Struktur und Eigenschaften von Cd4SiS6 Zur Kenntnis von Cd4SiSe6

The structure of Cd₄SiS₆ (crystal data see ?Inhaltsübersicht”?) has been solved from single crystal X-ray data. SiS₄ and distorted CdS₄ tetrahedra (Si–S 2.101–2.132 Å, Cd–S 2.510–2.694 Å) are linked three-dimensionally by sharing corners in such a way that 2/3 of the S atoms have 2 Cd + 1 Si as bonded neighbors (pyramidal coordination), the rest is tetrahedrally cordinated by 4 Cd. The new defect tetrahedral structure, which is also representative for the isotypic series Cd₄GeS₆, Hg₄SiS₆ Hg₄GeS₆ and Cd₄GeSe₆, has a strongly distorted packing of the S atoms and shows no close relationship to known types. The Si–S bonds contain only very little π-contributions, similar to the Ge–S and Sn–S bonds in thiogermanates and -stannates. The vibrational spectra of Cd₄SiS₆ are reported and discussed. Cd₄SiSe₆, isotypic with Cd₄SiS₆, has been prepared by reaction of CdSe, Si and Se at 800–1000°.     

Thiosilicate der Selten‐Erd‐Elemente: I. Die isotypen Verbindungen KCe[SiS4] und Eu2[SiS4]

Thiosili‐Thiosilicates of the Rare‐Earth Elements: I. The Isotypic Compounds KCe[SiS₄] and Eu₂[SiS₄] Both isotypic thiosilicates KCe[SiS₄] (a = 649.15(6), b = 656.18(6), c = 863.96(8) pm, β = 107.531(9)°) and Eu₂[SiS₄] (a = 651.71(6), b = 659.54(6), c = 821.93(8) pm, β = 108.437(9)°) crystallize monoclinically in the space group P2₁/m and Z = 2. By the reaction of KCl, Ce₂S₃ and SiS₂ in the ratio 1 : 1 : 1 using a sixfold molar amount of KCl as flux in evacuated silica tubes (7 d, 850°C) brownish yellow, plate‐shaped single crystals, resistant both to air and water are obtained. The conversion of Eu, S and SiS₂ in molar ratios of 2 : 2 : 1 with an excess of CsCl as flux in evacuated silica tubes (7 d, 850°C) leads to deep red, plate‐shaped single crystals, which remain air‐ and water‐stable for a few days. The crystal structure contains isolated ortho‐thiosilicate units, that together with the Ce³⁺ or (Eu2)²⁺ cations build corrugated anionic layers parallel (001) according to {(Ce[SiS₄])^—} and {(Eu2[SiS₄])^2—}, respectively. These layers are alternatingly piled with cationic layers consisting solely of K⁺ or (Eu1)²⁺ cations. The latter show coordination numbers of eight in the shape of a bicapped trigonal prism, whereas the cations of the position Ce³⁺ and (Eu2)²⁺ have a (2+1)‐fold capped trigonal prismatic environment with a coordination number of 8+1. The comparison of both compounds KCe[SiS₄] and Eu₂[SiS₄] (≡ EuEu[SiS₄]) demonstrates, that Eu²⁺ is able to substitute both K⁺ and Ce³⁺ isomorphically.     

A computational study into the (tetrahedral) distortion of TX2α-quartz materials: The effect of changing the chemical composition away from SiO2

We report a periodic density functional study into the tetrahedral distortion of a range of quartz-like TX₂ materials. The total tetrahedral distortion and its most sizable contribution, the angular part (angular tetrahedral distortion), are found to be strongly dependent on the chemical composition and to increase in the order of BeF₂<SiO₂<CO₂<GeO₂<SiS₂<GeS₂. The bond stretching contribution to the total tetrahedral distortion appears to be significantly smaller and to vary more erratically with changes in the chemical composition. The sulphide materials in the study were found to have tetrahedral distortion values commonly associated in silica with experimentally unrealizable hypothetical frameworks, suggesting that such frameworks might become realizable when stepping away from silica. None of the tetrahedra were found to be strictly regular, in line with the analysis of Smith [Acta. Cryst. 16 (1963) 542-545], demonstrating that regular tetrahedra are in principle possible in quartz but that distortions from ideality are energetically advantageous. The energetic reason for this distortion is still an open question; we propose a simple electrostatic model that explains the ease with which tetrahedra can be distorted in terms of charge transfer and the relative charge on the X (O,S,F) atom.     

