A method for recording X-ray diffraction patterns of milligram quantities of particulates

A method for recording X-ray diffraction patterns of milligram quantities of particulates has been developed. The resulting diffraction patterns are similar to patterns obtained by standard methods. Application of this method to analysis of airborne particulates in the vicinity of a lead smelting operation has demonstrated the practical use of the method. Sample sizes as low as 1.1 mg have produced useful diffraction patterns.     

Filter-spline backgrounds in gas-phase electron diffraction analysis

A filter-spline procedure is developed for a computer background adjustment in the gas-phase electron diffraction analysis of molecules. The procedure allows estimation of the principal internuclear distances and amplitudes of vibration without any a priori information on the molecular structure. The parameters thus found can be further used as starting points in structure analysis. The procedure is illustrated by treatment of intensity data for cyclooctane.     

Molecular structure of arsabenzene by gas-phase electron diffraction

Vapor-phase molecules of C₅H₅As were found, assuming C_2v symmetry, to have the following structure parameters and uncertainties (2.5σ): r_g(C-As)= 1.850 ± 0.003 Å, r_g(C₂–C₃) = 1.390 ± 0.032 /rA, r_g(C₃–C₄) = 1.401 ± 0.032 /rA, r_g(C-C_ave) = 1.395₄ ± 0.002 Å, ∠CAsC = 97.3 ± 1.7°, ∠AsCC = 125.1 ± 2.8°, and ∠C₃C₃C₄ = 124.2 ± 2.9°. Amplitudes of vibration were also determined. Auxiliary information is more restrictive than pure electron diffraction intensities as evidence that the molecule is rigorously planar. Structural characteristics of arsabenzene reinforce prior indications that the heterocyclic molecule is genuinely aromatic.     

A gas-phase electron diffraction study of the molecular structure of tetravinylsilane

The molecular structure of tetravinylsilane has been studied by gas-phase electron diffraction. The radial distribution curve suggests the absence of conformers having vinyl double bonds staggered with respect to the SiC₄ skeleton. Of the eclipsed or approximately-eclipsed conformers, the one with S₄ symmetry gives the best fit with experiment, although a small admixture of a C₁ conformation cannot be ruled out. Least-squares refinement gave the following values for the independent structural parameters (lengths, r_a basis; angles, r_α basis): C-H = 1.118 ± 0.003 Å, CC = 1.355 ± 0.002 Å, Si-C = 1.855 ±0.002 Å, ∠SiCC = 124.0 ± 0.3°, ∠SiCH = 118.4 ± 1.0°, torsion angles CSiCC are 17.5 ± 0.6° from the eclipsed conformation. During the refinement the vibrational amplitudes u and perpendicular amplitude corrections K were held constant at calculated values. The CC bond length provides evidence of interaction between the vinyl π-bonds and the vacant d-orbitals of silicon.     

A gas-phase electron diffraction study of the molecular structure of 1,1-difluoroethane

The molecular structure of 1,1-difluoroethane has been studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument. Effective least-squares refinement of the geometry was achieved with fixed values for vibrational amplitudes transferred from normal coordinate calculations on related molecules. In subsequent calculations, in which several amplitudes were also allowed to refine, only minor changes were noted. The refinements yielded the following main geometrical parameters (r_avalues with e.s.d. in parentheses): C—C = 1.498(4) Å, C—F = 1.364(2) Å, C—H(mean) = 1.081(3) Å, ∠CCH(mean) = 111.0(7)°, ∠CCF = 110.7(3)°, ∠FCF = 107.4(5)°. Dependent angles are ∠FCH = 108.5(8)° and ∠HCH = 107.9(7)°.     

A gas-phase electron diffraction study op the molecular structure of 1,1,1,2-tetrafluoroethane

The molecular structure of 1,1,1,2-tetrafluoroethane is studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument. Effective least-squares refinement of the geometry is achieved with values for vibrational amplitudes transferred from normal coordinate calculations on related molecules. The following values for the main independent geometrical parameters are obtained (r_a values with e.s.d. in parentheses): C-C = 1.501(4) Å, C-H = 1.077 (15) Å, C-F(CH₂F) = 1.389(6) Å, C-F(CF₃) = 1.334 (2) Å, ∠CCH= 106.1(12)°, ∠CCF(CH₂F)= 112.3(4) Å, ∠CCF(CF₃)= 110.4(2). Other angles are ∠FCF = 108.6 (2)° and ∠FCH = 111.4(15)°, with ∠HCH constrained at 109.4°. The r_a bond lengths of all the fluoroethanes are compared.     

