Electron scattering differential cross sections for C2H2, C2H4, and C2H6 by gas phase electron diffraction - binding effects

Experimental differential cross sections for 40 keV electrons scattered by C₂H₂, C₂H₄ and C₂H₆ molecules were measured using the gas electron diffraction method in the range of the scattering variable s from s = 1 A^?1 to s = 30 A^?1. The differential cross sections for neon were also measured and compared with calculated differential cross sections to calibrate the diffractograph. Experimental differential cross sections show significant deviations with respect to theoretical differential cross sections calculated from the Debye-Ehrenfest model, mainly in the range of small scattering angles. The observed differences are connected to chemical binding effects. From the experimental data, an estimation of the binding energy was carried out. The deduced values: ?0.58 ± 0.20 au for C₂H₂, ?0.94 ± 0.30 au for C₂H₄ and ?1.23 ± 0.40 au for C₂H₆ are in agreement with those obtained by thermochemical methods.     

IR multiple photon excitation of C2H5F and 1,1-C2H4F2 near dissociation threshold: vibrational emissions from parent and product species

The IR multiple photon excitation and dissociation of C₂H₅F and 1,1-C₂H₄F₂ has been studied by monitoring vibrational fluorescence in the region 2000–4000 cm^?1 following excitation with the focused output from a CO₂ TEA laser.     

Study of selective dissociation in C6F5H?C6F5D mixtures induced by intense CO2 laser pulses

Irradiation of a C₆F₅H–C₆F₅D mixture at 1 Torr by a tunable CO₂ pulse laser brings about chiefly dissociation of one component depending on the frequency of emission absorbed by the proper molecules. The addition of radical acceptors increases the selectivity by suppressing secondary reactions.

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C₆F₅H C₆F₅D 1 CO₂-, , . .

     

The molecular structure of beryllocene, (C5H5)2Be. A reinvestigation by gas phase electron diffraction

The electron scattering pattern of gaseous dicyclopentadienylberyllium, Cp₂Be, has been recorded from s = 2.00 to 39.00 Å⁻¹ with a nozzle temperature of about 120°C. Molecular models of D_5d symmetry or models containing one π-bonded and one σ-bonded Cp ring are not compatible with the data. The possibility the gaseous Cp₂Be consists fo a mixture D_5d and π-Cp, σ-Cp conformers is considered and rejected. A model of C_5v symmetry can be brought into satisfactory agreement with the data. It is also found that a slip sandwich model obtained from the C_5v model by moving sideways the ring which is at the greatest distance from Be, while keeping the two rings essentially parallel is compatible with the electron diffraction data. The best fit between experimental and calculated intensity curves is obtained with a model with a sideways slip of 0.8(1) Å. This model is similar to that indicated by the X-ray diffraction investigations by Wong and coworkers [4,5]. It is suggested that the potential energy of the molecule does not change much as the magnitude of the slip changes and that the molecule thus undergoes large amplitude vibration.     

Influence of secondary chemical reactions on the selectivity of the multiphoton dissociation process in C6F5H?C6F5D mixtures

Radical acceptors significantly increasing the dissociation selectivity in C₆F₅H–C₆F₅D mixtures have been selected by studying the mechanism of C₆F₅H dissociation in the field of a pulsed CO₂-laser. The contribution of secondary chemical reactions to the total rate of consumption of the initial molecules has been determined.

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C₆F₅H CO₂- , C₆F₅H C₆F₅D. .

     

Gas phase electron diffraction study of basketene,C10H10

A gas phase electron diffraction study of the cage hydrocarbon, basketene, is reported. A least squares treatment of molecular intensities has been carried out in terms of a geometrically consistent r_α structure. The mean amplitude values and shrinkage corrections have been calculated using the force field parameters estimated from the data on simpler molecules.Structure refinement of the C_2v molecular model yields the following parameter values (bond lengths, r_a, in nm; angles, r_α in degrees): <C₂—C_3, C₄—C₅?_av 0.1609(14); C₃—C₄ 0.1563(6); C₉C₁₀ 0.1360(9); C₁—C₁₀ 0.1511(13); C₁—C₂ 0.1517(9); <C-H>_av. 0.1092(8); <C₃C₄C₇ 88.5(1.0); dihedral angle C₃C₄C₇/C₃C₅C₇ 153.8(1.0). Parenthesized are three times the standard deviation values, 3σ.In addition to the geometric parameters listed, the mean amplitudes for all bonded and C· C nonbonded distances have been determined by the least squares method. All the other amplitudes (C· H and H· H) have been fixed at the values estimated from the spectral data.Comparison of the results obtained with the literature data on similar polycyclic molecules points to the stronger internal strain in the basketene molecule.     

