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.     

Molecular structure and ring distortions of p-dichlorobenzene as determined by electron diffraction

The molecular structure of p-dichlorobenzene in the vapour phase has been studied by electron diffraction. Least-squares refinement of a model with D_2h symmetry has led to the accurate determination of the small deviations of the benzene ring from D_6h symmetry caused by the chlorine substituents. The most appreciable effect is an increase from 120° to 121.6 ± 0.2° of the internal angle at the ipso carbon, associated with a shortening of the distance between the two ipso atoms. A less pronounced effect is a shortening of the C-C bonds that originate from the ipso carbons as compared to the central C-C bonds (r_g = 139.0 ± 0.3 pm vs. 139.5 ± 0.4 pm). Other bond distances are r_g (C-Cl) - 173.0 ± 0.4 pm and r_g (C-H) = 109.4 ± 1.0 pm. The observed ring distortions are in agreement with those obtained by low-temperature X-ray crystallography on three different crystal phases of p-dichlorobenzene. They are also consistent with those obtained for chloro-benzene by gas-phase electron diffraction and by NMR spectroscopy in a nematic solvent. The r_s structure of chlorobenzene obtained in a recent study by micro-wave spectroscopy is shown to need revision, as far as the ipso region of the ring is concerned.     

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.     

The molecular structure of perfluoro-thiirane in the presence of perfluoro-cyclobutane: a gas-phase electron diffraction study

The molecular structure of perfluoro-thiirane has been studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument at UMIST. As samples are unavoidaby contaminated with perfluoro-cyclobutane, it proved possible to obtain structural parameters for this molecule simultaneously. The three-membered sulphur ring has CS = 1.799(3), CC = 1.45(1) Å, with ∠CSC = 47.5(5), ∠SCC = 66.2(3)°, and the fluorine parameters are CF = 1.322(2) Å, ∠SCF = 121.5(6) and ∠CCF = 116.2(5)°, For perfluoro-cyclobutane, values of ∠CCC = 88.8(1) and ∠FCF = 110(1)° are obtained, the fold-angle expressing the non-planarity of the ring being 23(1)°, when refined the CC distance converged to 1.58(1) Å. These results compare favourably with those obtained previously for perfluoro-cyclobutane. The percentages of the latter present in the samples studied at the 100 and 50 cm camera distances averaged about 25%, whereas around 40% was present in experiments at the 25 cm distance, which require longer exposure times.     

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 new method of recording diffraction patterns in gas-phase electron diffraction

A new design of the recording unit of an electron diffractometer for gas-phase electron diffraction has been suggested, in which a fixed luminescent screen is used instead of photo plates. The diffraction pattern of a compound displayed on the screen is input to a reading device with contact photographic recording and is transmitted to a computer as digital two-dimensional intensity maps.     

Molecular structure of phenylsilane: a study by gas-phase electron diffraction and ab initio molecular orbital calculations

The molecular structure of phenylsilane has been determined accurately by gas-phase electron diffraction and ab initio MO calculations at the MP2(f.c.)/6-31G* level. The calculations indicate that the perpendicular conformation of the molecule, with a Si–H bond in a plane orthogonal to the plane of the benzene ring, is the potential energy minimum. The coplanar conformation, with a Si–H bond in the plane of the ring, corresponds to a rotational transition state. However, the difference in energy is very small, 0.13 kJ mol⁻¹, implying free rotation of the substituent at the temperature of the electron diffraction experiment (301 K). Important bond lengths from electron diffraction are: <r_g(C–C)>=1.403±0.003 Å, r_g(Si–C)=1.870±0.004 Å, and r_g(Si–H)=1.497±0.007 Å. The calculations indicate that the C_ipso–C_ortho bonds are 0.010 Å longer than the other C–C bonds. The internal ring angle at the ipso position is 118.1±0.2° from electron diffraction and 118.0° from calculations. This confirms the more than 40-year old suggestion of a possible angular deformation of the ring in phenylsilane, in an early electron diffraction study by F.A. Keidel, S.H. Bauer, J. Chem. Phys. 25 (1956) 1218.     

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.     

