6.22(1) g/cm³, Z=8, R1=0.029 for 80 parameters with 1069 independent reflections; 3, monoclinic, space group P2₁/n, a=8.083(4) Å, b=28.724(5) Å, c=9.012(4) Å, β=102.14(1)°, ρ_cal=5.83(2) g/cm³, ρ_mes=5.90(2) g/cm³, Z=8, R1=0.037 for 171 parameters with 1471 reflections; 4, monoclinic, space group P2₁/n, a=8.234(1) Å, b=28.740(3) Å, c=9.378(1) Å, β=104.59(1)°, ρ_cal=6.13(2) g/cm³,  g/cm³, Z=8, R1=0.037 for 171 parameters with 1452 reflections; 5, monoclinic, space group C2/c, a=24.359(5) Å, b=23.506(5) Å, c=6.8068(14) Å, β=94.85(3)°, ρ_cal=6.42(2) g/cm³,  g/cm³, Z=8, R1=0.036 for 306 parameters with 5190 independent reflections. The crystal structure of 2 contains linear one-dimensional chains formed from edge-sharing UO₇ pentagonal bipyramids connected by two octahedra wide (W₂O₈) ribbons formed from two edge-sharing WO₆ octahedra connected together by corners. This arrangement leads to [UW₂O₁₀]²⁻ corrugated layers parallel to (001). Owing to the unit-cell parameters, compound 1 probably contains similar sheets parallel to (100). Compounds 3 and 4 are isostructural and the structure consists of bi-dimensional networks built from the edge- and corner-sharing UO₇ pentagonal bipyramids. This arrangement creates square sites occupied by W atoms, a fifth oxygen atom completes the coordination of W atoms to form WO₅ distorted square pyramids. The interspaces between the resulting [U₂WO₁₀]²⁻ layers parallel to plane are occupied by K or Rb atoms. The crystal structure of compound 5 is particularly original. It is based upon layers formed from UO₇ pentagonal bipyramids and two edge-shared octahedra units, W₂O₁₀, by the sharing of edges and corners. Two successive layers stacked along the [100] direction are pillared by WO₄ tetrahedra resulting in sheets of double layers. The sheets are separated by Na⁺ ions. The other Na⁺ ions occupy the rectangular tunnels created within the sheets. In fact complex anions W₅O₂₀¹⁰⁻ are built by the sharing of the four corners of a WO₄ tetrahedron with two W₂O₁₀ dimmers, so, the formula of compound 5 can be written Na₁₀(UO₂)₈(W₅O₂₀)O₈.     

New details in the polarized spectrum of naphthalene by means of linear dichroism studies in oriented polymer matrices

Linear dichroism (LD) spectra are presented for naphthalene oriented in stretched polyethylene and polypropylene matrices at 77 K and 296 K. From the calculated spectrum LD(λ)/A(λ), where A(λ) is the corrected absorbance spectrum of the sample by unpolarized light, orientational parameters are calculated and component spectra, 235–315 nm, are resolved corresponding to polarization parallel to the long (B_3u = x) and the short (B_2u = y) axes in the molecular plane (D_2h). The orientational parameters indicate different orientational mechanisms in polyethylene and polypropylene, but the resolving procedure yields mainly identical component spectra. It is suggested that the polarization (B_3u) predominating in the 245–275 nm region isdue to a B_1g vibronic perturbation of the ¹B_2u state.     