Current research in gas-phase electron diffraction at the University of Antwerp

The gas-phase molecular structures of norbornane and methyl vinylether have been investigated by joint analysis of electron diffraction, infrared, Raman and microwave spectroscopic data. Constraints were taken from the completely relaxed ab-initio (4–21G) geometry. A range of models was investigated which fit to all the available data. For methyl vinylether the quadratic force field was determined by numerical differentiation of the energy gradient and used to calculate vibrational quantities. Also, features of our new electron diffraction unit are illustrated. A new scheme of densitometric data collection is used, based on a modified ELSCAN 2500 densitometer, and a Z8-microprocessor.     

The gas-phase molecular structure of 1-fluorosilatrane from electron diffraction

The molecular geometry of 1-fluorosilatrane has been determined by gas-phase electron diffraction. The distance between the nitrogen and silicon atoms is much longer in the gas phase, viz., 2.324±0.014 Å, than in the crystal, 2.042 (1) Å [5]. This indicates a weakened donor-acceptor interaction possibly as a consequence of the absence of intermolecular interactions in the gas phase. The five-membered rings take envelope conformations with the carbon atoms adjacent to nitrogen at the envelope tips. The following bond distances ( _g , Å) and bond angles (°) were obtained with their estimated total errors: N-C, 1.481±0.008; C-C, 1.514±0.011; O-C, 1.392±0.004; Si-O, 1.652±0.003; Si-F, 1.568±0.006; C-H, 1.118±0.005; N-C-C, 104.5±0.6; C-C-O, 117.0±0.7;C-O-Si, 123.7±0.6; O-Si-F, 98.7±0.3; O-Si-O, 117.8±0.1; C-N-C, 115.0±0.3.     

The molecular structures of mono-substituted cl-cyclohexene by gas-phase electron diffraction

The molecular structures of mono-substituted chlorocyclohexene are determined by gas-phase electron diffraction. The structural parameters are obtained by applying leastsquares analysis to the molecular scattering intensities. The bond distances (r_g) and bond angles are: (1) 1-Cl-cyclohexene: C₁C₂ = 1.336 ± 0.006 Å. C₂-C₃ = 1.500 ± 0.009 Å, C₃-C₄ = 1.533 ± 0.010 Å, C₄-C₅ = 1.537 Å, C₅-C₆ = 1.527 ± 0.010 Å, C₁-C₆ = 1.504 ± 0.009 Å. C-Cl = 1.747 ± 0.005 Å, C-H_av = 1.138 ± 0.010 Å, ∠Cl-cc = 126.3 ± 0.5°, ∠C₆C₁C₂ = 123.9 ± 0.8°. ∠C₁C₂C₃= 124.6 ± 0.8°, ∠C₄C₃C₂ = 111.8 ± 1.2° and ∠-C₅C₆C₁ = 111.3 ± 1.1°; (2) 3-Cl-cyclohexene: C₁=C₂ = 1.336 Å, C₂-C₃ = 1.501 ± 0.010 Å, C₃-C₄ = 1.513 ± 0.008 Å, C₄-C₅ = 1.542 Å, C₅-C₆, = 1.516 ± 0.007 Å, C₁-C₆ = 1.505 ± 0.006 Å, C-C1 = 1.801 ± 0.005 Å, C-H_av = 1.120 ± 0.008 Å, ∠C₆C₁C₂ = 123.2 ± 1.0°, ∠C₁C₂C₃ = 124.1 ± 1.7°, ∠C₅C₆C₁ = 113.0 ± 1.3°, ∠C₂C₃C₄ = 112.5 ± 1.5° ∠ClC₃C₂ = 110.3 ± 0.8°, ∠H-C=C = 123.0 ± 3.0° and ǒH-C-C = 109.5 ± 2.0°, with a mixture of 55% axial and 45% equatorial conformers; (3) 4-Cl-cyclohexene: C₁=C₂ = 1.336 Å, C₂-C₃ = 1.507 ± 0.007 Å, C₃-C₄ = 1.516 ± 0.008 Å, C₄-C₅ = 1.544 Å, C₅-C₆ = 1.523 ± 0.010 Å, C₁- C₆ = 1-507 Å, C-Cl = 1.799 ± 0.005 Å, C-H_av = 1.116 ± 0.005 Å, ∠C₆C₁C₂ = 123.3 ± 1.5°, ∠C₅C₆C₁ = 113.0 ± 1.0°, ∠C₂C₃C₄ = 112.3 ± 1.0°, ∠ClC₄C₃ = 110.2 ± 2.0°, ∠H-CC = 117.1 ± 4.5° and ∠H-C-C = 109.5 ± 1.0°, with a mixture of 45% axial and 55% equatorial conformers.     

