Gas electron diffraction study of the molecular structure of NbCl4

The structure of the NbCl₄ molecule is studied experimentally by the synchronous electron diffraction and mass spectrometry methods. The model molecular geometries of C_2v, C_3v, D_2d, and T_d symmetries are verified. The advantages of the tetrahedral model over the other models are established. The thermally averaged parameters of the effective configuration of the NbCl₄ molecule are as follows: r_g(Nb?Cl)=2.279(5) Å, l(Nb?Cl)=0.073(2) Å, r_g(Cl?Cl)=3.692(17) Å, l(Cl?Cl)=0.275(11) Å, ∠_g(Cl-Nb-Cl)=108.2(5)°, and δ(Cl?Cl)=0.030(19) Å.     

Molecular structure of WO2Br2

Molecular structure of WO₂Br₂ has been studied by electron diffractometry. Structural parameters for the molecule with C_2v symmetry are: r_α(W=O)=1.710(6) Å, r_α(W?Br)=2.398(5) Å, r_α(O?O)=2.815(30) Å, r_α(Br?Br)=4.021(16) Å, r_α(O?Br)=3.347(10) Å. The OWO and BrWBr bond angles are close to tetrahedral:L _αOWO=110.8(2.0)°, L_αBrWBr=113.9(1.0)°. The W=O bond was found to be characteristic in the series of tungsten dioxyhalides.     

Structure and force field of the NbOCl3 molecule

The structure of the NbOCl₃ molecule is studied by electron diffraction at 743(6) K. It is established that the molecule is characterized by C_3v symmetry and the following structural parameters: r_α(Nb=0) 1.682(6) Å, r_α(Nb?Cl) 2.276(5) Å, ?(ONbCl) 107.5(5)°, and ?(ClNbCl) 111.3(4)°. Comparison with the other niobium oxytrihalide molecules shows that these values of the Nb=O bond and the bond angles are characteristic. The assignment of the ?₃ frequency of the NbOCl₃ molecule is refined, and the vibration frequencies of the NbOF₃ molecule are estimated.     

Molecular structure and force field of cerium tetrafluoride

Combined interpretation of electron diffraction and spectroscopic data for the CeF₄ molecule is reported. It is shown that the diffraction pattern corresponds to a tetrahedral structure of the molecule with the following effective configuration parameters (T=1180(50) K): r_g(Ce-F)=2.036(5) Å, r_g(F-F)=3.312(25) Å, l(Ce-F)=0.074(3) Å, l(F-F)=0.261(17) Å. The total force field of the CeF₄ molecule is found in a harmonic approximation. Possible participation of the Ce f-electrons in chemical bonding is discussed.     

Molecular structure of arsabenzene by gas-phase electron diffraction

Vapor-phase molecules of C₅H₅As were found, assuming C_2v symmetry, to have the following structure parameters and uncertainties (2.5σ): r_g(C-As)= 1.850 ± 0.003 Å, r_g(C₂–C₃) = 1.390 ± 0.032 /rA, r_g(C₃–C₄) = 1.401 ± 0.032 /rA, r_g(C-C_ave) = 1.395₄ ± 0.002 Å, ∠CAsC = 97.3 ± 1.7°, ∠AsCC = 125.1 ± 2.8°, and ∠C₃C₃C₄ = 124.2 ± 2.9°. Amplitudes of vibration were also determined. Auxiliary information is more restrictive than pure electron diffraction intensities as evidence that the molecule is rigorously planar. Structural characteristics of arsabenzene reinforce prior indications that the heterocyclic molecule is genuinely aromatic.     

Structure and vibration frequencies of gaseous europium dibromide

Molecular structure of EuBr₂ was studied by electron diffraction and mass spectrometry at 1373(20) K. The molecule has a nonlinear equilibrium configuration and is characterized by the following effective parameters: r_g(Eu-Br) = 2.767(6) Å, r_g(Br-Br) = 5.11(5) Å, l_g(Eu-Br) = 0.109(2) Å, l_g(Br-Br) = 0.388(5) Å, Z_g(Br-Eu-Br) = 135.0(3.5)°. The vibration frequencies v₁ = 225(10) cm^-1 and v₂ = 40(4) cm^-1 were found using electron diffraction data.     