Monotonicity properties of the atomic charge density function

The present knowledge of the monotonicity properties of the spherically averaged electron density ρ(r) and its derivatives, which comes mostly from Roothan-Hartree-Fock calculations, is reviewed and extended to all Hartree-Fock ground-state atoms from hydrogen (Z = 1) to uranium (Z = 92). In looking for electron functions with universal (i.e., valid in the whole periodic table) monotonicity properties, it is found that there exist positive values of α so that the function g_o(r; α) = ρ(r)/r^α is convex, and g₁(r;α) = −ρ′(r)/r^α is not only monotonically decreasing from the origin but also convex. This is, however, not the case for the function g₂(r; α) = ρ′(r)/r^α. Additionally, the conditions which specify values for β such that the function g_n(r; β) = (−1) ′ρ⁽ⁿ⁾(r)/r^β is logarithmically convex are obtained and numerically calculated for n = 0,1 in all neutral atoms below uranium. The last property is used to obtain inequalities of general validity involving three radial expectation values which generalize all the similar ones known to date, as well as other relationships among these quantities and the values of the electron density and its derivatives at the nucleus. © 1996 John Wiley & Sons, Inc.     

Vibrational spectra of alkali metal peroxoborates

Raman and i.r. spectra of solid Na₂[B₂(O₂)₂(OH)₄]·6H₂O (normal, ¹⁰B, ¹¹B, and ²H-substituted), Na₂[B₂(O₂)₂(OH)₄]·nH₂O (n=4, 0), Li₂[B₂(O₂)₂(OH)₄] and M^I₂[B₂(O₂)₂(OOH)₂(OH)₂] (M^I= K, Rb, Cs) are reported and the vibrational modes assigned.     

Equilibrium structure and vibrational frequencies of the O4 molecule

SCF closed shell calculations were performed to determine the equilibrium structure and vibrational frequencies of the O₄ molecule by means of Payne's method and with the help of the molecule's symmetry coordinates. The equilibrium geometry corresponds to symmetry group D_2d with R = 1.505 Å and h = 0.094 Å. The vibrational frequencies are: ν₅(E) = 885.5 cm^?1, ν₃(B₁) = 1051.9 cm^?1, ν₁(A₁) = 1018.3 cm^?1, ν₄(B₂) = 880.3 cm^?1. The second vibrational coordinate (A₁) corresponds to a double-well potential. The first vibrational levels were calculated by a variational method.     

Effects of substituents and counterions on the structures of silenolates: a computational investigation

The structures and charge distributions of substituted silenolates [H₂SiC(O)X]⁻ (X=H, SiH₃, Me, t-Bu, OMe, NMe₂; group A), [Y₂SiC(O)H]⁻ (Y=H, F, Me, Ph, SiH₃, SiMe₃; group B), and [Y₂SiC(O)X]⁻ (Y=Me, X=t-Bu, and Y=SiMe₃; X=t-Bu, OMe, NMe₂; group C) were examined through density functional theory calculations. The effects of the solvated counterion (K⁺, Li⁺, or MgCl⁺) and coordination site (O or Si) on the properties of group C silenolates were also studied. The variation in the degree of π-conjugative reverse SiC bond polarization, ΣΦ_RP(π), calculated by natural resonance theory, was determined. The ΣΦ_RP(π) correlated with r(SiC) for both group A and B silenolates, and the correlation between ΣΦ_RP(π) and the sum of valence angles at Si, Σα(Si), was good for group A but poor for group B due to strong influence of the inductive effect. The SiC charge difference correlated well with ΣΦ_RP(π) for group A, but not for group B, again an effect of inductive substituent effects. The group C silenolates were coordinated to Li(THF)₃⁺, MgCl(THF)₄⁺, and K(THF)₅⁺ either via the O or Si atom. The coordination energies show that coordination to the hard O is preferred for Li⁺ and MgCl⁺, but the K⁺ ion coordinated simultaneously to Si and O. Coordination of the solvated metal ion to O resulted in shorter SiC bond length, an increased Σα(Si) value, and lower Δq(SiC) when compared to the naked silenolate. Choice of counterion and substituent provides a means to extensively vary the properties of silenolates such as their reactivity.     

