Molecular Structure of BiBr3: An Electron Diffraction Study

The molecular structure of BiBr₃ was determined by gas-phase electron diffraction. The principal geometrical parameters are r (Bi—Br) = 2.567 ± 0.005 Å and _221D;Br—Bi—Br = 98.6 ± 0.2°. The force field of the molecule was obtained by a normal coordinate analysis utilizing both experimental vibrational frequencies and electron diffraction mean amplitudes of vibration. The variation of bond lengths and bond angles within the Group 15 trihalides is consistent with the expected trend, except that all bismuth trihalide bond angles appear to be somewhat large.     

On the crystal chemistry of three copper(II)-arsenates: Cu3(AsO4)2-III,Na4Cu(AsO4)2, and KCu4(AsO4)3

The three copper(II)-arsenates were synthesized under hydrothermal conditions; their crystal structures were determined by single-crystal X-ray diffraction methods:Cu₃(AsO₄)₂-III:a=5.046(2) Å,b=5.417(2) Å,c=6.354(2) Å, =70.61(2)°, =86.52(2)°, =68.43(2)°,Z=1, space group ,R=0.035 for 1674 reflections with sin / 0.90 Å^–1.Na₄Cu(AsO₄)₂:a=4.882(2) Å,b=5.870(2) Å,c=6.958(3) Å, =98.51(2)°, =90.76(2)°, =105.97(2)°,Z=1, space group ,R=0.028 for 2157 reflections with sin / 0.90 Å^–1.KCu₄(AsO₄)₃:a=12.234(5) Å,b=12.438(5) Å,c=7.307(3) Å, =118.17(2)°,Z=4, space group C2/c,R=0.029 for 1896 reflections with sin / 0.80 Å^–1.Within these three compounds the Cu atoms are square planar [4], tetragonal pyramidal [4+1], and tetragonal bipyramidal [4+2] coordinated by O atoms; an exception is the Cu(2)^[4+1] atom in Cu₃(AsO₄)₂-III: the coordination polyhedron is a representative for the transition from a tetragonal pyramid towards a trigonal bipyramid. In KCu₄(AsO₄)₃ the Cu(1)^[4]O₄ square and the As(1)O₄ tetrahedron share a common O—O edge of 2.428(5) Å, resulting in distortions of both the CuO₄ square and the AsO₄ tetrahedron. The two Na atoms in Na₄Cu(AsO₄)₂ are [6] coordinated, the K atom in KCu₄(AsO₄)₃ is [8] coordinated by O atoms.Die drei Kupfer(II)-Arsenate wurden unter Hydrothermalbedingungen gezüchtet und ihre Kristallstrukturen mittels Einkristall-Röntgenbeugungsmethoden ermittelt:Cu₃(AsO₄)₂-III:a = 5.046(2) Å,b = 5.417(2) Å,c = 6.354(2) Å, = 70.61 (2)°, = 86.52(2)°, = 68.43(2)°,Z = 1, Raumgruppe ,R = 0.035 für 1674 Reflexe mit sin / 0.90 Å^–1.Na₄Cu(AsO₄)₂:a = 4.882(2) Å,b = 5.870(2) Å,c = 6.958(3) Å, = 98.51(2)°, = 90.76(2)°, = 105.97(2)°,Z = 1, Raumgruppe ,R = 0.028 für 2157 Reflexe mit sin / 0.90 Å^–1.KCu₄(AsO₄)₃:a = 12.234(5) Å,b = 12.438(5) Å,c = 7.307(3) Å, = 118.17(2)°,Z = 4, Raumgruppe C2/c,R = 0.029 für 1896 Reflexe mit sin / 0.80 Å^–1.Die Cu-Atome in diesen drei Verbindungen sind durch O-Atome quadratisch planar [4], tetragonal pyramidal [4 + 1] und tetragonal dipyramidal [4 + 2]-koordiniert; eine Ausnahme ist das Cu(2)^{[4 + 1]}-Atom in Cu₃(AsO₄)₂-III: Das Koordinationspolyeder stellt einen Vertreter des Übergangs von einer tetragonalen Pyramide zu einer trigonalen Dipyramide dar. In KCu₄(AsO₄)₃ haben das Cu(1)^[4]O₄-Quadrat und das As(1)O₄-Tetraeder eine gemeinsame O—O-Kante von 2.428(5) Å, was eine Verzerrung der beiden Koordinationsfiguren CuO₄-Quadrat und AsO₄-Tetraeder bedingt. Die zwei Na-Atome in Na₄Cu(AsO₄)₃ sind durch O-Atome [6]-koordiniert, das K-Atom in KCu₄(AsO₄)₃ ist [8]-koordiniert.