Gas-phase structure and conformational properties of copper (I) pivalate dimer (CuC5H9O2)2

The molecular structure and conformational properties of gaseous dimer of copper (I) pivalate, Cu₂piv₂, have been studied by gas electron diffraction (GED) at 413(5) K and quantum chemical calculations (DFT and MP2). The molecule possesses a planar eight-membered skeleton. Two conformers, “staggered” of C _2h symmetry and “eclipsed” of C _2v symmetry, were found for Cu₂piv₂ in the gas phase. The following geometric parameters of the skeleton ring and the tert-butyl groups have been determined from the GED experiment for the “staggered” form: r_g(Cu···Cu) = 2.520(8) Å, r_g(Cu–O)_ave = 1.871(4) Å, r_g(C–O)_ave = 1.273(3) Å, r_g(C–C)_ring-tert = 1.531(4) Å, r_g(C–C)_{tert-out-of-plane-ring} = 1.536(4) Å, r_g(C–C)_{tert-in-the-plane-ring} = 1.527(4) Å, r_g(C–H)_ave = 1.087(5) Å, (O–Cu–O) = 172.12°(3). Computations predict the internal rotation of the tert-butyl groups to be independent. The value of calculated Wiberg bond index for Cu···Cu testifies the existence of weak bonding between two copper atoms.

     

Simultaneous electron diffraction and mass-spectrometric study of the structure of the MoF5 molecule

Electron diffraction data for the MoF₅ molecule are analyzed in terms of an r_a structure. Three models of the geometrical structure, which have D_3h, C_4v, and C_2v symmetry, are considered. It is confirmed that a distorted bipyramid of C_2v symmetry is the best model that is in agreement with experimental electron diffraction data. The model has three different types of nuclear Mo-F distances: r_α(Mo-F_1eq) = 1.720(5) Å, r_α(Mo-F_2eq) = 1.826(7) Å, r_α(Mo-F_ax) = 1.825(7) Å. The bond angle between the pseudoaxial bonds is 168.1(0.6)?, and the angle between the Mo-F_2eq pseudoequatorial bonds is 122.6(0.8)?. The r_a(Mo-F_leq) and r_a(Mo-F_2eq) distances differ significantly. Possible reasons for this are 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)°.     

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.     

Gas-phase electron diffraction study of the molecular structure of tungsten oxytetrafluoride,WOF4

The molecular structure of tungsten oxytetrafluoride has been studied in the gas phase by electron diffraction. A square pyramidal model with molecular symmetryC_4v, as indicated by vibrational spectroscopy, gives a good fit to the experimental data. Least squares refinement on the molecular intensity curves gives the following results for the principal geometrical parameters (uncertainties in parentheses are 2σ):r_a(W=O) = 1.666 (0.007)Å,r_a(W-F)= 1.847 (0.002)Å, ∠OWF = 104.8 (0.6)°, ∠FWF = 86.2(0.3)°.     

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.     

Crystal Structure of Tl<Subscript>2</Subscript>[NbCl<Subscript>6</Subscript>] and Tl<Subscript>2</Subscript>[NbBr<Subscript>6</Subscript>]

Single crystals of Tl₂[NbCl₆] (1) and Tl₂ [NbBr₆] (2) are obtained as black needles on heating TlCl, Nb, S₂Cl₂ (1) and Tl, Nb, and Br₂ at 400°C (2). Tl₂NbBr₆ also forms in the reaction of TlBr, Nb, Br₂, and S at 500°C. Both compounds crystallize in the K₂[PtCl₆] structure type to form non-distorted octahedral [NbХ₆]^2– anions (Nb–Cl 2.397(4) Å and Nb–Br 2.516(2) Å). The magnetic properties of Tl₂[NbBr₆] in a range 5-300 K indicate an antiferromagnetic interaction between Nb⁴⁺ ion spins (d¹, S = 1/2). On cooling, the compound becomes a noncollinear ferromagnet with T_c = 23 K.     