Bond indices,enthalpies and relative stabilities of real and hypothetical closo-BnHc−n clusters (n = 5–12; c = 0, 2 or 4) as revealed by the molecular-orbital bond index method

Bond indices (I) have been calculated, using the CNDO-based molecular-orbital bond index method, for real and hypothetical closo borane species, B_nH^c−_n (n = 5–12; c = 0, 2 or 4), and used to infer their relative stabilities by means of the bond index (I)bond enthalpy [E (kJ mol⁻¹)] equations E(BB) = 297.9I(BB) and E(BH) = 374.8I(BH). For the species able to tolerate n + 2 skeletal electron pairs (n = 8, 9 or 11) in closed-shell electronic configurations, estimates of the relative stabilities of alternative nido structures for the anions B_nH⁴⁻_n have been made. Detailed assessments of the changes in bond index with electron numbers of particular edge types for B_nH^c−_n species (n = 8, 9 or 11; c = 0, 2 or 4) have been carried out, providing quantitative confirmation of earlier qualitative predictions, and showing that generally for the “normal” closo B_nH²⁻_n species addition or removal of an electron pair leads to the same type of polyhedral distortion, because, where the HOMO of B_nH²⁻_n is bonding for a particular edge, the LUMO is antibonding.     

Low-temperature heat capacity of sodium erbium molybdophosphate Na<Subscript>2</Subscript>Er(MoO<Subscript>4</Subscript>)(PO<Subscript>4</Subscript>)

Adiabatic calorimetry is used to measure the low-temperature heat capacity of Na₂Er(MoO₄)(PO₄)from 6.41 to 347.87 K. Experimental data are used to calculate the thermodynamic functions of Na₂Er(MoO₄)(PO₄), which at 298.15 K are as follows: C _p ⁰ (298.15 K) = 243,3 ± 0.4 J/(K mol), S ⁰(298.15 K) = 312.8 ± 0.8 J/(K mol), H ⁰(298.15 K) ? H ⁰(0 K) = 45280 ± 90 J/mol, and Φ⁰(298.15 K) = 136.1 ± 0.3 J/(K mol). A diffuse heat-capacity anomaly associated with splitting of the Stark levels (Schottky anomaly) is discovered in the low-temperature region.     

New examples of ternary rare-earth metal boride carbides containing finite boron-carbon chains: The crystal and electronic structure of RE15B6C20 (RE=Pr, Nd)

The ternary rare-earth metal boride carbides RE₁₅B₆C₂₀ (RE=Pr, Nd) were synthesized by co-melting the elements. They exist above 1270 K. Their crystal structures were determined from single-crystal X-ray diffraction data. Both crystallize in the space group P1¯, Z=1, a=8.3431(8) Å, b=9.2492(9) Å, c=8.3581(8) Å, α=84.72(1)°, β=89.68(1)°, γ =84.23(1)° (R1=0.041 (wR2=0.10) for 3291 reflections with I_o>2σ(I_o)) for Pr₁₅B₆C₂₀, and a=8.284(1) Å, b=9.228(1) Å, c=8.309(1) Å, α=84.74(1)°, β=89.68(1)°, γ=84.17(2)° (R1=0.033 (wR2=0.049) for 2970 reflections with I_o>2σ(I_o)) for Nd₁₅B₆C₂₀. Their structure consists of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with unprecedented B₂C₄ finite chains, disordered C₃ entities and isolated carbon atoms, respectively. Structural and theoretical analyses suggest the ionic formulation (RE³⁺)₁₅([B₂C₄]⁶⁻)₃([C₃]⁴⁻)₂(C⁴⁻)₂·11ē. Accordingly, density functional theory calculations indicate that the compounds are metallic. Both structural arguments as well as energy calculations on different boron vs. carbon distributions in the B₂C₄ chains support the presence of a CBCCBC unit. Pr₁₅B₆C₁₈ exhibits antiferromagnetic order at T_N=7.9 K, followed by a meta-magnetic transition above a critical external field B>0.03 T. On the other hand, Nd₁₅B₆C₁₈ is a ferromagnet below T_C≈40 K.