Thiosilicate der Selten‐Erd‐Elemente: III. KLa[SiS4] und RbLa[SiS4] – Ein struktureller Vergleich

Thiosilicates of the Rare‐Earth Elements: III. KLa[SiS₄] and RbLa[SiS₄] – A Structural Comparison Pale yellow, platelet shaped, air‐ and water resistant single crystals of KLa[SiS₄] derived from the reaction of lanthanum (La) and sulfur (S) with silicon disulfide (SiS₂) in a molar ratio of 2 : 3 : 1 with an excess of potassium chloride (KCl) as flux and source of potassium ions in evacuated silica ampoules at 850 °C within seven days. The analogous reaction utilizing a melt of rubidium chloride (RbCl) instead also leads to yellow comparable single crystals of RbLa[SiS₄]. The potassium lanthanum thiosilicate crystallizes monoclinically with the space group P2₁/m (a = 653.34(6), b = 657.23(6), c = 867.02(8) pm, β = 107.496(9)°) and two formula units per unit cell, while the rubidium lanthanum thiosilicate has to be assigned orthorhombically with the space group Pnma (a = 1728.4(2), b = 667.23(6), c = 652.89(6) pm) and four formula units in its unit cell. In both compounds the La³⁺ cations are surrounded by 8+1 sulfide anions in the shape of tricapped trigonal prisms. The Rb⁺ cations in RbLa[SiS₄] show a coordination number of 9+2 relative to the S^2? anions, which form pentacapped trigonal prisms about Rb⁺. This coordination number, however, is apparently too high for the K⁺ cations in KLa[SiS₄], so that they only exhibit a bicapped trigonal prismatic environment built up by eight S^2? anions. The isolated thiosilicate tetrahedra [SiS₄]^4? of the rubidium compound are surrounded by La³⁺ both edge‐ and face‐capping, but terminal as well as edge‐ and face‐spanning by Rb⁺. In the potassium compound there is no change for the La³⁺ environment about the [SiS₄]^4? tetrahedra, but the K⁺ cations are only able to attach terminal and via edges. The whole structure is built up by anionic equation/tex2gif-stack-1.gif{La[SiS₄]}^? layers that are separated by the alkali metal cations. In direct comparison the two thiosilicate structures can be regarded as stacking variants.     

Kristallstruktur und magnetische Eigenschaften eines neuen Thiosilicats des Terbiums: Tb4[SiS4]3

Crystal Structure and Magnetic Properties of a New Thiosilicate of Terbium: Tb₄[SiS₄]₃ Terbium thiosilicate Tb₄[SiS₄]₃ was obtained by reaction of the elements in a bromine atmosphere in silica ampoules. The crystal structure was determined by single crystal X—ray diffraction methods (monoclinic, P2₁/n, Z = 4, a = 983.6(2) pm, b = 1096.4(2) pm, c = 1639.1(3) pm, b = 102.76(2)°). The structure is characterized by four crystallographically independent terbium positions with coordination numbers seven and eight as well as isolated [SiS₄]^4—‐tetrahedra. The magnetic behaviour of powdered crystals was interpreted by theoretical calculations where the influence of the crystal field was taken into account by applying the angular overlap model and magnetic exchange by the molecular field approximation.     

Thiosilicate der Selten‐Erd‐Elemente: II. Die nichtzentrosymmetrischen Caesium‐Derivate CsM[SiS4] (M = Sm — Tm)

