Molecular structure and conformational preferences of 1-chloro-1-silacyclohexane,CH<Subscript>2</Subscript>(CH<Subscript>2</Subscript>CH<Subscript>2</Subscript>)<Subscript>2</Subscript>SiH-Cl,as studies by gas-phase electron diffraction and quantum chemistry

The molecular structure of axial and equatorial conformer of the 1-chloro-1-silacyclohexane molecule, CH₂(CH₂CH₂)₂SiH-Cl, as well as thermodynamic equilibrium between these species were investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/AUG-cc-PVTZ level of theory. According to electron diffraction data, the compound exists in the gas-phase as a mixture of conformers possessing the chair conformation of the six-membered ring and Cs symmetry and differing in the axial and equatorial position of the Si-Cl bond at 352 K. NBO analysis revealed that axial conformer of 1-chloro-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view steric effects and effects of conjugations and that stabilization is achieved due to electrostatic interactions.     

Equilibrium structure and spectroscopic constants of maleic anhydride

The quadratic, cubic, and semi-diagonal quartic force fields of maleic anhydride have been calculated at the MP2 level of theory employing the cc-pVTZ basis set. The spectroscopic constants derived from the force field are in excellent agreement with the corresponding experimental values. The semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational corrections calculated from the cubic force field. This semi-experimental equilibrium structure is in excellent agreement with the ab initio structures computed at the CCSD(T) level of theory and it is closer to the ab initio structure than the purely experimental (or empirical) structures r ₀, r _m⁽¹⁾, and r _m⁽²⁾ obtained by microwave spectroscopy as well as the equilibrium structure derived from gas-phase electron diffraction data.     

The structure Tl3SbS3, Tl3SbSe3, Tl3SbS3−xSex, and Tl3SbyAs1−ySe3

The space group symmetry and crystal structure of Tl₃SbS_3−xSe_x compounds in the composition range 0 < x < 3 have been determined by a combination of powder X-ray diffraction, electron diffraction, and high-resolution electron microscopy. The incongruently melting compound Tl₃SbSe₃ has been shown to crystallize in cubic space group P2₁3 with a = 9.435Å in a structure related to that of Langbeinite. The convergent beam electron diffraction pattern of Tl₃SbS₃ is in accord with the space group R3m determined by X-ray diffraction. The cubic Langbeinite-type structure is found for Tl₃SbS_3−xSe_x for 0.5 < x < 3 and for Tl₃Sb_yAs_1−ySe₃ for 0.077 < y < 1.0. A five-component compound Tl₃Sb_0.5As_0.5Se_1.5S_1.5 was also found to be cubic.     

A Structural Investigation of Ga3−xIn5+xSn2O16

The structure of the recently reported transparent conductor, Ga_3−xIn_5+xSn₂O₁₆(0.3<x<1.6), was established by a combination of high-resolution electron microscopy, convergent-beam electron diffraction, and Rietveld analysis of powder diffraction data (X-ray and time-of-flight neutron methods). This “T-phase” compound has an anion-deficient fluorite-derivative structure whose space group isI4₁/a. Although there are similarities to the parent oxide structures, the T-phase lacks one of the distorted InO₆octahedra observed in In₂O₃, which may account for its inability to be donor-doped by Sn.     

Comparison of the molecular structures of five benzene derivatives as determined independently by gas-phase electron diffraction in two different laboratories: a perspective

The merits and limitations of gas-phase electron diffraction as a technique for determining accurately the structures of small organic molecules are demonstrated by comparing the molecular structures of five benzene derivatives studied in two different laboratories. The existence of many possible representations of the structure of a molecule (r _a, r _g, r _α, r _α⁰, r _e, …), implying differences in the physical meaning of geometrical parameters, is discussed in some detail as it may cause difficulties in a comparison. Also illustrated are different ways of estimating and expressing uncertainties in the geometrical parameters from a gas-phase electron diffraction study.     

Unconventional Metal–Framework Interaction in MgSi5

The silicon‐rich cage compound MgSi₅ was obtained by high‐pressure high‐temperature synthesis. Initial crystal structure determination by electron diffraction tomography provided the basis for phase analyses in the process of synthesis optimization, finally facilitating the growth of single crystals suitable for X‐ray diffraction experiments. The crystal structure of MgSi₅ (space group Cmme, Pearson notation oS24, a=4.4868(2) Å, b=10.1066(5) Å, and c=9.0753(4) Å) constitutes a new type of framework of four‐bonded silicon atoms forming Si₁₅ cages enclosing the Mg atoms. Two types of smaller Si₈ cages remain empty. The atomic interactions are characterized by two‐center two‐electron bonds within the silicon framework. In addition, there is evidence for multi‐center Mg?Si bonding in the large cavities of the framework and for lone‐pair‐like interactions in the smaller empty voids.     

