Molecular structure of hexafluorobicyclo [2.2.0]hexa-2,5-diene (hexafluoro-dewar-benzene) in the vapour phase

Hexafluoro-Dewar-benzene has been studied by the electron-diffraction method. A model with C_2v symmetry gives excellent agreement between experimental and theoretical data. The structural parameters with error limits are (cf. Fig. 1): r(C₁-C₄)= 1.598 ±0.017 Å, r(C₁-C₂) = 1.505 ±0.005 Å, r(C₂-C₃) = 1.366 ± 0.015 Å, r(C₁-F₁) = 1.328±0.015 Å, r(C₂-F₂) = 1.319±0.007 Å, ∠F₁C₁C₄ = 118.7±0.7°, ∠F₂C₂C₃ = 133.6±0.7°, τ= 121.8±2.0°, and δ = -7.5±2.0°. Molecular orbital calculations by the CNDO/2 method gave τ = 119.8° and δ = ?4.2°.     

The structure of 1-chloro-1-silabicyclo(2.2.2)octane as determined by gas-phase electron diffraction

The structure of 1 -chloro-1 -si labicyclo( 2.2.2 )octane is determined by gas-phase electron diffraction. The molecule is found to have a large amplitude twisting motion with a double minimum quartic potential function of the form V(φ) = V_o[1 + (φ/φ_o)⁴ - 2(φ/φ_o)²]. Least-squares analysis of the experimental data gives values of 1.4(0.8) kcal mole^? for V_o and 17.5(2.5)° for φ_o. Other structural parameters for the “quasi-C_3v” cage-like molecule include: r_g(Si-Cl) = 2.061(3) Å, r_g(Si-C) = 1.863(3) Å, r_g(C-C_av) = 1.559(2) Å, and r_g(C-H_av) = 1.098(7) Å. Several valence angles exhibit large deviations from tetrahedral values, e.g. ∠Cl-Si-C₂ = 114.6(0.2)°, ∠Si-C₂-C₃ = 105.8(0.4)°, ∠C₂-C₃-C₄ = 114.2(1.2)°, ∠C-₃-C₄-C₅ = 111.4(0.8)° and ∠C₂-Si-C₆= 103.9(0.2)°. Many of the structural features in this strained polycyclic compound. Including the nature of the quartic potential function, can be rationalized in terms of a simple molecular mechanics model. A new method for the calculation of an analytical Jacobian of the intensity function with respect to parameters of the potential function is also discussed.     

Molecular structure and conformational composition of gaseous 1,1-dichloro-2-bromomethyl-cyclopropane and 1,1-dichloro-2-cyanomethyl-cyclopropane as determined by electron diffraction

The molecular structure of the title compounds have been investigated by gas-phase electron diffraction. Both molecules exist as about equal amounts of the two gauche conformers. There is no evidence for the presence of a syn conformer, but small amounts of this form cannot be excluded. Some of the important distance (r_a) and angle (∠_α) parameters for 1,1-dichloro-2-bromomethyl-cyclopropane are: r(CH) = 1.095(19) Å, r(C₁C₂) = 1.476(11) Å, r(C₂C₃) = 1.517(31) Å, r(CCH₂Br) = 1.543(32) Å, r(CCl) = 1.752(6) Å, r(CBr) = 1.950(13) Å, ∠CCBr = 110.5(1.9)°, ∠ClCCl = 111.9(6)°, ∠CCC = 117.5(1.3)°, σ₁ (CC torsion angle between CBr and the three-membered ring for gauche-1) = 116.2(5.6)°, σ₂ = −132.7(7.6). For 1,1-dichloro-2-cyanomethyl-cyclopropane the parameter values are: r(CH) = 1.101(16) Å, r(C₁C₂) = 1.498(9) Å, r(C₂C₃) = 1.544(21) Å, r(C₂C₄) = 1.497(33) Å, r(CCN) = 1.466(26) Å, r(CN) = 1.165(8) Å, r(CCl) = 1.754(5) Å, ∠CCCN = 113.7(2.0)°, ∠CCC = 122.8(1.6)°, ClCCl = 112.5(4)°, σ₁ = 113(13)°, σ₂ = −124(10)°.     