Selective oxidation of organic compounds using pyridinium-1-sulfonate fluorochromate, C5H5NSO3H [CrO3F] ( PSFC )

Efficient selective oxidation of primary, secondary, and benzylic alcohols to the corresponding carbonyl compounds by a new chromium oxidizing reagent, pyridinium-1-sulfonate fluorochromate, C₅H₅NSO₃H [CrO₃F] (PSFC) is reported. Various cholesterol derivatives were easily converted to related oxocholesterol from allylic oxidation at lower temperature in comparison to other general oxidants. This oxidation procedure is simple and affords good yields.     

Crystal and molecular structures of diphenylethylarsine sulfide,C14H15AsS,and p-tolyldiethylarsine sulfide,C11H17AsS

In order to elucidate the influence of aryl or alkyl substituents on the nature of the ? bond, which controls the complexing properties of the substances (Alk)n(Ar)_3-nAsS (n = 0–3), the X-ray investigation of (p-MeC₆H₄)Et₃AsS (TDEAS) and Ph₂EtAsS (DPEAS) has been carried out. It was shown that the bond length AsS in these compounds has intermediate values (2.0896 and 2.0812 Å) in respect to those previously found by us in Et₃AsS and Ph₃AsS (2.115 and 2.077 Å). This confirms the conclusion that the increase of the number of aryl substituents in tertiary arsine sulfides leads to a decrease of electron density on the As atom along with simultaneous strengthening of the π-interaction between As and S.     

Mass spectra of the thioethers C6F5SC6H4X

The mass spectra of a number of thioethers of the type p-XC₆H₄SC₆F₅ (X = H, Cl, NO₂, SC₆F₅, CO₂C₂H₅, C₆H₅) have been examined. These spectra have been compared with those of (C₆H₅)₂S and (C₆F₅)₂S. Fragmentation patterns have been deduced from metastable peaks. The spectrum of p-ClC₆H₄SC₆F₅ has been studied in more detail.     

A widely varying range of products in reactions of C6F5BrF2, C6F5IF2, and C6F5IF4 with Lewis acids of different strength

The relative fluoride donor ability: C₆F₅BrF₂ > C₆F₅IF₂ > C₆F₅IF₄ was outlined from reactions with Lewis acids of graduated strength in different solvents. Fluoride abstraction from C₆F₅HalF₂ with BF₃·NCCH₃ in acetonitrile (donor solvent) led to [C₆F₅HalF·(NCCH₃)_n][BF₄]. The attempted generation of [C₆F₅BrF]⁺ from C₆F₅BrF₂ and anhydrous HF or BF₃ in weakly coordinating SO₂ClF gave C₆F₅Br besides bromoperfluorocycloalkenes C₆BrF₇ and 1-BrC₆F₉. In reactions of C₆F₅IF₂ with SbF₅ in SO₂ClF the primary observed intermediate (¹⁹F NMR, below 0 °C) was the 4-iodo-1,1,2,3,5,6-hexafluorobenzenium cation, which converted into C₆F₅I and 1-IC₆F₉ at 20 °C. The reaction of C₆F₅IF₄ with SbF₅ in SO₂ClF below −20 °C gave the cation [C₆F₅IF₃]⁺ which decomposed at 20 °C to C₆F₅I, 1-iodoperfluorocyclohexene, and iodoperfluorocyclohexane. Principally, the related perfluoroalkyl compound C₆F₁₃IF₄ showed a different type of products in the fast reaction with AsF₅ in CCl₃F (−60 °C) which resulted in C₆F₁₄. Intermediate and final products of C₆F₅HalF_n−1 (n = 3, 5) with Lewis acids were characterized by NMR in solution. Stable solid products were isolated and analytically characterized.     