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Zur Kristallchemie dreier Kupfer (II)-Arsenate: Cu₃(AsO₄)₂-III, Na₄Cu(AsO₄)₂ und KCu₄(AsO₄)₃

     

Vibrational spectra,conformational stability,barriers to internal rotation,r0 structural parameters and ab initio calculations of fluoromethyl methyl ether

The far-infrared spectrum of gaseous fluoromethyl methyl ether, FCH₂OCH₃, along with three of the deuterium isotopes, has been recorded at a resolution of 0.10 cm^–1 in the 350 to 50 cm^–1 region. The fundamental asymmetric torsional and methyl torsional modes are extensively mixed and have been observed at 182 and 132 cm^–1, respectively, for the stablegauche conformer with the lower frequency band having several excited states falling to lower frequency. An estimate is given for the potential function governing the asymmetric rotation. On the basis of a one-dimensional model the barrier to internal rotation of the methyl moiety is determined to be 527±9 cm^–1 (1.51±0.03 kcal/mol). A complete assignment of the vibrational fundamentals for all four isotopic species observed from the infrared (3500 to 50 cm^–1) spectra of the gas and solid and from the Raman (3200 to 10 cm^–1) spectra of the gas, liquid, and solid is proposed. No evidence could be found in any of the spectra for the high-energytrans conformer. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing the 3-21G and 6-31G* basis sets along with the 6-31G* basis set with electron correlation at the MP2 level. Additionally, completer ₀ geometries have been determined from the previously reported microwave data and carbon-hydrogen distances determined from infrared studies. The heavy-atom structural parameters (distances in Å, angles in degrees) arer(C₁-F) = 1.395 ± 0.005;r(C₁-O) = 1.368 ± 0.007;r(C₂-O) = 1.426 ±0.003; FC₁O = 111.33 ± 0.25; C₁OC₂ = 113.50 ± 0.18 and dih FC₁OC₂ = 69.12 ± 0.26. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.     

Molecular Structure of an Isocytosine Analog: Combined X-ray Structural and Computational Study of 2-Diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone

The structure of 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone has been studied by X-ray crystallography and quantum-chemical calculations. X-ray analysis established that 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone exists exclusively as the lactam tautomer protonated at the N3 ring nitrogen in the solid state. Crystals of 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone are monoclinic (space group P2₁/n); the unit-cell dimensions are: a = 11.0460(8) Å, b = 5.0064(4) Å, c = 22.8358(17) Å, = = 90°, = 90.521(1)°. In the crystal, molecules of 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone are assembled in planar centrosymmetric dimers by strong resonance-assisted N—H···O intermolecular hydrogen bonds from the NH group of one molecule to the C=O of the adjacent molecule (N—H···O distance 2.804 Å). Bond distances and angles are generally similar to those reported for the corresponding tautomer of isocytosine and derivatives. Quantum-chemical calculations on 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone are also reported in order to estimate the relative energies of the possible tautomeric forms; ab initio and DFT results predict the coexistence of the N3 and AH tautomers in the gas phase. There is excellent correspondence between the crystal and the HF/6-311G** or B3LYP/6-31G* calculated structures of the N3 lactam form; the largest deviations between the experimental and computed structures are mostly the effects of strong intermolecular H bonds in the crystal.     