The story of gas-phase electron diffraction (GED) in Norway

This commentary is addressing gas electron diffraction in Norway from the late 1930s up to 2017. The account is about the people involved, the methods developed, and the chemical questions addressed. The development was based on three strong characteristics: Academic excellence, a holistic strategy, and a comprehensive international cooperation and publication. Two strong personalities—Odd Hassel and Otto Bastiansen—established the fundament; their contributions were pivotal. Their ability to obtain funding, to recruit highly qualified co-workers, and their international network were central to the development. The investments in structure chemistry and electron diffraction were exceptionally visionary and daring. The story is rather unique. It is about how a small university at Europe’s periphery in the late 1930s was able to establish a world-leading research group.

     

The molecular geometry of a heptacyclotetradecane from gas-phase electron diffraction

Electron diffraction established firmly the structure of a heptacyclo [6.6.0.0^2,6.0^3,13.0^4,11.0^5,9.0^10,14] tetradecane molecule in which two norbornane units are rotated 90° relative to each other and linked by four methine bridges. The longest bond is the ethano bridge (1.586 ± 0.004 Å, r_g) and the methine bridge is considerably shorter (1.528 ± 0.006 Å r_g) than the mean CC bond length of the norbornane unit (1.557 Å). The molecular structure is generally consistent with the geometry of its crystalline di-tert-butoxy derivative as well as with that of gaseous norbornane.     

Anomeric effect in N-azidomethylpyrrolidine: gas-phase electron diffraction and theoretical study

Gas-phase electron-diffraction data and high-level quantum chemical calculations have been used to study the conformational behaviour of N-azidomethylpyrrolidine. The two most stable conformers with a relative abundance of about 80% at 298 K possess gauche orientation of the azidomethyl group around the C-N(pyr) bond (C-N(azido)gauche with respect to the endocyclic N(pyr)-C bond). This orientation is a strong manifestation of an anomeric effect. The influence of the anomeric effect is also reflected in shortening of the C-N(pyr) bond and lengthening of the C-N(azido) bond as compared to such bonds in other compounds.     

The molecular structure of trifluoromethyl manganese pentacarbonyl. A study by gas-phase electron diffraction

The results of an SCF-MO calculation on the CH₂CCH radical are presented: population analysis indices and several one-electron properties are reported and the electronic structure of the radical is discussed. The spin density is almost equally associated with the terminal carbon atoms, and there is a large negative spin density associated with the central carbon atom.     

Molecular structure of 1,1-dimethylsilacyclopentene-3,4-oxide from gas-phase electron diffraction

The molecular structure of 1,1-dimethylsilacyclopentene-3,4-oxide has been determined by electron diffraction in the gas phase. The experimental data are consistent withC _s molecular symmetry and boat conformation with a flattened end at the silicon atom. The flap angles characterizing the orientation of C-Si-C and C-O-C planes with respect to the four coplanar carbon atoms of the ring are 16.6 ± 0.6 and 73.3 ± 0.6, respectively. Bond lengths (r_g) are Si-C₆, 1.866 ±0.008; Si-C₂, 1.899 ± 0.008; C₂-C₃, 1.513 ± 0.005; C₃-C₄ (bridge), 1.477 ± 0.013; C-O, 1.443 ± 0.007; (C-H)_mean 1.116 ± 0.003 å. Bond angles are <C₅-Si-C₂, 96.2 ± 0.4; <Si-C₂-C₃, 103.9 ± 0.3; <C₂-C₃-C₄, 116.5 ± 0.3; <C₃-C₄-O, 59.2 ± 0.5; zC₄-C₃-H₉, 109.0 ± 3.5; <C₂-C₃-H₉, 132.9 ± 3.1; <C₆-Si-C₁₂, 114.6 ± 0.8; <Si-C₆-H₁₅, 109.7 ± 0.9.     

