Molecular structures of isobutene and 2,3-dimethyl-2-butene as studied by gas electron diffraction

The structures of isobutene and 2,3-dimethyl-2-butene have been studied by gas electron diffraction. For isobutene the rotational constants obtained by Laurie by microwave spectroscopy have also been taken into account. Leastsquares analyses have given the following r_g bond distances and valence angles (r_av for isobutene and r_α for dimethylbutene): for isobutene, r(CC) = 1.342±0.003 Å, r(C-C)= 1.508±0.002Å, r(C-H, methyl) = 1.119±0.007 Å, r(C-H, methylene) = 1.095±0.020 Å, ∠(C-CC) = 122.2±0.2°, ∠(H-C-H) = 107.9±0.8°, and ∠(C-C-H) 121.3±1.5°; for dimethylbutene, r(CC)= 1.353 ±0.004 Å, r(C-C) = 1.511±0.002 Å, r(C-H) = 1.118± 0.004 Å, ∠(C-CC)= 123.9±0.5°, and ∠(H-C-H)= 107.0±1.0°, where the uncertainties represent estimated limits of experimental error. The bond distances and valence angles in these molecules and in related molecules are compared with one another. The CC and C-C bond distances increase almost regularly with the number of methyl groups, and the C-C bonds in isobutene and dimethylbutene are shorter than those in acetaldehyde and acetone by about 0.01 Å. Systematic variations in the C-CC angles suggest the steric influence of methyl groups.     

An electron diffraction study of the molecular structure of gaseous cyclobutylgermane

The molecular structure and conformation of cyclobutylgermane have been determined by gas-phase electron diffraction. Like its counterpart cyclobutylsilane (CBS) it possesses quasi-equatorial and quasi-axial conformers. The most interesting aspect of the structure of CBG is the influence of the germyl group on the ratio of equatorial to axial conformers. The predominance of the quasi-equatorial conformer (ΔG = 3.1(1) kJ mol⁻¹), the near equality of the skeleton C---C bond lengths (C---C = 1.557(3)A) (r_a value) and the values of the puckering angles for the equatorial angles form and the axial one of 25.3(3.1)° and −20.4(3.6)° respectively, all support the predictions made by Jonvik and Boggs concerning the correlation between electronegativity and structural parameters in four-membered rings. From a consideration of these predictions, a comparison of the most prominent structural factors in CBG and cyclobutylsilane indicates that the germanium atom is more electronegative than silicon. This result could be considered as the first structural evidence for the previously postulated inversion of the electronegativity order within group IV.     

An electron diffraction study of the molecular structure of gaseous allyl silane

The molecular structure of allyl silane has been studied by gas-phase electron diffraction. The experimental radial distribution curve has only four prominent peaks, resulting in serious resolution problems in the structure determination. A single conformer whose dimensions resemble those of related molecules fits the diffraction data. The torsion angle φ_siccc is102 ± 1°, measured from the conformation having Si-C and CC eclipsed.     

An electron diffraction investigation of the molecular structure of dichloromaleic anhydride

The molecular structure of gaseous dichloromaleic anhydride has been investigated by electron diffraction at a nozzle-tip temperature of 164–170°C. The molecule is planar to within experimental error, but small deviations from planarity corresponding to torsion up to about 10° around the carbon-carbon single bonds cannot be ruled out. Values of the more important r_α distances and angles with estimated 2σ uncertainties are r(CO) = 1.188(2) Å, r(CC) = 1.332(5) Å, r(C-O) = 1.389(3) Å, r(C—C) = 1.495(3) Å, r(C—Cl) = 1.685(2) Å, ∠CC-Cl = 129.4(2)°, ∠C-CO = 128.5(4)° and ∠CC—C = 107.9(2)°. The shortening of the carbonyl bond relative to that in maleic anhydride itself is discussed in terms of a possible general effect of vicinal substitution.     

Structure determination of gaseous 1,3-dimethyl-2chloro-diazaboracyclopentane by electron diffraction

By means of gas phase electron diffraction it has been shown that the five-membered ring in 1,3-dimethyl-2-chloro-diazaboracyclopentane is essentially planar, while there seems to be a slight deviation from planarity about the N atoms. The most important bond lengths (r_a) and bond angles are (standard deviations in parentheses): r(B-N) = 1.413(3) Å; r(C-N)_av = 1.455(2) Å; r(B-Cl) = 1.770(4) Å; ∠NBN = 110.8(3)°; ∠B2N3C4 = 108.6(3)°; ∠N3C4C5 = 105.7(3)°.     

An investigation of the molecular structure of cycloheptanone by gas phase electron diffraction

The electron diffraction data of cycloheptanone, collected at 371 K, can be explained using a model of partial pseudorotation, with the symmetrical twist—chair as the mean structure. Ther_g, r_α-structure is characterized by r(C-C) = 1.536 Å, r(C=O) = 1.219 Å, r(C-H) = 1.124 Å, xxxCC(sp²)C = 117.3°, xxx(CCC = 115.5° and xxx(HCH = 103.2°. Approximate values for the constants of the pseudorotation potential are included.     