Die Kristallstruktur von TlFeSe2 und TlFeS2

TlFeSe₂ is monoclinic, space groupC2/m–C _2h ³ ,a=11.973 Å,b=5.490 Å,c=7.110 Å, =118.2°,Z=4.TlFeS₂ is isotypic witha=11.636 Å,b=5.304 Å,c=6.799 Å, =116.7°.The crystal structure of TlFeSe₂ has been determined from single crystal diffractometer data. Isotypy of the sulfide has been confirmed from powder diffraction data. The crystal structure containing infinite linear chains of edgesharing FeX ₄-tetrahedra, and its relationship to the thio- and selenoferrates of the alkali metals are discussed. The mineral raguinite is very probably isotypic to synthetic TlFeS₂.

     

Die Phase La2U2N5

In the system U–La–N a new phase of composition La₂U₂N₅ was observed. The differaction pattern of this phase can be indexed with a tetragonal unit cell:a=8.43 Å,c=8.50 Å andc/a=1.008. The pseudocubic sub-cell withaca/2 is closely related to the CsCl-type.

Auszug aus der von der Technisch-Naturwissenschaftlichen Fakuktät der TU Wien approbierten Diplomarbait des Herrn Dipl.-Ing.J. Waldhart.     

Equilibrium Structure of Beryllium Dibromide from Combined Gas Electron Diffraction and Vibrational Spectroscopy Analysis

The molecular structure of BeBr₂ has been investigated by gas-phase electron diffraction at the temperature 800(10) K. The conventional analysis yielded the following values: r _g(Be–Br) = 1.944(6)Å, l(Be–Br) = 0.068(4)Å, r _g(Br–Br) = 3.848(8)Å, l(Br–Br) = 0.109(3)Å, k(Be–Br) = 1.1(1.1) × 10^–5 ų, (Br–Br) = 2.1(1.0) × 10^–5 ų. Three models of nuclear dynamics were used to simulate the conventional analysis values—infinitesimal vibrations and two models, which take into account the kinematic and dynamic anharmonicity of the bending vibration. All models give similar values of bond angle, amplitudes, and shrinkage, excluding the harmonic model, which yields too low value l(Br–Br). The equilibrium bond distance r _e(Be–Br) = 1.932(11) Å was estimated, taking into account the anharmonicity corrections for stretching and bending vibrations and centrifugal distortion.     

Comparison of Ab Initio MP2/6-311+G(d,p) Predicted Carbon–Hydrogen Bond Distances with Experimentally Determined r 0 (C–H) Distances

Fifty different carbon–hydrogen distances have been predicted from ab initio MP2/6-311+G(d,p) calculations, which range from a short value of 1.0611 Å for HCNO to a long value of 1.1044 Å for H₂CO. The values include those predicted for a series of methyl (CH₃) moieties where the two different C–H distances vary by as much as 0.005 Å. These predicted values are compared to r ₀(C–H) distances obtained from the isolated carbon–hydrogen stretching frequencies, as well as to r ₀ or r _s parameters obtained from microwave data. Except for the very short C–H bonds, the ab initio values from the MP2/6–311+G(d,p) calculations can be used for the carbon–hydrogen distances with error limits of ± 0.003 Å. By utilizing the spectral data from CD₃CClO, it is shown that combination bands in the C–H stretching region could cause problems in the identification of the isolated C–H stretching frequency from the CD₂HCClO isotopomer. The value of the ab initio predicted C–H distances for checking unusually long or short r _s (C–H) or r ₀ values is demonstrated.     

Femtosecond dynamics of relaxation of photoexcited meso-tetraferrocenylporphyrin in the nonprotonated and diprotonated forms (Fc4PH2 and Fc4PH42+)

The relaxation of the Q¹(—*) excited state of the nonprotonated Fc₄PH₂ and diprotonated Fc₄PH₄ ²⁺ forms of meso-tetraferrocenylporphyrin was studied by femtosecond laser absorption spectroscopy. Transition from the Q¹(—*) state to the charge-transfer state was shown to occur within 208±10 fs for Fc₄PH₂ and 9±3 ps for Fc₄PH₄ ²⁺. A fast vibrational relaxation with a characteristic time of 120—140 fs was found for both forms. The relaxation time of Fc⁺—P^– charge-transfer state for Fc₄PH₂ was 17±4 ps.     