Molecular structure of N-chlorosuccinimide studied by gas-phase electron diffraction and quantum-chemical methods

The molecular structure of N-chlorosuccinimide has been studied by GED method at the nozzle temperature of 116 °C. Vibrational corrections to the r _a parameters, Δ(r _a − r _e), have been calculated using the scaled quadratic and cubic force constants from B3LYP/6-31G(df,p) calculations. The force field scaling has been carried out using the IR and Raman spectra of the solid N-chlorosuccinimide. The molecular skeleton and the bond conformation around nitrogen were found to be planar within large experimental errors. The equilibrium geometrical parameters derived from the experimental data assuming C _2v molecular symmetry and those from MP2(fc)/6-311G(3df,2pd) calculations are in a good agreement.     

The gas-phase molecular structure of silyl chloroacetylene,determined by electron diffraction

The molecular structure of SiH₃CCCl in the gas phase has been investigated using electron diffraction. Mean amplitudes of vibration and perpendicular amplitude correction factors calculated from spectroscopic data enabled refinement of both r_a and r_α structures to be carried out. The r_α refinement leads to a linear skeleton, with r_α parameters: r(Si-C) 181.2(5) pm, r(CC) 123.4(6) pm, r(C-Cl) 162.0(5) pm, r(Si-H) 148.8(12) pm, ∠HSiC 109.4(20)°. The r_a structure shows an apparent angle in the skeleton of 172(3)° (∠CCCl) owing to shrinkage.     

The gas-phase molecular structure of methyl formate,determined by electron diffraction

The structure of methyl formate in the gas phase has been reinvestigated by electron diffraction. The results confirm that the molecular skeleton is cis-planar, with bond lengths and angles in close agreement with those found by microwave techniques. Principal parameters (r_a) are: r(CO) 120.2(2) pm, r(C-O) 134.0(2), and 143.5(3) pm; ∠ (OC-O) 125.4(5)°, and ∠ (C-O-C) 115.9(5)°.     

Molecular structure of 2,4-dicarba-closo.heptaborane-7 by gas-phase electron diffraction

Gas phase electron diffraction patterns of 2,4-dicarba-closo-heptaborane were recorded at room temperature. Least squares analysis of the reduced intensity data show that the molecule is a pentagonal bipyramid and confirms the C_2v symmetry in the gas phase. The carbon atoms are located in the pentagonal belt and are separated by one boron atom. No significant departures from planarity of the pentagonal belt are indicated. The bond lengths in the belt are C(4)-B(3), 1.537 Å, C(4)-B(5), 1.558 Å, and B(5)-B(6), 1.659 Å. The distance from the apical B(l) atom to the B(3) is 1.852 Å, from the B(1) to the B(5) atom is 1.772 A, and from the B(1) to the C(2) atom is 1.717 Å. The bond angles in the pentagonal belt are C(2)B(3)C(4) = 100.5°, B(3)C(4)B(5) = 116.7° and C(4)B(5)B(6) = 103.0°. The angle between B(1)B(3)B(7) = 84.7°. Comparisons are made with structural data and moments of inertia determined by microwave spectroscopy.     

The molecular structure of pyrazine as determined from gas-phase electron diffraction data

The structure of pyrazine (1,4 diazabenzene, C₄H₄N₄) has been determined at 333 K by means of gas-phase electron diffraction. The r_g parameters are as follows: r(C-C) = 1.339 ± 0.002 Å. r(C-N) = 1.403 ± 0.004 Å, r(C-H) = 1.115 ± 0.004 Å. ∠C-C-N = 115.6 ± 0.4°, and ∠C-C-H = 123.9 ± 0.6° (error limits are 2.5σ). At a 10% level the r_α structure does not differ significantly from the structure in the solid state, so long as high order X-ray, results corrected for librational motion are used; otherwise significantly different results are found even at the 1% level. Calculated and observed mean square amplitudes compare favourably.