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.     

Molecular structure of 1,2,4-triazole

The molecular structure of 1,2,4-triazole has been determined by gas phase electron diffraction. The intemuclear distances and bond angles were obtained by applying a least-squares analysis to the experimental intensity. The bond distances (r_g) and bond angles were N₁-N₂ = 1.380 ± 0.010 Å, N₂C₃ = 1.329 ± 0.009 Å, C₃-N₄ = 1.348 ± 0.009 Å, N₁-C₅ = 1.377 ± 0.004 Å, N₄C₅ = 1.305 Å (calculated value). N-H = 0.990 Å, C-H = 1.054 Å, ∠N₁N₂C₃ = 102.7± 0.5°, ∠N₂C₃N₄ = 113.8 ± 1.3°, ∠N₂N₁C₅ = 108.9 ± 0.8°, ∠H₁N₁N₂ = 110.9°, ∠H₂C₃N₄ = 119.2°, ∠H₃C₅N₁ = 131.0°, ∠C₃N₄C₅ = 105.7° (calculated value) and ∠N₄C₅N₁ = 108.7° (calculated value).     

Molecular structures of acetyl fluoride and acetyl iodide

The molecular structures of acetyl fluoride and acetyl iodide have been determined by making use of the average distances obtained in the present study together with the moments of inertia reported in the literature. The large amplitude theory for a molecule with an internal top was used in the joint analysis. The thermal-average values of internuclear distances r_g and the bond angles in the zero-point average structure Φ_z are as follows: r_g(C-O) = 1.185 ±0.002 $\text{}\text{?}$rA, r_g(C-F) = 1.362± 0.002 Å, r_g(C-C) = 1.505±0.002 Å, r_g(C-H) = 1.101 ±0.004 Å, Φ_z(OCF) = 120.7°±0.4°,Φ_z(CCF) = 110.5° ± 0.5°, Φ_z(HCH) = 109.3°±0.6° tilt(CH₃) = 0.1°±1°, for acetyl fluoride; r_g(C=O) = 1.198±0.013 $\text{}\text{?}$rA, r_g(C-I) = 2.217±0.009 Å, r_g(C-C) = 1.492±0.015 $\text{}\text{?}$rA, r_g(C-H) = 1.101 ± 0.004 Å, Φ_z(OCI) = 119.5°± 0.8°,Φ_z(CCI) = 111.7°±0.9°, Φ_z(HCH) = 110.8°±0.8° and tilt(CH₃) = 1.7°+5.4° for acetyl iodide. The uncertainties represent the estimated limits of error. The barriers V₃ to internal rotation have been reanalyzed making use of the effective moments of inertia of the methyl top estimated on the basis of the large amplitude theory and resulted in 1039 and 1176 cal mol^?1 for acetyl fluoride and acetyl iodide, respectively. The structure parameters have been compared with those of other CH₃COX (X = Cl, Br, H, CH₃) type molecules.     

Electron diffraction study of molecular structure of mebicar

The structure of the mebicar molecule has been studied by gas-phase electron-diffractometry using quantum chemical calculations. An eclipsed conformation along the C-C bond (torsion angle ?(H-C-C-H) = 10°) and flattened semi-chair conformations of cyclic fragments have been found. The bond lengths (r _g ) and angles (∠α) show the following average values: r(C-C) 1.576(3) Å, r(C-N) 1.460(3) Å, r(C(O)-N) 1.390(4) Å, r(C=O) 1.211(5) Å, r(C-H) 1.090(5) Å, ∠CCN 103.0(5)°, ∠CNC(O) 112.2(1)°, ∠CNC 122.4(1)°. The dihedral angle between the cyclic fragments is 116.6°.     