Evidence of Noncentrosymmetry of Human Tooth Hydroxyapatite Crystals

Herein, we investigate human single hydroxyapatite crystals (enamel and dentine) by convergent‐beam electron diffraction (CBED) and automated electron‐diffraction tomography (ADT). The CBED pattern shows the absence of the mirror plane perpendicular to the c axis leading to the P6₃ space group instead of the P6₃/m space group considered for larger‐scale crystals, this is confirmed by ADT. This experimental evidence is of prime importance for understanding the morphogenesis and the architectural organization of calcified tissues.     

Electron Beam Induced Transformation of MoO3 to MoO2 and a New Phase MoO

The transformation of MoO₃ induced by electron beam irradiation was studied by electron energy‐loss spectroscopy (EELS) in combination with electron diffraction and high‐resolution transmission electron microscopy (HRTEM) techniques. The routes of structure transformation were dependent on the applied electron current density. In case of low current density, MoO₂ was obtained. In case of high current density, MoO with a rock‐salt structure is suggested to be the final phase. The change in oxidation states of the Mo oxides was deduced from the features in energy‐loss near edge structure (ELNES) of the O K‐edge. Quantitative analysis was successfully employed on Mo M₃‐edge and O K‐edge to obtain the O/Mo ratio of the reduced phases. The mechanisms of different structure transformation behaviors were suggested in the frame of radiolysis enhanced diffusion.     

Mass spectrometry of heterocyclic compounds. XV—The mechanism of the fragmentation under electron impact of 3-p-NH2-phenyl-5-phenyl-1,2,4-oxadiazole. A CNDO/2 treatment

Molecular orbital calculations using the semi-empirical CNDO/2 method were carried out on the molecular ion of 3-p-NH₂-phenyl-5-phenyl-1,2,4-oxadiazole, whose structure had been determined by X-ray diffraction. The calculated diatomic interaction energy values are consistent with the mechanistic proposals made previously in terms of the quasi-equilibrium theory concerning the cleavage of the heterocycle ring after electron impact.     

Electron microscopy and structural studies of Nd1/3NbO3

The crystal structure of the deficient perovskite Nd_1/3NbO₃ has been determined at room temperature, using electron microscopy observations and X-ray powder diffraction data. Electron diffraction study and HRTEM images evidence the doubling of a and b parameters and confirm one of the c-parameter with respect to perovskite unit cell. The structure refinement has been carried out using the orthorhombic Cmmm space group. The Nd³⁺ cations occupy randomly and alternatively the (00l) planes and Nb⁵⁺ cations distorted octahedral sites. This structure is characterized by the presence of empty Nd³⁺ sites in every second layer and the tilting of the octahedral sites around the b-axis. In addition, electron diffraction patterns exhibit weak additional diffuse reflections, which supposes some ordering of the Nd vacancies.     

Electron crystallography of epitaxially grown paraffin

Three-dimensional electron diffraction data from the epitaxially crystallized paraffin n-C₃₆H₇₄ are investigated in a structure analysis and are found to give a reasonable image of the structure, in agreement with the earlier x-ray determination. An inconsistency in the refined isotropic temperature factors from different zonal projections, however, indicates the necessity for a more accurate physical model for epitaxially grown crystals which will account for a large spread of diffraction peaks along reciprocal lattice lines parallel to the projection axis.     

A Novel High-Pressure Tin Oxynitride Sn2N2O

We report the first oxynitride of tin, Sn₂N₂O (SNO), exhibiting a Rh₂S₃-type crystal structure with space group Pbcn. All Sn atoms are in six-fold coordination, in contrast to Si in silicon oxynitride (Si₂N₂O) and Ge in the isostructural germanium oxynitride (Ge₂N₂O), which appear in four-fold coordination. SNO was synthesized at 20 GPa and 1200–1500 °C in a large volume press. The recovered samples were characterized by synchrotron powder X-ray diffraction and single-crystal electron diffraction in the TEM using the automated diffraction tomography (ADT) technique. The isothermal bulk modulus was determined as B_o=193(5) GPa by using in-situ synchrotron X-ray diffraction in a diamond anvil cell. The structure model is supported by DFT calculations. The enthalpy of formation, the bulk modulus, and the band structure have been calculated.     

Phase transformation and structure of n-C50H102/n-C60H122 solid solutions formed from the vapor phase

Binary solid solutions of n-paraffins (n-C₅₀H₁₀₂/n-C₆₀H₁₂₂) epitaxially prepared on potassium hydrogen phthalate substrate from the vapor phase have been studied by electron diffraction to characterize their phase transitions and structure. The continuity of solid solution in the n-C₅₀H₁₀₂/n-C₆₀H₁₂₂ system is demonstrated once lamellar ordering of the crystal packing is achieved. However, such ordering is achieved only by annealing and proceeds through a series of intermediate chain packings. At first, the electron diffraction patterns from all samples prepared at room temperature resemble those from polyethylene, in which no spots corresponding to the interlayer spacings appear. Longitudinal chain translations are induced by annealing to cause the lamellar reflections to appear, while the “polyethylene” subcell reflections remain unsplit until the crystal structure with well-defined methyl end planes is reached.     