Thiosilicates of the Rare‐Earth Elements: II. The Noncentrosymmetric Cesium Derivatives CsM[SiS₄](M = Sm — Tm) The cesium lanthanoid thiosilicates CsM[SiS₄] (M = Sm — Tm) all crystallize orthorhombically in the noncentrosymmetric space group P2₁2₁2₁ with four formula units per unit cell. The lattice constants show values within the following ranges: a = 630 — 640 pm, b = 665 — 673 pm and c = 1763 — 1778 pm. The reaction of lanthanoid metal (M) with sulfur (S) and silicon disulfide (SiS₂) with an excess of cesium chloride (CsCl) serving both as flux medium and as reactand (Cs⁺ source) in evacuated silica ampoules for seven days at 850 °C leads to air‐ and water‐resistant platelet‐shaped single crystals that exhibit the colour of the lanthanoid trication (M³⁺) with a slight yellowish shade. The crystal structure arranges in layers since anionic {M[SiS₄]}^— sheets get alternatingly piled with those of Cs⁺ cations. The M³⁺ cations are surrounded capped trigonal prismatically by seven sulfide anions whereas the Cs⁺ cations have an environment of nine plus two S^2— in the shape of a fivefold overcapped trigonal prism. All sulfide anions belong to almost ideal tetrahedral ortho‐thiosilicate units [SiS₄]^4—.     

Theoretical study of silicon–sulfur clusters (SiS2)n (n = 1–6)

 The possible geometrical structures and relative stability of (SiS₂) _n (n=1–6) silicon–sulfur clusters are explored by means of density functional theory quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. The electronic structures and vibrational spectra of the most stable geometrical structures of (SiS₂) _n are analyzed by the same method. As a result, the regularity of the (SiS₂) _n cluster growth is obtained, and the calculation may used for predicting the formation mechanism of the (SiS₂) _n cluster. Received: 17 November 1999 / Accepted: 3 November 2000 / Published online: 3 May 2001     

Theoretical study of silicon–sulfur clusters (SiS2)n− (n=1–6)

The possible geometrical structures and relative stability of silicon–sulfur clusters (SiS₂) (n=1–6) are explored by means of density functional theory (DFT) quantum chemical calculations. We also compare DFT with second‐order Møller–Plesset (MP2) and Hartree–Fock (HF) methods. The effects of polarization functions, diffuse functions, and electron correlation are included in MP2 and B3LYP quantum chemical calculations, and B3LYP is effective in larger cluster structure optimization, so we can conclude that the DFT approach is useful in establishing trends. The electronic structures and vibrational spectra of the most stable geometrical structures of (SiS₂)_n⁻ are analyzed by B3LYP. As a result, the regularity of the (SiS₂)_n⁻ cluster growing is obtained, and the calculation may predict the formation mechanism of the (SiS₂)_n⁻ cluster. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 280–290, 2001     

Züchtung und röntgenographische Untersuchung von Einkristallen der Thiosilicate (Mn,Mg)2SiS4, (Mn,Fe)2SiS4 und (Mg,Fe)2SiS4 mit Olivinstruktur

Crystal Growth and Structure of Olivine Type Compounds (Mn, Mg)₂SiS₄, (Mn,Fe)₂SiS₄ and (Mg,Fe)₂SiS₄ Single crystals of (Mn, Mg)₂SiS₄, (Mn,Fe)₂SiS₄ and (Mg,Fe)₂SiS₄ were obtained by chemical transport reactions. Characterization of the crystals was done by electron microprobe analysis and X-ray methods. The compounds have the olivine structure with space group Pnma. Cell dimensions are a = 1267.6(1), b = 743.0(2), c = 592.7(2) pm for Mn_1,4Mg_0,6SiS₄, a = 1265.1(4), 736.1(2), c = 589.9(3) pm for Mn_1,4Fe_0,6SiS₄ and a = 1258.6(7), b = 732.9(3), c = 587.0(3) pm for Mg_1,14Fe_0,86SiS₄. Crystal structure refinement yielded different distributions of the two metal cations on the octahedral sites of the olivine structure: while Mn²⁺ and Mg²⁺ are evenly distributed over the M(1) and M(2) positions in Mn_1,4Mg_0,6SiS₄, the M(1) position is enriched in Fe²⁺ relative to M(2) in Mg_1,14Fe_0,86SiS₄.     