Molecular structures,vibrational assignments and barriers to pseudorotation of gaseous niobium pentachloride and tantalum pentachloride from electron diffraction data

The pentachlorides of niobium and tantalum have trigonal bipyramidal structures (D_3hsymmetry) with thermal average axial bonds, r_α, longer than the equatorial ones by 0.097(9) and 0.142(5) Å, respectively. The equatorial bonds are r(Nb—Cl) = 2.241(4) and r(Ta—Cl) = 2.227(3) Å. Standard deviations are given. Calculated amplitudes of vibration for the e' type of bending frequencies assigned as v₆ (equatorial in-plane bend) < v₇ (axial—equatorial bend) agree significantly better with the experimental vibrational amplitudes than do amplitudes computed for the opposite assignment. Assuming an analytical quartic-harmonic potential for the pseudorotational motion of the molecules, barriers to pseudorotation of 1.5(0.7) and 1.2(0.6) kcal mol^?1 are estimated from the electron diffraction data for NbCl₅ and TaCl₅, respectively. Effects from interatomic multiple scattering are included in the theoretical intensities, and are found to be of some importance to the results.     

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 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°.     

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.     

Darstellung,Schwingungsspektren und Normalkoordinatenanalyse von Hexachlororhenat(V) sowie Kristallstruktur von [P(C6H5)4][ReCl6]

Preparation, Vibrational Spectra, and Normal Coordinate Analysis of Hexachlororhenate(V) and Crystal Structure of [P(C₆H₅)₄][ReCl₆] By oxidation of A₂[ReCl₆], A = [(n-C₄H₉)₄N]⁺, [P(C₆H₅)₄]⁺, with Cl₂ in dichloromethane/trifluoracetic acid A[ReCl₆] is formed. [P(C₆H₅)₄][ReCl₆] crystallizes with tetragonal symmetry, space group P4/n-C, a = 12.967(4), c = 7.6992(8) Å, Z = 2. The octahedral complexion [ReCl₆]^? is compressed (C_4v) with the bond lengths, axial Re? Cl1 = 2.28 and Re? Cl3 = 2.24 Å, equatorial Re? Cl2 = 2.31 Å. The infrared active antisymmetric Re? Cl stretching vibration is split into v′₃ = 346 an v″₃ = 326 cm^?1. The assignment of all IR and Raman modes is confirmed by a normal coordinate analysis. The different valence force constants, f_d(ReCl1) = 2.09, f_d(ReCl3) = 2.10, f_d(ReCl2) = 1.88 mdyn/ Å result from the distortion of the octahedron. On excitation with the Ar laser line 514.5 nm a resonance Raman spectrum is observed, showing 8 overtones of v′(A₁) = 382 cm^?1, from which the harmonic frequency ω₁ = 382.1 cm^?1, the anharmonicity constant X₁₁ = ?0.76 cm^?1, and the maximum bond dissociation energy of the [ReCl₆]^? ion to be 138 kcal/mol, are calculated. The vibrational fine structure of the intraconfigurational transitions in the near infrared has been resolved by measuring the absorption spectrum of [(n-C₄H₉)₄N][ReCl₆] at low temperature (10 K), resulting in the assignment of the following electronic origins: Γ₃(³T_1g) → Γ₄(³T_1g): 7 512, Γ₃(³T_1g) → Γ₁(³T_1g): 7 624 and Γ₃(³T_1g) → Γ₅(¹T_2g), Γ₃(¹E_g): 8 368 cm^?1.     

Molecular structure and microwave spectra of isotopic chloro-, bromo-, and iodocyanoacetylene

The microwave spectra of the halogeno-cyanoacetylenes, X-CC-CN (X = ¹²⁷I, ⁸¹Br, ⁷⁹Br, ³⁷Cl, ³⁵Cl), have been investigated. The molecules were found to be linear. The vibration-rotation constants of the three bending vibrations and the lower stretching vibration were determined. Lines belonging to the monosubstituted ¹³C and ¹⁵N species in their natural abundances were measured and the rotational constants obtained. The bond distances based on the substitution coordinates were: for I-CC-CN r(I-C) = 1.984₆ Å, r(CC) = 1.207_o Å, r(C-C) = 1.370₂ Å, r(CN) = l.l60₄ Å; for Br-CC-CN, r(Br-C) = 1.785₈ Å, r(CC) = 1.204₁ Å, r(C-C) = 1.369₉ Å, r(CN) = 1.159₃ Å; and for C1-CC-CN, r(Cl-C) = 1.624₅ Å, r(CC) = 1.209_o Å, r(C-C) = 1.369_o Å, r(CN) = 1.160₂ Å.