The gas-phase molecular structure of CF3HgCH3 - an electron diffraction and microwave study

The molecular structure of CF₃HgCH₃ in the gas phase is determined by a joint analysis of electron diffraction and microwave data. The following geometric parameters (r_av values) are derived: r(Hg—CH₃) = 2.052(5) Å, r(Hg—CF₃) = 2.116(4) Å. r(C-F) = 1.354(2) Å. r(C—H) = 1.079(14) Å, ∠.FCF = 105.7(0.2)° and ∠HCH = 107.0(1.5)°. Error limits are twice the standard deviations.     

Electron diffraction and CNDO/2 investigation on the molecular structure of tetrafluoro-1,3-diselenetane (F2CSe)2

The molecular structure of tetrafluoro-1,3-diselenetane was determined in the gas phase by electron diffraction. A planar ring configuration with the following geometric parameters (r_g-values) was obtained:r(Se-C) = 1.968 ± 0.004 Å, r(C-F) = 1.353 ± 0.003 Å, ∠SeCSe = 98.5° ± 0.4°, ∠FCF = 106.3 ± 0.8°. SCF-MO calculations in the CNDO/2 approximation confirm the planarity of the four membered ring and give a plausible explanation for the remarkably short Se-C bond length in the ring which in spite of ring strain is shorter than in Se(CF₃)₂. There exists a strong bonding interaction between the diagonal selenium atoms which amounts to about one fourth of a normal single bond strength.     

Gas phase molecular structure of bis(trifluoromethyl). mercury (CF3)2 Hg

The molecular structure of bis(trifluoromethyl)mercury has been determined by electron diffraction of gases. The best agreement between experiment and model was obtained for freely rotating CF₃ groups and the following geometric parameters (r°_α values): C-F = 1.345(3) Å, Hg-C = 2.101(5) Å and <FCF = 106.8°(0.2). The effect of CH₃/CF₃ substitution on the Hg-C bond length is discussed.     

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.     

Gas-phase electron diffraction studies of the molecular structures of pentafluoroethane,C2F5H,and monofluoroethane,C2H5F

The molecular structures of C₂F₅H and C₂H₅F have been studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument. The following values for the main independent geometrical parameters were obtained (r_a values with e.s.d. in parentheses): in C₂F₅H, C-C = 1.525(4) Å, C-F(CHF₂) = 1.347 Å, C-F(CF₃) = 1.327 Å [C-F(av.) = 1.335(2) Å], ∠CCF(av.) = 110.0(2)°; in C₂H₅F, C-C = 1.502(5) Å, C-F = 1.397(4) Å, C-H = 1.097(2) Å. ∠CCF = 110.4(2)°, ∠CCH(av.) = 113.6(4)°. Evidence is presented to show that the electron diffraction data for C₂H₅F are not compatible with values for the bond angles deduced spectroscopically.     

Molecular structures of propene and 3,3,3-trifluoropropene determined by gas electron diffraction

The structures of propene and 3,3,3-trifluoropropene have been studied by electron diffraction intensities measured in the present study and rotational constants reported in the literature. The following average structures have been determined: For propene, r_g(CC) = 1.342 ± 0.002 Å, r_g(C-C) = 1.506 ± 0.003 Å, r_g(C-H)_vinyl = 1.104 ± 0.010 Å, r_g(C-H)_methyl = 1.117 ± 0.008 Å, ∠(C-CC) = 124.3 ± 0.4°, ∠(CC-H) = 121.3 ± 1.4°, and ∠(C-C-H) = 110.7 ± 0.9°; for trifluoropropene, r_g(CC) = 1.318 ± 0.008 Å, r_g(C-C) = 1.495 ± 0.006 Å, r_g(C-H)= 1.100 ± 0.018 Å, r_g(C-F) = 1.347 ± 0.003 Å, ∠(C-CC) = 125.8 + 1.1°, ∠(C-C-F) = 112.0 ± 0.2°, where the valence angles refer to the r_av structure, and the uncertainties represent estimated limits of experimental error. A simple set of quadratic force constants for each molecule has been estimated. Regular trends have been observed in the CC and C-C bond distances and the C-CC angles in these and related molecules. Significant differences between the CC, C-C and C-F distances and the C-C-F angle in trifluoropropene and in hexafluoroisobutene reported by Hilderbrandt et al. have been indicated.     