VO3/CeO2(111)催化剂上丙烷氧化脱氢反应活性和选择性的密度泛函理论研究

低碳烯烃一直以来都是化工行业非常重要的基础原料,一般采用烷烃直接热裂解制得,但该方法耗能很大,经济价值有限.近年来,人们开始尝试利用氧化脱氢反应(ODH)方法制备低碳烯烃,并取得了巨大的研究进展,其中稀土氧化物负载钒氧化物催化剂具有良好的低碳烷烃氧化脱氢性能.本文分析了前人对于钒氧化物负载在CeO2表面的计算研究结果,并选取了最具代表性的VO3/CeO2(111)作为烷烃ODH制烯烃的模型催化剂,详细研究了丙烷在该催化剂体系中发生ODH反应机理.通过使用密度泛函理论,对丙烷在VO3/CeO2(111)催化剂上断裂第一根和第二根碳氢键的反应过程进行了理论模拟,并对比了丙烷制丙烯中碳氢键断裂先后的活化能及VO3/CeO2(111)催化剂材料自身的电子性质.结果表明,该催化剂的电子结构在丙烷氧化脱氢反应中扮演关键角色.在丙烷分子断裂第一根碳氢键的反应过程中,会产生两个自由电子,对其电子结构分析发现,其中的一个自由电子会局域在由VO3/CeO2(111)催化剂中五个相关氧原子的2p轨道所形成的新发生局域空轨道(NELS)上,这个独特的新发生局域空轨道只能接受一个电子,另一个电子则会通过丙基在CeO2表面发生吸附将电子传递到CeO2表面的Ce原子上;当丙烷分子进一步发生第二根碳氢键断裂反应时,同样会产生两个新的局域电子,其中一个电子局域在Ce的4f轨道上,此时CeO2表面存在两个局域电子,相互排斥,导致该催化剂上丙烷断裂第二根碳氢键所需的活化能远高于第一根碳氢键.综上,本文对VO3/CeO2(111)催化剂上低碳烷烃ODH反应独特的催化活性和选择性给出了较为细致的分析和解释.     

Estimates for systematic empirical corrections of consistent 4–21G ab initio geometries and their correlations to total energy group increments

The structural parameters of the completely relaxed 4–21G ab initio geometries of more than 30 basic organic compounds are compared to experimental results. Some ranges for systematic empirical corrections, which relate 4–21G bond distances to experimental parameters, are associated with total energy increments. In general, for the currently feasible comparisons, the following corrections can be given which relate calculated distances to experimental r_g parameters and calculated angles to r_s-structures For CC single bond distances, deviations between calculated and observed parameters (r_g) are in the ranges of ?0.006(2) to ?0.010(2) Å for normal or unstrained hydrocarbons; ?0.011(3) to ?0.016(3) Å for cyclobutane type compounds; and +0.001(5) to +0.004(4) Å for CH₃ conjugated with CO. For CO single bonds the ranges are ?0.006(9) to +0.002(3) Å for CO conjugated with CO; and ?0.019(3) to ?0.027(9) Å for aliphatic and ether compounds. A very large and exceptional discrepancy exists for the highly strained ethylene oxide, r_s — r_e = ?0.049(5) Å and in CH₃OCH₃ and C₂H₅OCH₃ the r_s — r_e differences are ?0.029(5), ?0.040(10) and ?0.025(10) Å. Some of these discrepancies may also be due to deficiencies of the microwave substitution method caused by atomic coordinates close to inertial planes. For CN bonds, two types of NCH₃ corrections are from +0.005(6) to ?0.006(6) and from ?0.009(2) to ?0.014(6) Å; and the range for NCO is +0.012(3) to +0.028(4) Å. For isolated CC double bonds the range is + 0.025(2) to +0.028(2) Å. For conjugated CC double bonds the correction is less positive (+0.014(1) Å for benzene). For CO double bonds the corrections are ?0.004(3) to +0.003(3) Å. For bond angles of type HCH, CCH, CCC, CCO, CCO, OCO, NCO and CCC the corrections are of the order of magnitude about 1–2° (or better). Angles centered at heteroatoms are less accurate than that, when hydrogen atoms are involved. Differences in HOC and NHC angles were found in a range of ?2.3(5)° to ?6.2(4)°.     

Ab initio versus CNDO potential surface calculations for Li2O and Al2O

Potential surfaces for Li₂O and Al₂O have been calculated by an ab initio SCF-LCAO-MO method and by the semiempirical CNDO method. For both molecules the semiempirical methods incorrectly imply unreasonable structures with very acute apex angles and very long bond distances — rather more like diatomic Li₂ or Al₂ molecules with O-atoms attached to their bonds. Our ab initio treatment does correctly predict a symmetrical linear configuration for Li₂O with bond distances in excellent agreement with experiment. This method also predicts a linear symmetrical structure for Al₂O, in agreement with experimental gas phase measurements but in disagreement with matrix-isolation studies.     