The zero point average structure of isobutane as determined by electron diffraction and microwave spectroscopy

The molecular structure of isobutane in the gas phase was investigated by combining electron diffraction data with microwave spectroscopic rotational constants of Lide.The analysis indicated that the tertiary C-H distance (r_g = 1.122±0.006 Å) was substantially longer than the average methyl C-H distance (r_g = 1.113±0.002 Å). Other structural parameters obtained were: r_g(C-C) = 1.535±0.001 Å, ∠CCC = 110.8±0.2°, and the average ∠CCH (methyl) = 111.4±0.2°.     

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.     

Molecular structure and conformation of 2,3-dichloro-1-propene as determined by gas-phase electron diffraction

The molecular structure and conformation of 2,3-dichloro-1-propene have been determined by gas-phase electron diffraction at nozzle temperatures of 24, 90 and 273°C. The molecules exist as a mixture of two conformers with the chlorine atoms anti (torsion angle ∠φ = 0°) or gauche (∠φ = 109°) to each other and with the anti form the more stable. The composition (mole fraction) of the vapor with uncertainties estimated at 2σ was found to be 0.55 (0.08), 0.49 (0.08) and 0.41 (0.10) at 24, 90 and 273°, respectively. These values correspond to an energy difference with estimated standard deviation ΔE° = E°_g-E°_a = 0.7 ± 0.3 kcal mol^?1 and an entropy difference ΔS° = S°_g-S°_a = 0.6 ± 0.9 cal mol^?1 K^?1. Some of the diffraction results, together with spectroscopic observations, permit the evaluation of an approximate torsional potential function of the form 2V = V₁ (1 - cos φ) + V₂ (1 - cos 2φ) + V₃ (1 - cos 3φ); the results are V₁ = 4.4 ± 0.5, V₂ = ?2.9 ± 0.5 and V₃ = 4.8 ± 0.2, all in kcal mol^?1. The results at 24°C for the distance (r_a) and angle (∠_α) parameters, with estimated uncertainties of 2σ, are: r(C_sp²-H) = 1.098(0.020)Å, r(C_sp³-H) = 1.103(0.020)Å, r(CC) = 1.334(0.009)Å, r(C-C) = 1.504(0.013)Å, r(C_sp²-Cl) = 1.752(0.021)Å, r(C_sp³-Cl) = 1.776(0.020)Å, ∠C-CC = 127.6(1.1)°, ∠C_sp³-C_sp²-Cl = 110.2(1.0), ∠C_sp²-C_sp³-Cl = 113.1(1.2)°, ∠H-C_sp³-H = 109.5° (assumed), ∠CC-H = 120.0° (assumed) and ∠φ = 108.9(3.4)°.     

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.     

Molecular structure of n-butane: calculation of vibrational shrinkages and an electron diffraction re-investigation

A normal coordinate analysis was carried out based on the force field of Schachtschneider and Snyder in order to calculate all amplitudes of vibration and shrinkage corrections for n-butane. The results are tabulated to aid diffraction analyses of related substances. A vapor-phase electron diffraction reinvestigation of n-butane led to experimental measurements of the principal amplitudes of vibration and to the following molecular parameters (± 3σ ): r_g(C-C) = 1.531(2)Å, r_g(C-H)= 1.117(5)Å, ∠CCC (trans. gauche average) = 113.8(4)°, ∠CCH (ave) = 111.0(5)° , gauche CCCC dihedral angle 65(6)°, % trans conformer = 54 ± 9%, and ΔG° (gauche— trans) = 497 ± 220 cal mol^?1.     

On the molecular structure of (CH3)2NSO2N(CH3)2 as studied by gas electron diffraction

The following bond lengths and bond angles have been deduced from a vapour phase electron diffraction study of (CH₃)₂NSO₂N(CH₃)₂: r(C-H) 1.114 ± 0.005 Å, r(S-O) 1.432 ± 0.010 Å, r(N-C) 1.475 ± 0.013 Å, r(S-N) 1.651 ± 0.003 Å, ∠N-C-H 109.3 ± 2.0°, ∠C-N-C 118.0 ± 302°, ∠S-N-C 115.2 ± 1.1°, ∠N-S-N 110.5±1.3° and ∠O-S-O 114.7±2.5°. The sulphur bond configuration and the prevailing conformation, which was identical to that in the crystal, are discussed in relation to analogous sulphide and sulphoxide derivatives.     