Crystal structures of Co3(SeO3)3·H2O and Ni3(SeO3)3·H2O,two new isotypic compounds

Summary The crystal structures of the new, hydrothermally synthesized, isotypic compounds Co₃(SeO₃)₃·H₂O and Ni₃(SeO₃)₃·H₂O were determined by direct and Fourier methods and refined toR _w=0.023, 0.032 using single crystal X-ray data up to sin/=0.81 Å^–1 [space group P ,a=8.102 (2), 7.986 (3) Å;b=8.219 (2), 8.133 (3) Å;c=8.572 (2), 8.422 (3) Å, =69.15 (1), 69.50 (1)°; =62.88 (1), 62.50 (1)°; =67.23 (1), 67.64 (1)°;Z=2]. The structures are built up from [Me ₅(SeO₃)₆·2H₂O]^2– sheets containing three crystallographically different types of octahedrally coordinatedMe ²⁺ and trigonal pyramidal coordinated Se⁴⁺ atoms, respectively. These sheets are linked only by a fourth type ofMe ^2+[6] atom. All coordination polyhedra deviate significantly from their ideal shapes, bond lengths within the extremly distortedMe(4)O₆ polyhedra range from 1.983 (2) Å to 2.403 (2) Å in Co₃(SeO₃)₃·H₂O and from 1.987 (4) Å to 2.301 (3) Å in the Ni compound, O-Se-O bond angles were found between 92.8 (2)° and 104.9 (1)°. Hydrogen bond lengths are 2.802 (3)Å and 2.600 (4)Å in the Co compound, and 2.762 (6) Å and 2.561 (6) Å in Ni₃(SeO₃)₃·H₂O. The latter is one of the shortest known hydrogen bonds donated by a water molecule.

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Die Kristallstrukturen von Co₃(SeO₃)₃·H₂O und Ni₃(SeO₃)₃·H₂O, zwei neue isotype Verbindungen

Zusammenfassung Die Kristallstrukturen der neuen, hydrothermal synthetisierten, isotypen Verbindungen Co₃(SeO₃)₃·H₂O und Ni₃(SeO₃)₃·H₂O wurden mit direkten und Fourier-Methoden bestimmt und unter Verwendung von Einkristallröntgendaten bis sin/=0.81 Å^–1 aufR _w-Werte von 0.023, 0.032 verfeinert [Raumgruppe P ,a=8.102 (2), 7.986 (3) Å;b=8.219 (2), 8.133 (3) Å;c=8.572 (2), 8.422 (3) Å, =69.15 (1), 69.50 (1)°; =62.88 (1), 62.50 (1)°; =67.23 (1), 67.64 (1)°;Z=2]. Die Strukturen werden von [Me ₅(SeO₃)₆·2H₂O]^2– Schichten aufgebaut, die je drei kristallographisch unterschiedliche Arten von oktaedrisch koordiniertenMe ²⁺ und trigonal pyramidal koordinierten Se⁴⁺ Atomen enthalten. Diese Schichten sind nur durch eine vierte Art vonMe ^2+[6] Atomen verknüpft. Alle Koordinationspolyeder weichen deutlich von ihren Idealformen ab, Bindungslängen in den extrem verzerrtenMe(4)O₆ Polyedern variieren zwischen 1.983 (2) Å und 2.403 (2) Å in Co₃(SeO₃)₃·H₂O und zwischen 1.987 (4) Å und 2.301 (3) Å in der Ni-Verbindung, O-Se-O-Bindungswinkel liegen zwischen 92.8 (2)° und 104.9 (1)°. Wasserstoffbrückenlängen sind 2.802 (3) Å und 2.600 (4) Å in der Co-Verbindung, und 2.762 (6) Å und 2.561 (6) Å in Ni₃(SeO₃)₃·H₂O. Letztere ist eine der kürzesten bekannten Wasserstoffbrücken eines Wassermoleküls.

     

The Structure of Gaseous Carbon Tetraiodide from Electron Diffraction and All Carbon Iodides,CIn (n = 1–4), and Their Dimers,C2I2n (n = 1–3) from High-Level Computation. Any Other Carbon-Iodide Species in the Vapor?