Elucidating heteroatom influence on homonuclear 4J(H,H) coupling constants by DFT/NMR approach

We report the structural dependency of long range scalar J-coupling constant across four bonds as function of the dihedral angles Φ1 and Φ3. The calculated homonuclear coupling constants ⁴J(_H,H), obtained at a density functional theory level, were measured between C(1)─X(2) and X(2)─C(3) bonds in three-term models, where C, N, O, and S were systematically used as the second atom of the alkyl structures ( 1 - 4 ). The ⁴J_(H,H) calculated values, tabulated for variation of 30° for both Φ1 and Φ3, have disclosed an unexpected detectable coupling constant (⁴J(_H,H) ≥ 1 Hz) across heteroatoms, useful to provide valuable structural information. A 2-methyl-1,3-dithiane sulfide ( 5 ) was used as a case study to prove the applicability and reliability of the calculated values to real issues. The ⁴J(_H,H) values obtained at density functional theory for the system 4 have reproduced with good accuracy an unexpected experimental ⁴J(_H2ax-H4ax) = 1.01 Hz of sulfide molecule ( 5 ), suggesting these calculated coupling constant values as a new powerful tool for the organic synthesis and stereochemical analysis.     

Synthese und Kristallstrukturen von Bromid—Thiosilicaten Ln3Br[SiS4]2 der Lanthanide (Ln = La,Ce, Pr,Nd, Sm,Gd)

Synthesis and Crystal Structures of Lanthanide Bromide Thiosilicates Ln₃Br[SiS₄]₂ (Ln = La, Ce, Pr, Nd, Sm, Gd) Single crystals of the bromide—thiosilicates Ln₃Br[SiS₄]₂ were prepared by reaction of lanthanide metal (Ln = La, Ce, Pr, Nd, Sm, Gd), sulfur, silicon and bromine in quartz glass tubes. The thiosilicates crystallize in the monoclinic spacegroup C2/c (Z = 4) isotypically to the iodide analogues Ln₃I(SiS₄)₂ and the A—type chloride—oxosilicates Ln₃Cl[SiO₄]₂ with the following lattice constants: La₃Br[SiS₄]₂: a = 1583.3(4) pm, b = 783.0(1) pm, c = 1098.2(3) pm, β = 97.33(3)° Ce₃Br[SiS₄]₂: a = 1570.4(3) pm, b = 776.5(2) pm, c = 1092.2(2) pm, β = 97.28(2)° Pr₃Br[SiS₄]₂: a = 1562.6(3) pm, b = 770.1(2) pm, c = 1088.9(2) pm, β = 97.50(2)° Nd₃Br[SiS₄]₂: a = 1561.4(4) pm, b = 766.0(1) pm, c = 1085.3(2) pm, β = 97.66(3)° Sm₃Br[SiS₄]₂: a = 1555.4(3) pm, b = 758.5(2) pm, c = 1079.9(2) pm, β = 98.28(2)° Gd₃Br[SiS₄]₂: a = 1556.5(3) pm, b = 750.8(1) pm, c = 1074.5(2) pm, β = 99.26(2)° In the crystal structures the bromide ions form chains along [001] with trigonal planar coordination by lanthanide cations, while the [SiS₄]^4‐—building units display isolated distorted tetrahedra.     

Glass-forming region and structure in SiS2Li2SLiX (X = Br,I)

The glass-forming region in the SiS₂Li₂SLiX (X = Br, I) system, as determined by X-ray diffraction, has been extended to higher lithium content by the use of a direct liquid nitrogen quench. The base glass, 0.4SiS₂0.6Li₂S, was synthesized and doping up to 0.28SiS₂0.42Li₂S0.30LiBr and 0.24SiS₂0.36Li₂S0.40LiI was achieved. The properties of these glasses are compared to the glasses formed by previously reported quenching methods.Vibrational assignments have been made from infrared investigation of the glasses using FT-IR. The basic glass network consists of SiS₄ tetrahedra while the addition of Li₂S as a network modifier produced nonbridging sulfur. Doping with lithium halide did not significantly change the glass network, i.e., no changes in IR peak positions were observed. However, the higher lithium ion concentration resulted in higher mobility and ionic conduction.     