The molecular structure of gaseous 2-cyclopentene- 1,4-dione as determined by electron diffraction

The molecular structure of gaseous 2-cyclopentene-1,4-dione has been studied by electron diffraction. The molecule is planar to within the experimental error. The results obtained for some of the more important parameters with estimated uncertainties of 2σ are r(C-H) = 1.093 Å (0.013), r(C0) = 1.208 Å (0.002), r(CC) = 1.341 Å (0.005), r(CH-CO) = 1.493 Å (0.005), r(CO-CH₂) = 1.525 Å (0.005), ∠CC-C = 110.4° (0.3), ∠CH-CO = 124.9° (1.1), ∠CC-H. = 118.7° (5.8), ∠H-C-H = 113.2° (8.7) l(C-H) = 0.0853 A (0.0113), l(CO) = 0.0428 Å (0.0021), l(CC) = 0.0448 Å (0.0037) and l(C-C) = 0.0561 Å (0.0029). The structure is discussed in connection with the structures of related molecules.     

An electron diffraction study of the molecular structure of meta-difluorobenzene

The molecular structure of meta-difluorobenzene in the gas phase has been determined by electron diffraction at room temperature. The carbon ring deviates slightly from D_6h symmetry. The four C-C distances adjacent to the F atoms are 1.384 Å, the two other C-C distances are slightly longer, i.e. 1.405 Å. The C-F and C-H distances are 1.324 and 1.107 Å. The C-C-F valency angle is 119.5°.     

An electron diffraction study of the molecular structure of hexamethyldisiloxane

An electron diffraction determination of the molecular geometry of hexamethyldisiloxane has removed much of the uncertainty concerning this structure. The length of the SiO bond and the SiOSi bond angle were determined to be 1.631 ± 0.003 Å and 148 ± 3°, respectively. The experimental data are consistent with a staggered conformation (C_2v symmetry) while a model with twist angles around the SiO bonds of about 30° cannot be excluded. The molecule is probably performing large amplitude intramolecular motion.     

An electron diffraction determination of the molecular structure of trifluorosilyl isocyanate

The molecular structure of F₃SiNCO has been determined in the vapour phase by the sector microphotometer method of electron diffraction. The data are consistent with an Si-N-C angle of 160.7±1.2°, and Si-F and Si-N bond lengths of 1.553±0.004 and 1.648±0.010 Å respectively; the N-C and C-O bond lengths are strongly correlated with each other. The problems of interpretation of the electron diffraction structure are discussed in the light of a possible low Si-N-C deformation frequency.     

An electron diffraction study of the molecular structure of 1,2-difluoroethane

The molecular structure of 1,2-difluoroethane in the gas phase has been determined by electron diffraction at room temperature. Only the gauche conformation was found, the dihedral angle F-C-C-F is 74.5°. The bond lengths r_g(1) are: r(C-C) = 1.535 Å, r(C-F) = 1.394 Å, r(C-H) = 1.13 Å. The valency angles are: α(C-C-F) = 108.3, α(C-C-H) = 108.3. The dihedral angle between the C-C-F and C-C-H planes is 113.6°.     

The molecular structure of gaseous tetrachloro-p-benzoquinone and tetrachloro-o-benzoquinone by electron diffraction

The structures of tetrachloro-p-benzoquinone and tetrachloro-o-benzoquinone (p- and o-chloranil) have been investigated by gas electron diffraction. The ring distances are slightly larger and the carbonyl bonds slightly smaller than in the corresponding unsubstituted quinones. The molecules are planar to within experimental error, but small deviations from planarity such as those found for the para compound in the crystal are completely compatible with the data. Values for the geometrical parameters (r_a distances and bond angles) and for some of the more important amplitudes (l) with parenthesized uncertainties of 2σ including estimated systematic error and correlation effects are as follows. Tetrachloro-p-benzoquinone: D_2h symmetry (assumed); r(CO) = 1.216 Å(4), r(CC) = 1.353 Å(6), r(C-C) = 1.492 Å(3), r(C-Cl) = 1.701 Å(3), ∠C-C-C = 117.1° (7), ∠CC-C1 = 122.7° (2), l(CO)= 0.037 Å(5), l(CC) = l(C-C) - 0.008 Å(assumed) = 0.049 Å(7), and l(C-Cl) = 0.054 Å(3). Tetrachloro-o-benzoquinone: C_2v symmetry (assumed); r(CO) = 1.205 Å(5), r(CC) = 1.354 Å(9), r(C_cl-C_cl) = 1.478 Å(28), r(Co-C_cl) = 1.483 Å(24), r(C_o-C_o) = 1.526 Å(2), r(C-Cl)= 1.705 Å(3), <C_o-CO = 121.0° (22), ∠C-C-C = 117.2° (9), ∠C_co, ClC-Cl = 118.9° (22), ∠C_ccl, ClC-Cl = 122.2°(12), l(CO) = 0.039 Å(5), and l(C_cl-C_cl) = l(C_o-C_cl) = l( Co-Co) = l(CC) + 0.060 Å(equalities assumed) = 0.055 Å(9). Vibrational'shortenings (shrinkages) of a few of the long non-bond distances have also been measured.     

An electron diffraction determination of the molecular structure of trifluoromethyl selenium cyanide

The molecular structure of F₃CSeCN has been determined, in the vapour phase by the sector microphotometer method of electron diffraction. Two molecular conformations are consistent with the experimental data; for the physically more reasonable structure, the principal parameters are: C-F 1.332 ± 0.007, Se-C(F₃) 1.984± 0.020, Se-C(N) 1.854 ± 0.016, N-C 1.152±0.020 Å and angle C-Se-C, 92.2 ± 2.0°.