The geometry of a series of carbon iodides have been determined, CI₄ by gas-phase electron diffraction and CI _n (n = 1–4) and C₂I_2n (n = 1–3) by high-level quantum chemical calulations. The bond length of the tetrahedral CI₄ molecule from electron diffraction is (r _g):2.157(10) Å. The indication of about 20% I₂ in the vapor suggests partial decomposition and it has been thoroughly investigated what other carbon iodide species might be present beside CI₄. There is no appreciable amount of either of the dimeric species in the vapor phase, in spite of the suggestion from thermodynamics. On the other hand, the electron diffraction data are compatible with the presence of about 18% of either of the monomeric free radicals, CI₃ or CI₂, beside CI₄ and I₂. Possible reasons for these observations are discussed. Our correlated level computations, in agreement with other high level computations, found the singlet ¹A₁ state to be the ground state for CI₂. This is in contrast with a recent photoelectron spectroscopic study according to which the triplet state is the ground state though with a large margin of error (1 ± 3 kcal/mol energy difference). The computed singlet-triplet separation strongly depends on the level of the computation, but it is at least 9 kcal/mol. Geometrical parameters, singlet-triplet separations, and dipole moments have been calculated for the CX₂ series (X = F, Cl, Br, I, H) and their variations are discussed. The thermodynamic stability of different carbon iodide species has also been investigated.     

Darstellung und Kristallstruktur von Pb2(NO2)(NO3)(SeO3)

Crystals of Pb₂(NO₂)(NO₃)(SeO₃) were synthesized by partial reduction of nitrate ions with native copper under hydrothermal conditions. The crystal structure [a=5.529 (2) Å,b=10.357 (3) Å,c=6.811 (2) Å, space group Pmn2₁,Z=2] was determined from 1 707 independent X-ray data up to sin /=0.81 Å^–1 and was refined toR _w =0.028. The Pb(1) atom is ten coordinated to O atoms [Pb(1)-O from 2.51 Å to 2.96 Å], the Pb(2) atom has three nearest O atoms [Pb(2)-O=2.41 Å (1 ×) and 2.45 Å (2 ×)] and six next-nearest O atoms [Pb(2)-O from 2.80 Å to 3.22 Å].

Herrn Prof. Dr.K. Komarek zum 60. Geburtstag gewidmet.     

The Molecular Structure and the Puckering Potential Function of Octamethylcyclotetrasilane,Si4Me8, Determined by Gas Electron Diffraction and Relaxation Constraints from Ab Initio Calculations

Gas electron diffraction data are applied to determine the geometrical parameters of the octamethylcyclotetrasilane molecule using a dynamic model in which the ring puckering is treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined, taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree–Fock level of theory using a 6-311G** basis set. The potential function has been described as V() = V ₀[(/ _e )² – 1]² with V ₀ = 1.0 ± 0.5 kcal/mol and _e = 28.3 ± 1.9°, where is the puckering angle of the ring. The geometric parameters at the minimum of V() (r _a in Å, in degrees and errors given as three times the standard deviations including a scale error) are as follows: r(Si—C)_av = 1.894(3), r(Si—Si) = 2.363(3), r(C—H) = 1.104(3), CSiC = 109.5(6), SiSiSi = 88.2(2), SiCH = 111.7(6), C = 4.1, where the tilt C was estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds.     

A complex of buckminsterfullerene with sulfur,C60·2S8: synthesis and crystal structure

The C₆₀·2S₈ complex was prepared by reaction of buckminsterfullerene C₆₀ with sulfur in trichloroethylene and its single-crystal X-ray structure was studied at room temperature. Crystals of this compound are monoclinic, space groupC 2/c, a=20.90(1),b=21.10(1),c=10.537(9) Å, =111.29(7)°,Z=4,d _calc=1.89 g·cm^–3. The crystal structure of the C₆₀·2S₈ complex consists of packed fullerene molecules that form hexagonal channels along thec axis with eight-membered crown-shaped S₈ cyclic molecules inside the channels. The distances between the centers of neighboring fullerene molecules are 10.036(7), 10.636(7), and 10.537(9) Å. Each C₆₀ molecule is linked to eight S₈ molecules with ten shortened intermolecular contacts C...S 3.41(1)–3.52(2) Å. The average values of the C=C and C-C bond lengths are 1.32(3) and 1.47(3) Å, which attest to a significant degree of localization of electron density in the c₆₀ molecule.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 262–266, February, 1994.     