Molecular structure of bis(acetylacetonato)nickel(II) in the gas phase as determined from electron diffraction data

The molecular structure of bis(acetylacetonato)nickel(II) has been determined by a sector-microphotometer gaseous electron-diffraction method. The experimental data were found to be consistent with a monomeric square-planar structure. The structural parameters of the chelate were determined as follows: ∠ ONiO = 93.6 ± 1.1°, r_g(Ni-O) = 1.876±0.005A Å, r_g(C-0) = 1.273± 0.007 Å, r_g(C-C_ring) = 1.401 ± 0.010 Å, r_g(C-C_methyl) = 1.504 ± 0.013 Å. The mean amplitudes of vibration and the shrinkage effects were calculated from normal-vibration treatment using the Urey-Bradley force field.     

Molecular structure of Pt(PF3)4 by gas-phase electron diffraction

The structure of Pt(PF₃)₄ was reinvestigated making use of a new theory of intramolecular dynamic scattering. Derived molecular parameters were insensitive to the dynamic corrections. Refinements for this tetrahedral molecule yielded r_g(Pt-P) = 2.229(5) Å, r_g(P-F) = 1.550(4) Å, and ∠PtPF = 118.9°(0.4), with the indicated uncertainties representing 2.5σ. Amplitudes of vibration were also determined. Diffraction patterns were consistent with freely rotating PF₃ groups.     

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.     

The structure and conformation of dichloroacetyl chloride as determined by gas-phase electron diffraction

The structure and conformation of dichloroacetyl chloride have been determined by gas-phase electron diffraction at nozzle temperatures of 20 and 119°C. The molecules exist as a mixture of two conformers with the hydrogen and oxygen atoms syn and gauche to each other. The composition (mole fraction of syn form) of the vapor was found to be 0.72 ± 0.06 and 0.73 ± 0.12 at 20 and 119°C, respectively, corresponding to almost equal energy for the two forms. The results for the distance (r_g), angle ∠α and r.m.s. amplitude (l) parameters obtained at the two temperatures are entirely consistent. At 20°C the more important parameters, with estimated uncertainties of 3σ are: r(C-H) = 1.062(0.049)Å, r(C0) = 1.189(0.003)Å, r(C-C) = 1.535(0.008)Å, r(CO-Cl) = 1.752 (0.009)Å, r(CHCl-Cl) = 1.771(0.004)Å, ∠C-CO = 123.3(1.3)°, ∠C-CO-Cl = 113.9 (5.9)°, ∠C-CHCl—Cl = 109.5(1.5)°, ∠C1-C-Cl = 111.7(0.5)°, ∠Cl-C-H = 108.0(1.5), φ₁ (HCCO torsion angle in the syn conformer) = 0.0° (assumed), φ₂ (HCCO torsion angle in the gauche conformer) = 138.2(5.1)°.     

The molecular structure and conformation of acetamides in the vapour phase: Part II. 2-Iodoacetamide

2-Iodoacetamide has been studied by electron diffraction, utilizing a new nozzle construction. A skew conformation with a dihedral angle of 126.3(1.1)° from syn (C-I bond eclipsing the C-N bond), and a gauche conformation with a dihedral angle of 42.3(1.6) both fit the experimental data almost equally well. However, comparison with the X-ray structure and the results for the two models indicate a slight preference for the skew form.The most important structural parameters are: r_g(CO) = 1.222(3)Å, r_g(C-N) = 1.370(3)Å, r_g(C-C) = 1.515(4) Å, r_g(C-I) = 2.160(4) Å, ∠_αOCC = 120.0(6)°, ∠_αNCC = 116.9(4)° and ∠_αCCl = 117.3(4)°. Parenthesized values are one standard deviation.     

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.     

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.