Molecular structure of carbonyl bromide as studied by gas electron diffraction

The molecular structure of COBr₂ has been determined as follows by an analysis of electron diffraction intensity: r_g(CO) = 1.178 ± 0.009 Å, r_g(C-Br) = 1.923 ± 0.005 Å and θ°_α(Br-C-Br) = 112.3 ± 0.4°. The uncertainties represent estimated limits of error. The observed systematic trends in the bond lengths and bond angles in carbonyl and thiocarbonyl halides are discussed.     

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 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.     

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.     

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.     

Molecular structure of gaseous vanadyl(V) fluoride as studied by electron diffraction,and its modified valence force field

The molecular structure of gaseous OVF₃ has been determined by electron diffraction to be: r_g(V-O) = 1.570(5) Å, r_g(V-F) = 1.729(2) Å and ∠_α(OVF) = 107.5(4)°. A modified force field has been fitted to results from spectroscopic as well as diffractional studies. A similar attempt to determine the force field for OVCl₃ was not as successful as for OVF₃, probably because the Coriolis constants are less accurately determined for that molecule.     

The molecular structure of 1,1,1-trimethoxyethane in the gas phase as determined by electron diffraction,molecular mechanics calculations and vibratio

The structure of 1,1,1-trimethoxyethane has been studied by electron diffraction in the gas phase. Although this technique cannot discriminate between a GGG (point symmetry C₃) and a TGG (C₁) conformation, vibrational spectra indicate that in the gas phase the C₁ conformer is predominant. Constraints necessary for a satisfactory leastsquares refinement were obtained from molecular mechanics calculations. The molecular geometry as obtained from r_α-refinements is as follows (r_g distances, r_α angles; standard deviations in parentheses): r(C-O central = 1.398 (6) Å, r(C-O)_terminal = 1.431(6)Å, r(C-C) = 1.527 (6) Å, r(C-H) = 1.114 (1) Å, ∠(C-O-C) = 114.0 (4)°, ∠(O-C-H) = 110.7 (4)°; the C-C-O and O-C-0 angles around the central carbon range between 106.6° and 113.1°.     

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.     

Structure and conformation of 1,3-dithlane: an electron diffraction study

A gas-phase electron diffraction study of 1,3-dithiane, carried out at 100° C, has found no statistically significant evidence for the presence of any conformer in the vapor other than the chair, within an estimated uncertainty of 10%. An index of the degree of ring puckering in 1,3-dithiane is the average torsional angle which was found to be 61.3°, appreciably greater than that in cyclohexane, but somewhat less than that in 1,4-dithiane and 1,3,5-trithianc. The C-C-C, C-C-S and S-C-S valency angles, 113.6(33)°, 114.9(4)° and 115.0(3)° respectively, were all larger than the C-C-C valency angles in cyclohexane. The C-S-C valency angle, 98.1(7)°, was slightly smaller than that of dimethyl sulfide. Observed bond lengths were r_g(C-H) = 1.116(10) Å, r_g(C-H) = 1.533(5)Å, and r_g(C-S) = 1. 812(3)Å and mean amplitudes of vibration were l_g(C-H) = 0.081(12)Å, l_g(C-C) = 0.052(6)Å and l_g(C-S) = 0.052(4) Å (parenthesized quantities correspond to 3σ). Curiously, nonbonded distances between the axial hydrogen atoms in 1,3-dithiane are virtually identical to those in cyclohexane, even though these molecules have greatly different bond lengths, valency angles, and torsional angles.     

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.     