Molecular structure and benzene ring deformation of three cyanobenzenes from gas-phase electron diffraction and quantum chemical calculations

The molecular structures of cyanobenzene, p-dicyanobenzene, and 1,2,4,5-tetracyanobenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations. The equilibrium structures of these molecules are planar, but their average geometries in the gaseous phase are nonplanar because of large-amplitude vibrational motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in electron diffraction analysis is necessary to yield ring angles consistent with the results of MO calculations. The angular deformation of the benzene ring in the three molecules is found to be much smaller than obtained from previous electron diffraction studies, as well as from microwave spectroscopy studies of cyanobenzene. While the deformation of the ring CC bonds and CCC angles in p-dicyanobenzene is well interpreted as arising from the superposition of independent effects from each substituent, considerable deviation from additivity occurs in 1,2,4,5-tetracyanobenzene. The changes in the ring geometry and C ipso-C cyano bond lengths in this molecule indicate an enhanced ability of the cyano group to withdraw pi-electrons from the benzene ring, compared with cyanobenzene and p-dicyanobenzene. In particular, gas-phase electron diffraction and MP2 or B3LYP calculations show a small but consistent increase in the mean length of the ring CC bonds for each cyano group and a further increase in 1,2,4,5-tetracyanobenzene. Comparison with accurate results from X-ray and neutron crystallography indicates that in p-dicyanobenzene the internal ring angle at the place of substitution opens slightly as the molecule is frozen in the crystal. The small geometrical change, about 0.6 degrees , is shown to be real and to originate from intermolecular C identical withN...HC interactions in the solid state.     

Molecular structure and benzene ring deformation of three ethynylbenzenes from gas-phase electron diffraction and quantum chemical calculations

The molecular structures of ethynylbenzene and s-triethynylbenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations and are compared to that of p-diethynylbenzene from a previous study [Domenicano, A.; Arcadi, A.; Ramondo, F.; Campanelli, A. R.; Portalone, G.; Schultz, G.; Hargittai, I. J. Phys. Chem. 1996, 100, 14625]. Although the equilibrium structures of the three molecules have C2v, D3h, and D2h symmetry, respectively, the corresponding average structures in the gaseous phase are best described by nonplanar models of Cs, C3v, and C2v symmetry, respectively. The lowering of symmetry is due to the large-amplitude motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in the electron diffraction analysis yields ring angles consistent with those from MO calculations. The molecular structure of ethynylbenzene reported from microwave spectroscopy studies is shown to be inaccurate in the ipso region of the benzene ring. The variations of the ring C-C bonds and C-C-C angles in p-diethynylbenzene and s-triethynylbenzene are well interpreted as arising from the superposition of independent effects from each substituent. In particular, experiments and calculations consistently show that the mean length of the ring C-C bonds increases by about 0.002 A per ethynyl group. MO calculations at different levels of theory indicate that though the length of the C[triple bond]C bond of the ethynyl group is unaffected by the pattern of substitution, the C(ipso)-C(ethynyl) bonds in p-diethynylbenzene are 0.001-0.002 A shorter than the corresponding bonds in ethynylbenzene and s-triethynylbenzene. This small effect is attributed to conjugation of the two substituents through the benzene ring. Comparison of experimental and MO results shows that the differences between the lengths of the C(ipso)-C(ethynyl) and C(ipso)-C(ortho) bonds in the three molecules, 0.023-0.027 A, are correctly computed at the MP2 and B3LYP levels of theory but are overestimated by a factor of 2 when calculated at the HF level.     

High-resolution electron diffraction: accounting for radially and angularly invariant distortions

The distortions present in an electron diffraction pattern can be classified into two categories: one is radially invariant and the other is angularly invariant. We report a method to compensate these displacements undergone by diffraction features promoted by any kind of artifacts generated in parallel beam electron diffraction conditions. This approach is not aimed at quantifying these distortions but only intends to aid in the measurement of lattice parameters of crystals with a significant increase of accuracy and precision as compared to previous approaches. It is based on statistical estimations of the relative positions between diffraction rings and/or spots after performing a transformation of the digitalized patterns to polar coordinates. The analytical method is based on fitting a Gaussian type profile to intensity distributions. This makes it possible to determine the lattice parameters of a polycrystal or single crystal with relative errors smaller than 0.1% for diffractograms acquired in photographic films and below 0.01% for those collected in imaging plates.     

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)°.     

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 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.     

Tris(pyrazol-1-yl)-s-triazine (TPT): X-ray crystallographic,computational, and gas-phase electron diffraction studies

The conformation of the TPT molecule has been analyzed using experimental and computational techniques. The solid-state molecular structure shows similar conformational features to those in the 2-pyrimidine and phenyl derivatives although a different pattern of bond angles in the triazine ring was observed. The AM1 calculations predicted two conformations of comparable stability (E=1.8 kcal/mol) differing in the orientation of one pyrazole ring. While the minimum energy conformation corresponds to a model displayingC _3h symmetry ( ₁= ₂= ₃=0°), the other minimum ( ₁= ₂=0°, ₃=180°) is close to that observed in the solid state. The electron diffraction results are consistent with a planar or nearly planar conformation in agreement with the preceding studies.On leave from the Depto. Química. Universidade Federal Rural do Rio de Janeiro. Itaguai (RJ) 23851 Brazil.     

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.     

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 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 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.