The reaction of F atoms with C2H4. Vibrational Spectrum of the C2H4F Intermediate trapped in solid argon

When the products of the reaction between F atoms produced in a microwave discharge and C₂H₄ are frozen in a large excess of argon at 14 K, new infrared absorptions appear which can be assigned to the 2-fluoroethyl radical. Studies of the dependence of the product distribution on the F-atom concentration have confirmed that the stabilization of C₂H₄F₂ plays only a minor role under the sampling conditions typical of these experiments. Isotopic substitution experiments have demonstrated that the steric configuration about the CH bond is randomized as a result of the F-atom reaction. Upon irradiation of the sample with the full light of a medium-pressure mercury arc, absorptions of vinyl fluoride and acetylene and of the acetylene—HF complex grow in intensity, while those of FCD₂CH₂ and of FCH₂CD₂ diminish in intensity and those of FCH₂CH₂ a nd of FCH₂CD₂ are unchanged. The F-atom reactions and photolysis processes which occur in these experiments are discussed, and a tunnelling mechanism is proposed to explain the isotopic selectivity in the 2-fluoroethyl photodecomposition. The vibrational spectrum of FCH₂CH₂ is compared with that derived in a recent ab initio calculation.     

The synthesis, X-ray and DFT structures of the free ansa–cyclopentadiene ligand C5H5CMe2CMe2C5H5

The title compound 2,3-dicyclopentadiene-2,3-dimethylbutane (C₅H₅CMe₂CMe₂C₅H₅) 1 shows the typical staggered conformation of a highly substituted ethane derivative with the two largest substituents (C₅H₅) adopting a trans position. The molecule shows C₂ symmetry about the central C–C bond. Due to the high substitution, the central bond of the ethane is elongated to 160.0 pm (X-ray structure analysis) while the DFT calculation finds a value of 159.2 pm.     

Gas phase electron diffraction study of tetraphenyltin,Sn(C6H5)4

The geometrical parameters of the tetraphenyltin molecule have been determined by gas phase electron diffraction at about 310°. The S₄ and “open” D_2d molecular models with the tetrahedral bond configuration at tin were chosen for the structure analysis. The former gave the better fit. The thermal average bond lengths (r_g, in Å) are as follows:

The benzene ring geometry appears to be almost unaffected by bonding to tin. However, tin causes an increase in the endocyclic valence angle at the ipso-carbon atom to 121.0(0.9)° rather than a decrease of that angle as might be expected, tin being a σ-electron donor. The ring plane and the plane containing the

bond and S₄ axis make an angle, ?, of 34.1(2.1)°. The

bond length in tetraphenyltin is longer than not only the

bond in tetravinyltin (r_g = 2.117(4) Å) but also the

bond in tetramethyltin (r_g = 2.144(7) A).     

Matrix isolation study of the vacuum ultraviolet photolysis of allene and methylacetylene. Vibrational and electronic spectra of the species C3, C3H,C3H2, and C3H3

Upon hydrogen-discharge photolysis of normal or deuterium-substituted allene or methylacetylene in an argon or a nitrogen matrix at 14°K, infrared absorptions of all of the C₃H_n species with n < 4 appear. A hydrogen-deformation fundamental of C₃H₂ has been identified in the far infrared. Infrared studies of the partially deuterium- substituted methylacetylenes indicate that extensive photoisomerization occurs. The observed products are consistent with those predicted using the previously postulated gas-phase photolysis mechanism. The ultraviolet spectrum of C₃H₃ corresponds closely with that characteristic of the gas-phase molecule. Comparison of the spectrum between 1900 and 4000 A of photolyzed methylacetylene with that of matrix-isolated graphite vapor has indicated that any new electronic transition of C₃ in this region must be weak.     

The molecular structure of selenonyl fluoride,SeO2F2, and sulfuryl fluoride,SO2F2, as determined by gas-phase electron diffraction

The molecular structure of selenonyl fluoride (SeO₂F₂) and sulfuryl fluoride (SO₂F₂) has been studied by gas-phase electron diffraction. The geometries of both molecules are consistent with predictions of VSEPR (valence-shell electron-pair repulsion) theory. The results for the more important distance (r_a), bond angle, and r.m.s. amplitude (l) parameters with estimated uncertainties estimated at 2σ are for SeO₂F₂r(Se = 0) = 1.575 Å (0.002), r(Se-F) = 1.685 Å (0.002), ∠OSeO = 126.2° (0.5), ∠FSeF = 94.1° (0.5), l(Se = 0) = 0.0440 Å (0.0046), l(Se-F) = 0.0472 Å (0.0042), and for SO₂F₂r(S = 0) = 1.397 Å (0.002), r(S-F) = 1.530 Å (0.002), ∠OSO = 122.6° (1.2), ∠FSF = 96.7° (1.1), l(S = 0) = 0.0331 Å (0.0015), l(S-F) = 0.0393 Å (0.0018).     