Electron diffraction and quantum chemical study of the molecular structure of 4-methylbenzene sulfochloride

A combined electron diffraction and mass spectrometric study was carried out to investigate the molecular structure of 4-methylbenzene sulfochloride at 330(2) K. An analysis of the electron diffraction data was performed in terms of the r_α structure. Several models of geometrical structure having different orientations of the sulfochloride group relative to the plane of the benzene ring are treated. The following values of structural parameters were obtained: r_α(C-H)_meth= 1.104(41)Å, r_a(C-H)/_phen = 1.103(27)Å, r_a(C-C)_phen = 1.403(7) Å, r_a(C-C)_meth = 1.512(25) Å, r_a(C-S) =1.758(6) Å, r_a(S = O) = 1.419(3) Å,r _a(S-Cl) = 2.049(5) Å, ∠CCH_meth = 106.9(47)?, ∠CSO = 110.5(6)?, ∠CSCl = 101.3(6)°, ∠OSO = 120.5(9)°. The angle between the plane of the benzene ring and the plane of the S-Cl bond was found to be 83°. Ab initio and semiempirical quantum chemical calculations were accomplished to estimate the geometrical and energy parameters and compare them with electron diffraction data.     

Structure of SmCl3, DyCl3, and HoCl3 molecules based on the data of synchronous electron diffraction and mass-spectrometric experiment

Saturated vapors of SmCl₃, DyCl₃, and HoCl₃ have been studied in the framework of a synchronous electron diffraction and mass-spectrometric experiment at temperatures 1205 K, 1160 K, and 1148 K, respectively. In vapors of all compounds, along with monomer molecular forms, an insignificant (up to 2 mol.%) amount of dimers was detected. Parameters of the effective configuration of monomer molecules were determined. For molecules SmCl₃, DyCl₃, and HoCl₃ values of internuclear distances r _g(Ln-Cl) were 2.511(5) Å, 2.453(5) Å, and 2.444(5) Å, values of valence angles ∠_g(Cl-Ln-Cl) were 115.6(11)°, 116.8(10)°, and 116.6(10)°, respectively. It is shown that parameters of the r _g-structure are not incompatible with the notion of a planar equilibrium geometrical configuration of molecules SmCl₃, DyCl₃, and HoCl₃. Main tendencies in the change of structural and vibration characteristics in the series of lanthanide trichlorides are considered.     

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.     

An electron-diffraction study of the molecular structure of gaseous perbromyl fluoride and calculation of its force field and vibrational amplitudes

The molecular structure of FBrO₃ has been studied by gas-phase electron diffraction. Least-squares refinements of the molecular geometry using fixed spectroscopic amplitudes revealed two geometrical minima. Initially, the amplitudes employed were derived from diagonal force fields obtained by spectroscopic least-squares refinements to fit observed and calculated wave numbers; for each geometry there are two spectroscopic minima. In the lowest geometrical minimum the wave number agreement is poor, however, the introduction of the ∠OBrO/∠FBrO interaction force constant removed the discrepancies; the resulting force field is F(Br-O) = 6.92 ± 0.02 mdyn Å^?1F(Br-F) = 3.22 ± 0.03 mdyn Å^?1, F(∠OBrO) = 1.06 ± 0.02 mdyn Å, F(∠FBrO) = 0.81 ± 0.03 mdyn Å, F(∠OBrO/∠FBrO) = ?0.19 ± 0.02 mdyn Å. In the corresponding geometrical minimum r_g(Br-O) = 1.582 ± 0.001 Å, r_g(Br-F) = 1.708 ± 0.003 Å, r_α(∠OBrO) = 114.9 ± 0.3°, r_α(∠FBrO) = 103.3 ± 0.3°. Perpendicular amplitude correction coefficients, calculated for each force field employed, were used throughout to relate the interatomic distances through the r_α-structure. The geometries of the r_α^o- and r_e-structures are estimated.     

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.     

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.     

Electron diffraction study of the thionyl fluoride molecular structure

The electron diffraction study of thionyl fluoride yielded the following geometrical parameters (r_a structure): S-O 1.420±0.003 Å, S-F 1.583±0.003Å, O-S-F 106.2±0.2° and F-S-F 92.2±0.3°. The average structure (r_α°) is also given. Some of the variations in the molecular geometries of SOX₂ and SO₂X₂ molecules (X = F or Cl) involving the valence shell electron pair repulsion theory are discussed.     