Kristallstruktur und Infrarot-Absorptionsspektrum von synthetischem Monohydrocalcit,CaCO3·H2O

The crystal structure of synthetic monohydrocalcite was solved withPatterson andFourier syntheses and refined for the subcell withá=6.0931 (9) Å,=7.5446 (18) Å, space group P3₁21 or P3₂21,Z=3 toR _w=0.039. Very weak superstructure reflections define a cell with andc=, space group P3₁ or P3₂,Z=9. The superstructure result from ordering of the CO₃ groups and was refined with rigid carbonate groups toR _w=0.086. The interpretation of the IR spectrum is in agreement with the results of the X-ray structure analysis.

     

Thermodynamic Study of the Aqueous Rubidium and Manganese Bromide System

Crystallization of 2RbBr · MnBr₂ · 2H₂O, the only double salt obtained under standard conditions from saturated aqueous rubidium–manganese bromide solutions, was theoretically predicted using the hard and soft Lewis acids and bases concept and Pauling's rules. The RbBr—MnBr₂—H₂O system was thermodynamically simulated by the Pitzer model assuming a solubility diagram of three branches only: RbBr, 2RbBr · MnBr₂ · 2H₂O and MnBr₂ · 4H₂O. The theoretical result was experimentally proved at 25°C by the physicochemical analysis method and formation of the new double salt 2RbBr · MnBr₂ · 2H₂O was established. It was found to crystallize in a triclinic crystal system, space group –P1, a = 5.890(1) Å, b = 6.885(1) Å, c = 7.367(2) Å, = 66.01(1)°, = 87.78(2)°, = 84.93(2)°, V = 271.8(1) ų, Z = 1, D _x = 3.552 g-cm^–3. The binary and ternary ion interaction parameters were calculated and the solubility isotherm was plotted. The standard molar Gibbs energy of the synthesis reaction, _rG _m ^o , of the double salt 2RbBr · MnBr₂ · 2H₂O from the corresponding simple salts RbBr and MnBr₂ · 4H₂O, as well as the standard molar Gibbs energy of formation, _fG _m ^o , and standard molar enthalpy of formation _fH _m ^o of the simple and double salts were calculated.     

Crystal structure of synthetic Cu3SeO4(OH)4

Summary The crystal structure of synthetic Cu₃SeO₄(OH)₄ was determined by single crystal X-ray methods:a=8.382 (2) Å,b=6.087 (1) Å,c=12.285 (2) Å,V=626.8 ų,Z=4, space group Pnma,R=0.026,R _w =0.021 for 1255 independent reflections (sin / 0.8 Å^–1). The crystal structure is isotypic to that of the mineral antlerite, Cu₃SO₄(OH)₄. The copper atoms are Jahn-Teller distorted with Cu^[4+2]O₆ polyhedra forming triple chains along [010]. These chains are linked via SeO₄ tetrahedra and weak hydrogen bonds to a framework structure.

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Die Kristallstruktur von synthetischem Cu₃SeO₄(OH)₄

Zusammenfassung Die Kristallstruktur von synthetischem Cu₃SeO₄(OH)₄ wurde mittels Einkristall-Röntgenmethoden ermittelt:a=8.382 (2) Å,b=6.087 (1) Å,c=12.285 (2) Å,V=626.8 ų,Z=4, Raumgruppe Pnma,R=0.026,R _w =0.021 für 1255 unabhängige Reflexe (sin / 0.8 Å^–1). Die Kristallstruktur ist isotyp mit der des Minerals Antlerit, Cu₃SO₄(OH)₄. Die Kupferatome sind Jahn-Teller-verzerrt, die Cu^[4+2]O₆ Polyeder bilden Dreierketten entlang [010]. Diese Ketten sind über SeO₄-Tetraeder und schwache Wasserstoffbrücken zu einer Gerüststruktur verbunden.