On the geometry and internal rotation of XSCF3 compounds (X = F,Cl and CF3)

The molecular structures (r_α⁰ values) for XSCF₃ with X = F, Cl and CF₃ have been determined by electron diffraction of gases. While the geometry (C-F bond length and FCF angle) of the CF₃ groups and the bond angle at the sulfur atom depend very little on the substituent X, the S-C bond length increases with decreasing electronegativity of X from 1.805 (3) Å for X = F to 1.824 (6) Å for X = Cl. Torsional force constants for the CF₃ groups were derived from vibrational amplitudes. A strong increase of this force constant is observed between FSCF₃ (f_τ = 0.09 (2) mdyn Å) and CISCF₃ (f_τ = 0.18 (5) mdyn Å). The torsional frequencies derived from the electron diffraction experiment agree very well with the values observed in the far IR spectra for CISCF₃, and CF₃SCF₃. A force field for CF₃SCF₃ has been derived from IR and Raman data.     

A re-investigation of the molecular structure of trifluoroacetic acid by means of gas phase electron diffraction

Analysis of the electron diffraction patterns of trifluoroacetic acid at 140°C indicates the existence of one conformation with the CF₃-group rotated 17.3± 0.9° from a position with a fluorine atom eclipsed with respect to the CO bond. The data does not exclude the possibility of free internal rotation but it seems improbable.The important bond lengths, r_g(1), and bond angles with their standard deviations in parentheses, are: C-F: 1.325 (0.003), C-C: 1.546 (0.005), CO: 1.192 (0.003), C-O: 1.353 (0.014) Å, C-C-F: 109.5 (0.3), C-CO: 126.8 (0.8), C-C-O: 111.1 (0.9)°.     

The molecular structure of gaseous tetrafluoro-p-benzoquinone and tetramethyl-p-benzoquinone as determined by electron diffraction

The molecular structures of gaseous tetrafluoro-p-benzoquinone (p-fluoranil) and tetramethyl-p-benzoquinone (duroquinone) have been investigated by electron diffraction. Except for the methyl group hydrogen atoms, the molecules are planar to within experimental error, but small deviations from planarity are completely compatible with the data. Values for the geometrical parameters (r_adistances and r_α with parenthesized uncertainties of 2σ including estimated uncertainty in the electron wavelength and correlation effects, are as follows. Tetrafluoro-p-benzoquinone: D_2h symmetry (assumed); r(C0) = 1.211(6) Å, r(CC) = 1.339(12) Å, r(C-C) = 1.489(5') Å, r(C-F) = 1.323(5) Å, ∠C-C-C = 116.8(7)° and ∠C-C-F = 116.1(7)°. Tetramethyl-p-benzoquinone: C_2h symmetry (assumed);r(C-H) = 1.102(18) Å, r(CO) = 1.229(8) Å, r(CC) = 1.352(8) Å, r(C_sp2-C_sp2) = 1.491(11) Å, r(C_sp2-C_sp3) = 1.504(12) A, ∠C-CO-C = 120.8(8)°. ∠C-C-CH₃ = 116.1(8)°, ∠C-C-H = 110.5(34)° and α₁ = α₂ (methyl torsion = 30° (assumed).     

Molecular structure of 1,1-difluoroethylene as determined by gas phase electron diffraction and microwave data

The structure of 1,1-difluoroethylene was determined, from gas phase electron diffraction data obtained independently in Leiden and Tokyo and the rotational constants of F₂CCH₂, F₂CCHD and F₂CCD₂ derived from the microwave study by Chauffoureaux. The two electron diffraction data agreed without significant discrepancy. From a joint least squares analysis of the diffraction and microwave data, the following r_g bond distances and r_z bond angles were derived: CC = 1.340 ± 0.006 Å, C-F = 1.315 ± 0.003 Å, C-H = 1.091 ± 0.010 Å, ∠C-C-F = 124.7 ± 0.3°, ∠C-C-H = 119.0 ± 0.4°, where the uncertainties represent estimated limits of error.     