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

Chloroacetyl chloride is studied by gas-phase electron diffraction at nozzle-tip tempera- tures of 18, 110 and 215°C. The molecules exist as a mixture of anti and gauche confor- mers with the anti form the more stable. The composition (mole fraction) of the vapor with uncertainties estimated at 2σ is found to be 0.770 (0.070), 0.673 (0.086) and 0.572 (0.086) at 18, 110 and 215°C, respectively. These values correspond to an energy difference with estimated standard deviation ΔE^o = E^o_g -E^o_a = 1.3 ± 0.4 kcal mol^?1 and an entropy difference ΔS^o = S^o_g -S^o_a = 0.7 ± 1.1 cal mol^?1 K^?1. Certain of the diffraction results 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 = 1.19 ± 0.33, V₂ = 0.56 ± 0.20 and V₃ = 0.94 ± 0.12, all in kcal mol^?1. The results for the distance (r_a), angle (_∠α) and r.m.s. amplitude parameters obtained at the three temperatures are entirely consistent. At 18°C the more important parameters are, with estimated uncertainties of 2σ, r(C-H) = 1.062(0.030) Å, r(CO) = 1.182(0.004) Å, r(C-C) = 1.521(0.009) Å. r(CO-Cl) = 1.772(0.016) Å, r(CH₂-Cl) = 1.782(0.018) Å, ∠C-C-0 = 126.9(0.9)°, ∠CH₂-CO-C1 = 110.0(0.7)°,∠CO-CH₂-C1 = 112.9(1–7)°, ∠H-C-H = 109.5° (assumed), ∠φ (gauche torsion angle relative to 0° for the anti form) = 116.4(7.7)°, δ (r.m.s. amplitude of torsional vibration in the anti conformer) == 17.5(4.2)°.     

Electron diffraction study on the molecular structure of hexamethylcyclotrisilazane, [(CH3)2SiNH]3

A gas electron diffraction study yielded the following geometrical parameters for hexamethylcyclotrisilazane: r(Si-N) = 1.728 ± 0.004 Å, r(Si-C) = 1.871 ± 0.004 Å, r(C-H) = 1.124 ± 0.007 Å, ∠N-Si-N = 108.4 ± 1.0°, ∠Si-N-Si = 126.8 ± 0.8°, ∠C-Si-C = 108.9 ± 2.3°, ∠H-C-H = 111.6 ± 0.9°. The (SiN)₃ ring was found to be puckered but the deviation from planarity is relatively small. Details of the ring shape could not be determined. The degree of ring puckering in six-membered rings with alternating atoms can be roughly predicted from the bond angles in analogous non-cyclic molecules.     

Geometrical and electronic structure of LaI3 molecule from the gas electron diffraction data and quantum chemical calculations

The saturated vapor over LaI₃ has been studied using the electron diffraction method with mass-spectral monitoring. It was determined that at a temperature 1142(10) K, along with monomer molecules, dimers are present in the vapor in the quantity of 0.7 mol.%. Effective configuration parameters of LaI₃ molecule were obtained: r _g(La-I) 2.961(6) Å, ∠_g(I-La-I) 116.5(9)°, l(La-I) 0.106(1) Å and l(I…I) 0.412(7) Å. A small deviation of the valence angle ∠_g(I-L-I) from 120° can be totally caused by a contraction effect of the distance r _g(I…I) of LaI₃ molecule with planar equilibrium configuration. The electronic structure of LaI₃ molecule was examined by the B3LYP/SDD method. In terms of the NBO-analysis, the participation of lanthanum 4f-AO in bonding orbitals La-I is noted. It is shown that the NBO-analysis describes the bond La-I in LaI₃ molecule as predominantly ionic one with a noticeable covalence component. The energy of the heterolytic bond breakage E(La-I)_het = 1216 kJ/mole was calculated.