Synthesis and crystal structures of (C8h8)Nd(C5H9C5H4) (THF)2 and [(C8H8)Gd(C5H9C4H4(THF)][(C8H8)GD(C5H9C5H4(THF)2]

LnCl₃ (Ln=Nd, Gd) reacts with C₅H₉C₅H₄Na (or K₂C₈H₈) in THF (C₅H₉C₅H₄ = cyclopentylcyclopentadienyl) in the ratio of 1 : to give (C₅H₉C₅H₄)LnCl₂(THF)_n (orC₈H₈)LnCl₂(THF)_n], which further reacts with K₂C₈H₈ (or C₅H₉C₅H₄Na) in THF to form the litle complexes. If Ln=Nd the complex (C₈H₈)Nd(C₅H₉C₅H₄)(THF)₂ (a) was obtained: when Ln=Gd the 1 : 1 complex [(C₈H₈)Gd(C_%H₉)(THF)][(C₈H₈)Gd(C₅H₉H₄)(THF)₂] (b) was obtained in crystalline form.

The crystal structure analysis shows that in (C₈H₈)Ln(C₅H₉C₅H₄)(THF)₂ (Ln=Nd or Gd), the Cyclopentylcyclopentadieny (η⁵), cyclooctatetraenyl (η⁸) and two oxygen atoms from THF are coordinated to Nd³⁺ (or Gd³⁺) with coordination number 10.

The centroid of the cyclopentadienyl ring (Cp′) in C₅H₉C₅H₄ group, cyclooctatetraenyl centroid (COTL) and two oxygens (THF) form a twisted tetrahedron around Nd³⁺ (or Gd³⁺). In (C₈H₈)Gd(C₅H₉C₅H₄)(THF), the cyclopentyl-cyclopentadienyl (η⁵), cyclooctatetraenyl (η⁸) and one oxygen atom are coordinated to Gd³⁺ with the coordination number of 9 and Cp′, COT and oxygen atom form a triangular plane around Gd³⁺, which is almost in the plane (dev. -0.0144 Å).     

Kinetic isotope effects in the reaction H + C2H4 → C2H5

The high-pressure limiting rate constants of the reactions between H or D atoms and three isotopic ethylenes have been measured in the temperature range 206–461 K. Practically no isotope effects due to the differences between the ethylenes could be observed. This result does not agree with the prediction recently made by the activated complex theory.     

Ab Initio calculation of molecular, thermodynamic, and kinetic parameters of CF, CF2, CF3, and C2F5 radicals and CF4, CF3I, C2F4, and C2F6 molecules in gas

Enthalpies of formation of ground states of the gaseous particles CF, CF₂, C₂F₅, CF₄, CF₃I, C₂F₄, and C₂F₆ were calculated by ab initio method in the CCSD(T) approximation with extrapolation to the full basis and regard to the correlation energy. Their equilibrium geometrics, frequencies of normal vibrations, and other values were found by the B3LYP/aug-cc-pvdz method, from which thermodynamic functions within the range of 0–6000 K were calculated. Equilibrium constants were calculated from these functions, and then the information on the rate constants in the limit of high pressures was obtained.     

Spectral bandshape and intensity of the C-H chromophore in the infrared spectra of CF3H and C4F9H

The vapour phase IR spectra and integrated band intensities of the fundamental and first-overtone transition of the C-H chromophore in CF₃H and (CF₃)₃CH are reported. The vibrational coarse bandshapes for (CF₃)₃CH are consistent with lorentzians of width Γ = 3 cm^?1 (fundamental) and Γ = 12 cm^?1 (overtone). The results are discussed in connection with time-dependent molecular processes and IR photochemistry.     

The geometry, vibrational frequencies, thermochemistry, quadrupole moments and electronic structure of C2Na2: Comparison with C2Li2, C2H2, C2F2 and C2Cl2

The electronic structure of Na₂C₂ is studied using ab initio electronic structure methods and is compared to the companion molecule Li₂C₂. Both the linear D_∞h and planar structures are minima on the ground state potential surface with the planar D_2h conformation being the lowest energy form, similar to Li₂C₂. At the CCSD(t) level the planar form is more stable that the linear by 11.2 kcal/mol as compared with 7.34 kcal/mol for Li₂C₂. Both molecules are significantly ionic. The vibrational frequencies, atomization energy at 0 K, D₀, and the standard enthalpy of formation, are calculated and compared to those of Li₂C₂ as well as HCCH, FCCF and ClCCCl. We find D₀ and to be 331.1 and 84.92 kcal/mol for Li₂C₂ and 298.3 and 93.25 kcal/mol for Na₂C₂. We calibrate these by calculating the same quantities for HCCH, FCCF and ClCCCl.