Molecular structure and conformation of gaseous 2-chloro-3-fluoro-1-propene

2-Chloro-3-fluoro-1-propene has been studied by electron diffraction, and the molecule was found to exist in equilibrium between a syn and a gauche conformation, with the syn conformation as the most stable. The most important structure parameters with standard deviation are: r_g(CC) = 1.338(6) Å,r_g(C—C) = 1.505(5) Å, r_g(C—F) = 1.378(4) Å, r_g(C-Cl) = 1.743(3) Å, ∠CC—Cl = 123.0(7)°, ∠CC—C = 125.6(6)° and ∠C—C—F = 111.2(8)°.A force field was determined by a least-squares refinement to vibrational frequencies. Mean square amplitudes of vibration and perpendicular amplitude correction coefficients have been calculated. The mean square amplitudes of vibration from the electron diffraction data are in very good agreement with the values calculated from the spectroscopic data.     

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.     

Molecular structure and conformation of gaseous vinyldimethylchlorosilane as determined by electron diffraction

The molecular structure of vinyldimethylchlorosilane has been determined by gas phase electron diffraction at room temperature. The least squares values of the bond lengths (r_g) and bond angles (∠_α) are : r(CH) = 1.086(6) Å, r(CC) = 1.347(5) Å, r(SiC=) = 1.838(6) Å, r(SiC) = 1.876(3) Å, r(SiCl) = 2.078(2) Å, ∠CCSi = 127.8° (1.2) and ∠=CSiCl = 107° (1). Models with pure syn form and a mixture of syn and gauche gave equally good agreement with the diffraction data.     

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.     

The structure of bis(1,1,1,5.5,5-hexafluoro-2,4. pentadionato) copper(II) as determined by gas phase electron diffraction

The r_g structure of bis(1,1,1,5,5,5-hexafluoroacetylacetonato) copper(II) has been determined by gas phase electron diffraction. The experimental data were found to be consistent with a D_2h model in which the oxygens from the two ligands are arranged in an essentially square planar configuration about the copper atom (∠OCuO = 90.6° ± 1.2°). It was possible to obtain a precise value for the copper oxygen bond length, r_g = 1.919 ± 0.008 Å, since this distance appeared as an isolated peak in the radial distribution curve. Structural parameters for the ligand (r_g(C-O) = 1.276 ± 0.009 Å, r_g(C-C_ring) = 1.392 ± 0.015 Å, r_g(C-CF₃)= 1.558 ± 0.009 Å and r_g(C-F) = 1.339 ± 0.003 Å), while less precisely determined are, nevertheless, consistent with reported values for related molecules. A model for the rotational isomerism of the four CF₃ groups was invoked in order to explain various features in the radial distribution curve in a region from 2.5 to 5.5 Å.     

Molecular structure and conformation of gaseous bromoacetyl chloride and bromoacetyl bromide as determined by electron diffraction

Bromoacetyl chloride and bromoacetyl bromide are studied by gas phase electron diffraction at nozzle-tip temperatures of 70°C and 77°C, respectively. Both compounds exist as mixtures of anti and gauche conformers. The mole fraction anti, with uncertainties estimated at 2σ, was found to be 0.474(0.080) for bromoacetyl chloride and 0.615(0.069) for bromoacetyl bromide. The results for the distance (r_a)and angle (∠α) parameters, with parenthesized uncertainties of 2σ including estimated uncertainty in the electron wave length and correlation effects are as follows: (1) bromoacetyl chloride, r(C-H) = 1.086(0.062) Å, r(CO) = 1.188(0.009) Å, r(C-C) = 1.519(0.018) Å, r(C-Cl) = 1.789(0.011) Å, r(C-Br) = 1.935(0.012) Å, ∠C-CO = 127.6(1.3)°, ∠C-C-Cl = 111.3(1.1)°, ∠C-C-Br = 111.0(1.5)°, ∠H-C-H = 109.5°(assumed), $\text{}\text{?}$/o (gauche torsion angle relative to 0° for the anti form) = 110.0°(assumed); (2) bromoacetyl bromide, r(C-H) =1.110(0.088) Å, r(C=O) = 1.175(0.013) Å, r(C-C) = 1.513(0.020) Å, r(CO-Br) = 1.987(0.020) Å, r(CH₂-Br) = 1.915(0.020) Å, ∠C-CO = 129.4(1.7)°, ∠CH₂-CO-Br = 110.7(1.5)°, ∠CO-CH₂-Br = 111.7(1.8)°, ∠H-C-H = 109.5°(assumed), ∠ø (gauche torsion angle relative to 0° for the anti form) = 105.0°(assumed). The structural results are discussed in connection with the structures of related molecules.     