     

Synthese und Strukturanalyse von Na3SbSe3·3Sb2O3·0,5Sb(OH)3

Investigations in the system Sb-Se-NaOH-H₂O, hydrothermal conditions, yielded crystals of the compound Na₃SbSe₃·3Sb₂O₃·0,5Sb(OH)₃. The structure of this compound (a=14.40 Å,c=5.568 Å; space group P 6₃-C ₆ ⁶ ;Z=2) was determined from 985 independent X-ray intensities — collected on an automaticWeissenberg type diffractometer — by thePatterson method and refined by the least squares method toR=8.3% (with -weighting 5.9%). The structure consists of SbO₃ pyramids which are connected via common oxygen corners to tubes parallel [001]. These tubes and SbSe₃ pyramids are combined by Na atoms to a framework. The Sb(OH)₃ groups are statistically located within the channels of the tubes.

     

Crystal structure of Li2Cu3(SeO3)2(SeO4)2

Summary Single crystal X-ray data of the hydrothermally grown new phase Li₂Cu₃(SeO₃)₂(SeO₄)₂ were measured with a four-circle diffractometer up to sin /=0.81 Å^–1 [I2/a,Z=4,V=1175.5 ų,a=16.293(6),b=5.007(2),c=14.448(6) Å, = 94.21(1)°]. The structure was determined by direct and Fourier methods and refined toR=0.034,R _w =0.027 for 2 086 independent reflections.Cu(1)^[4+1]O₅ forms a tetragonal pyramid, Cu(2)^{[4 + 2]}O₆ is a strongly elongated octahedron. The Li atom is surrounded by four O atoms forming a distorted tetrahedron. Se(IV)O₃ and Se(VI)O₄ groups are in accordance to literature, mean Se-O bond lengths are 1.714 and 1.644 Å.

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Die Kristallstruktur von Li₂Cu₃(SeO₃)₂(SeO₄)₂

Zusammenfassung Einkristall-Röntgendaten der hydrothermal gezüchteten neuen Phase Li₂Cu₃(SeO₃)₂(SeO₄)₂ wurden mit einem Vierkreisdiffraktometer im Bereich bis zu sin /=0.81 Å^–1 gemessen [I2/a,Z=4,V=1175.5 ų,a=16.293(6),b=5.007(2),c=14.448(6) Å, =94.21(1)°]. Die Kristallstruktur wurde mittels direkter und Fourier-Methoden bestimmt und für 2 086 unabhängige Reflexe zuR=0.034,R _w =0.027 verfeinert.Cu(1)^[4+1]O₅ bildet eine tetragonale Pyramide, Cu(2)^[4+2]O₆ ist ein stark verlängertes Oktaeder. Das Li-Atom ist von vier O-Atomen in Gestalt eines verzerrten Tetraeders umgeben. Die Se(IV)O₃-und Se(VI)O₄-Gruppen entsprechen der Literatur, die mittleren Se-O-Abstände betragen 1.714 und 1.644 Å.

     

Toward a More Accurate Silicon Stereochemistry: An Electron Diffraction Study of the Molecular Structure of Tetramethylsilane

The present electron diffraction study of the molecular structure of tetramethylsilane, augmented with theoretical calculations, answers the need for accurate and detailed information on the most fundamental molecules containing silicon. The Si—C bond length is r _g = 1.877 ± 0.004 Å, in perfect agreement with a previous study (Beagley, B.; Monaghan, J. J.; Hewitt, T. G. J. Mol. Struct. 1971 8 401). The C—H bond length is r _g= 1.110 ± 0.003 Å and the Si—C—H angle is 111.0 ± 0.2°. The experimental data are consistent with a model of T _d symmetry and staggered methyl conformation. The barrier to methyl rotation is estimated to be 5.7 ± 2.0 kJ mol^–1 on the basis of the experimentally observed average torsion of the methyl groups.