Molecular Structure of ortho-Fluoronitrobenzene Studied by Gas Electron Diffraction and Ab Initio MO Calculations

The molecular structure of ortho-fluoronitrobenzene (o-FNB) has been investigated by gas-phase electron diffraction and ab initio MO calculations. The geometrical parameters and force fields of o-FNB were calculated by ab initio and DFT methods. The obtained force fields were used to calculate vibrational amplitudes required as input parameters in an electron diffraction analysis. Within the experimental error limits, the geometrical parameters obtained from the gas-phase electron diffraction analysis are mostly in agreement with the results obtained from the ab initio calculations. The main results are: the molecular geometry of o-FNB is nonplanar with a dihedral angle about C–N of 38(3)°. The r _g (C–F) bond is shortened to 1.307(13) Å in comparison with r _g (C–F) = 1.356(4) Å in C₆H₅F.     

Electron Diffraction and Quantum Chemical Study of the Molecular Structure of para-Methylbenzenesulfonyl Fluoride and para-Methylbenzenesulfonyl Bromide

A combined electron diffraction and mass-spectrometric experiment has been performed at 350(2) and 340(2) K, respectively, to study the molecular structure of para-methylbenzenesulfonyl fluoride and para-methylbenzenesulfonyl bromide. The electron diffraction data are analyzed in terms of r structure. Several geometrical models with different orientations of the sulfonyl halide group relative to the plane of the benzene ring are considered. The angle between the plane of the benzene ring and the plane of the S–Hal bond equals 90°, indicating that the molecules have C _s symmetry. The structural parameters of the molecules and the internal rotation barriers of the sulfonyl halide and methyl groups have been calculated by the ab initio and semiempirical quantum chemical methods. The results of calculations are compared with electron diffraction data.     

Crystal structure determination of BaNd2Ti3O10 using high-resolution electron microscopy

The crystal structure of BaNd₂Ti₃O₁₀ has been determined by electron diffraction and high-resolution electron microscopy. The unit cell is monoclinic with P2₁/m as the most probable space group and not orthorhombic as previously found by X-ray diffraction. However, the structure has an orthorhomic pseudosymmetry, but due to the small Nd³⁺ cations the octahedra are titled and the structure is monoclinic. The cell dimensions based on X-ray data are: a_m = 7.7310 ± 0.0006 Å; b_m = 7.6661 ± 0.0007 Å; c_m = 14.210 ± 0.002 Å; β_m = 97.82 ± 0.01°.     

Neutron Diffraction and TEM Studies of the Crystal Structure and Defects of Nd4Ni3O8

Powder neutron diffraction and electron microscopy studies confirmed the crystal structure of Nd₄Ni₃O₈(space groupI4/mmm;a=b=3.9168(2) Å,c=25.322(2) Å,Z=4; reliability factorsR_p=0.125,R_wp=0.108,χ²=2.81,R_Bragg=0.043,R_F=0.037) previously determined by powder X-ray diffraction [Ph. Lacorre,J. Solid State Chem.97, 495 (1992)]. It consists in the intergrowth between triple layers of nickel in square planar coordination with Nd in cubic coordination (infinite layer type) and fluorite type layers. Nd₄Ni₃O₈is thus the first known compound with a triple T′ type structure (relative to the single T′ type structure of Nd₂CuO₄). A study of the coherent diffraction lengths and defects in the structure has been carried out. Two kinds of defects have been detected by HREM, for which a model is proposed.     

Electrical transport properties of liquid alloys

Two complementary mechanisms have been proposed for relatively high temperature superconductor MgB₂. While the first is the electron–phonon mechanism of BCS theory, advocated strongly by Pickett and co-workers, the second, by Bianconi et al., invokes Feshbach shape resonances. While we cannot presently discount the second mechanism, and while both proposals exploit the multiband nature of the electronic structure of MgB₂, we show here that five body-centred cubic (bcc) transition metals, whose superconducting transition temperature correlate intimately with elastic constants and therefore are plainly BCS-like in character, lie on a curve which has MgB₂ at the high T _c end. Any alternative mechanism to electron–phonon interaction in MgB₂ will need to account quantitatively for this circumstance.     

A Novel Polymeric Lead(II)‐Azido Compound: Synthesis,Structural Characterization,and DFT Calculations of [Pb(dmp)(N3)2]n

A novel 1D Pb^II coordination polymer containing Pb₂‐(μ‐N₃)₂ unit [Pb(dmp)(N₃)₂]_n (dmp =  2,9‐dimethyl‐1,10‐phenanthroline) has been prepared and characterized. Single‐crystal X‐ray diffraction analyses show that the coordination number for Pb^II ions is six, PbN₆, with “stereochemically active” electron lone pairs and the coordination sphere being hemidirected. The single‐crystal X‐ray data show the chains interact with each other through the π–π stacking interactions, which create a 3D framework. The structure of title complex has been optimized by density functional theory. Structural parameters and IR spectra for the complex are in agreement with the crystal structure.