Effect of hydration on the stability of fullerene-like silica molecules

The hydration of fullerene-like silica molecules was studied by the density functional method (exchange-correlation functional B3LYP, basis set 6-31G**). It was demonstrated that completely coordinated structures transform to more stable hydroxylated ones during hydrolysis. These in turn react with H₂O molecules with the formation of hydrogen bonds.     

Researches by the method of low-temperature adiabatic calorimetry and quantum chemical computation of vibrational states

The temperature dependence of heat capacity of a natural zinc silicate, hemimorphite Zn₄Si₂O₇(OH)₂·H₂O, over the temperature range 5–320 K has been investigated by the method of low-temperature adiabatic calorimetry. On the basis of the experimental data on heat capacity over the whole temperature interval, its thermodynamic functions C _p (T), S(T) and H(T) ? H(0) have been calculated. The existence of a phase transition in the area of 90–105 K determined on the basis of vibrational spectra has been confirmed, and changes of entropy ΔS _tr. and enthalpy ΔH _tr. of the phase transition have been calculated. Hemimorphite heat capacity has also been determined by the calculation methods according to the valence force field model in LADY program. The values of force constants of valence bonds and angles have been calculated by semi-empirical method PM5. The calculated IR and Raman spectra concordant with the experimental spectra have been obtained. The heat capacity values calculated according to the found vibrational states satisfactorily agree with those experimentally obtained with an accuracy of ±1.7% in the area of 120–200 K, and not more than ±0.8% for the interval of 200–300 K. This fact testifies that the calculation of thermodynamic characteristics is correct.     

Structural determination of a recalcitrant molecule (S2F4)

The structure determination of S₂F₄ (or SF₃SF), the dimer of SF₂, is made difficult by the large variety of possible conformers and the instability of the compound. An electron diffraction and microwave study succeeded only with the help of a molecular model derived from ab initio calculations, after initial experimental attempts had failed. The force field required for a joint electron diffraction and microwave spectroscopy analysis was calculated by ab initio methods and adjusted to the experimental vibrational frequencies. The structure of S₂F₄ is a trigonal bipyramid with the electron lone pair, the SF group and one fluorine atom in equatorial positions. The SF₃ group is strongly distorted with the two axial SF bonds differing by 0.10 Å and bond angles between axial bonds and equatorial plane of about 77° and 92°, respectively. The geometric structure of S₂F₄ in combination with the ab initio calculations allows one to visualize the dissociation process SF₃SF → 2 SF₂ more clearly.     

Variation of calculated single bond lengths in saturated hydrocarbon and silicon hydride molecules

A series of saturated X_nH_m (X being either C or Si) molecules is studied using the MINDO /3 and MNDO quantum-chemistry procedures. We find that the proximity of H atoms tends to shorten X? X bonds, and that this effect is (especially for MINDO /3) much larger than a similar trend in experimental values. The connection between this and hybridization values calculated from localized orbitals is investigated. It is found that the hybridization model works well for X? H bonds, whereas for X? X bonds non-σ-bond effects also play a significant role. It is shown that the changes in electronic structure may be directly connected to differences in X and H atomic levels, so that similar bond length differences may be expected in other molecular orbital calculations.     

FT-IR and FT-Raman vibrational spectra and molecular structure investigation of nicotinamide: A combined experimental and theoretical study

In this work, the experimental and theoretical spectra of nicotinamide (C₆H₆N₂O) are studied. FT-IR and FT-Raman spectra of title molecule in the liquid phase have been recorded in the region 4000–100 cm^?1. The structural and spectroscopic data of the molecule in the ground state have been calculated by using Hartree–Fock and density functional method (B3LYP) with the 6-31+G*(d, p) and 6-31++G* (d, p)basis set. The vibrational frequencies have been calculated and scaled values have been compared with the experimental FT-IR and FT-Raman spectra. The observed and calculated frequencies are found in good agreement. The DFT-B3LYP/6-31++G (d, p) calculations have been found are more reliable than the ab initio HF/6-31+G (d, p) calculations for the vibrational study of nicotinamide. The optimized geometric parameters (bond lengths and bond angles) are compared with experimental values of the molecule. The alteration of vibrational bands due to the substitutions in the base molecule is also investigated from their characteristic region of linked spectrum.