The gas-phase structure of bis(fluorosulfonyl)difluoromethane, (SO2F)2CF2

New synthetic pathways and the infrared spectrum of bis(fluorosulfonyl)difluoromethane, (SO₂F)₂CF₂, are reported. The geometric structure and conformational properties of the title compound have been studied by gas electron diffraction. Depending on the rotational position of the two SO₂F groups, four conformers with different symmetries can occur in this compound: C_2v symmetry, if both S? F bonds stagger the CF₂ group. C₂ or C_s symmetry, if one S?O bond of each group staggers the CF₂ group. The experimental electron diffraction intensities can be fitted equally well with a C₁ conformer or with a mixture of C_2v, C₂ and C_s conformers, in a ratio of 3:2:5. The following geometric parameters (r_a distances, ∠_α angles with 3σ uncertainties) were derived: C? F = 1.340(6) Å, S?O = 1.412(2) Å, S? F = 1.550(3) Å, C? S = 1.848(4) Å, S? C? S = 113.6(7)°, F? C? F = 110.0(10)°, O?S?O = 124.6(18)°, C? S? F = 96.5(16)° and C? S?O = 108.4(14)°.     

The molecular structures of cis-3-hexene and trans-3-hexene in the gas phase by electron diffraction and molecular mechanical calculations

The molecular structures of cis-3-hexene and of trans-3-hexene in the gas phase have been determined by electron diffraction combined with molecular mechanical calculations. For cis-3-hexene the data indicate the presence of the (+ac, +ac) and the (?ac, +ac) forms. In trans-3 -hexene three rotamers were observed, with an energy sequence E(+ac, +ac) ≈ E(?ac, +ac) < E(ac, sp). The refined r_α⁰-structural parameters are: cis-3-hexene: C-H = 1.073 Å, CC = 1.330 Å, C(sp²)-C(sp³) = 1.505 Å, ∠CCH(in CH₂) = 111.1°, ∠CCC = 111.4°, ∠(CC-C) = 129.1° trans-3-hexene: C-H = 1.078 Å, CC = 1.342 Å, C(sp²)-C(sp³) = 1.506 Å, ∠CCH(in CH₂) = 109.3°, ∠CCC = 112.8, ∠CC—C = 124.1°The agreement between calculated and experimental geometries and vibrational amplitudes is good.     

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.     

Molecular structure and conformation of cis-1,3- dichloro-1-propene as determined by gas phase electron diffraction

The molecular structure and conformation of cis-1,3-dichloro-1-propene have been determined by gas phase electron diffraction at a nozzle temperature of 90°C. The molecule exists in a form in which the chlorine atom of the methyl group and the carbon-carbon double bond are gauche to one another. The results for the distance (r_g) and angle (∠_α) parameters are: r(C-H) = 1.078(10)Å, r(CC) = 1.340(5)Å, r(C-C) = 1.508(7)Å, r( =C-Cl) = 1.762(3)Å, r(C-Cl) = 1.806(3)Å, ∠Cl-C-C = 111.7°(1.8), ∠(CC-C) = 125.5°(1.5), ∠Cl-CC = 124.6°(1.6) and ∠H-C-Cl = 111°(5). The torsion-sensitive distances close to the gauche form can be approximated using a dynamic model with a quartic double minimum potential function of the form V(Φ) = V₀[1 + ($\text{Φ}\text{Φ}{}_0$⁴ - 2($\text{Φ}\text{Φ}{}_0$)²], where V_o = 1.1(8) kcal mol^?1 and Φ₀ = 56°(5) (Φ = 0 corresponds to the anti form).