Molecular Structure of tert-Butylazide: A Gas-Phase Electron Diffraction and Quantum Chemical Study

The molecular structure of tert-butylazide has been determined by gas-phase electron diffraction and quantum chemical calculations. The HF/6-31G* and B3LYP/6-31G** calculations yielded near C _s symmetry for the tert-butyl group, anti conformation of the (C)N—N bond with respect to one of the bonds, and an essentially free rotation around the bond with a 0.34 kcal/mol energy difference between syn and anti conformations of the CNNN moiety, the anti being the more stable form. The electron diffraction analysis was carried out by modeling a mixture of conformational isomers, generated by rotating the terminal nitrogen of the azide group, using a computed rotational potential. The data are consistent with C _s symmetry for the tert-butyl group. The bond, however, was found to be rotated out of the anti position, with respect to one of the bonds, by 12.5(12)°. The electron diffraction analysis yielded the following bond lengths (r _g), bond angles, and torsional angles: , .     

Molecular Structure of Bis(trimethylsilyl) Hypophosphite HP(OSiMe3)2 by Gas-Phase Electron Diffraction and Quantum Chemistry

Geometric parameters and conformation of the bis(trimethylsilyl)hypophosphite molecule were determined by gas-phase electron diffraction and quantum-chemical calculations. The molecule has an asymmetric structure, including an asymmetric P(OSiMe₃)₂ group. The principal geometric parameters are as follows: (r _a; in parentheses are standard deviations): bond lengths: P-O 1.616 and 1.633(1), Si-O 1.670(1), Si-C 1.892(1), C-H 1.097(3) Å; bond angles: OPO 100.8(8), POSi 133.3, and 138.4(3)°; torsion angles about P-O bonds 120(2) and 41.(3)°; and torsion angles about Si-O bonds are 145 and −178(4)°.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 6, 2005, pp. 897–902.Original Russian Text Copyright © 2005 by Naumov, Oberhammer, Tafipol’skii.     

Molecular Structure of Triphenylphosphine by Gas-Phase Electron Diffraction and ab initio Calculations

The molecular structure of triphenylphosphine was determined by gas-phase electron diffraction and quantum-chemical calculations. The symmetry of the molecule is C ₃ with the L_pPCC torsion angle of 32.2(3.0)°. The main geometrical parameters are as follows: bond distances, Å: P-Cl 1.839(2), C-Cav 1.400(1), and C-H 1.098(3); bond angles, deg: CPC 102.2(3), PCC 115.3(5), and CC _ipso C 119.4(4).     

Molecular Structure of Neodymium Tribromide from Gas-Phase Electron Diffraction Data

The structural investigation of molecules in the vapor over neodymium tribromide was performed by synchronous gas-phase electron diffraction and mass spectrometric (GED/MS) experiments at 1110(10) K. Besides the monomeric molecules (NdBr₃), a small amount (0.7%) of the dimer (Nd₂Br₆) was detected. For NdBr₃, the thermal-average bond length r _g (Nd–Br) of 2.675(6) Å was determined. The equilibrium structure was estimated to be planar (or nearly planar) with r _e (Nd–Br) of 2.659(7) Å. Three vibrational frequencies were estimated using the GED data: ₁ = 193 cm^–1, ₂ = 35 cm^–1, ₄ = 41 cm^–1. The structural parameters of Nd₂Br₆ could not be refined and were constrained at the estimated values during the analysis.     

Molecular Structure of Diphenylphosphine As Determined by Gas-Phase Electron Diffraction and Quantum-Chemical Calculations

Gas-phase electron diffraction was used to show the title molecule has an asymmetric structure with torsion angles around P-C bonds of 65° and 154(7)°. The principal geometric parameters are as follows: interatomic distances, Å: P-C 1.829 and 1.833(4), C-Cav 1.401(1); bond angles, deg: CPC 101.0(20), PCC 120.4 and 119.4(16). Quantum-chemical calculations of chlorodiphenylphosphine were performed.     

Molecular Structure of Phenylphosphine and Its Analogs by Gas-Phase Electron Diffraction and Quantum-Chemical Calculations

The molecular structures of phenylphosphine and its analogs, aniline and phenylarsine, were studied by gas-phase electron diffraction and quantum-chemical methods. The geometry and force constants were calculated at the HF/6-31G and B3LYP/6-31G^* levels of theory. Phenylphosphine and phenylarsine possess a bisector conformation with asymmetric phenyl rings. The main geometric parameters are as follows (r _a): P-C 1.833(6), C-Cav 1.397(1)Å. The structures of molecules like X₂YPh (X = H, F, Cl; Y = N, P, As) were discussed.     

Molecular Structure of MnI2: A Gas-Phase Electron Diffraction Study

The molecular structure of MnI₂ has been determined by gas-phase electron diffraction. The analysis confirmed the linearity of the equilibrium configuration of the monomer with a bond length (r _g) of 2.538 ± 0.008 Å. The presence of about 7% dimer was also indicated. The experimental data were consistent with the usual dimer model with two halogen bridges but they were insufficient to determine the dimer structure unambiguously.     

Molecular Structure of 1,2,4,5-Tetracyanobenzene from Gas-Phase Electron Diffraction and Theoretical Calculations

The molecular structure of 1,2,4,5-tetracyanobenzene has been determined by gas-phase electron diffraction and by ab initio calculations at several levels of theory. The electron diffraction study indicates an elongation of the aromatic ring along the (H)CC(H) axis, characterized by angular deformation of the benzene ring and lengthening of the (NC)C—C(CN) bonds. The following bond lengths (r _g) and bond angles were obtained by electron diffraction: .     

Gas-Phase Electron Diffraction and Quantum-Chemical Studies of the Molecular Structure of N,N-dimethyldiaziridine

Molecular structure of N,N-dimethyldiaziridine was studied by gas-phase electron diffraction and quantum-chemical methods. It is confirmed that the molecule has a trans conformation, but the structural parameters r(endo-C–N) 1.448(2) Å, r(exo-C–N) 1.468(2) Å, and r(N–N) 1.514(6) Å differ substantially from the results of our previous electron diffraction study.     

Molecular Structure and Conformation of p-Bis(trimethylsilyl)benzene: A Study by Gas-Phase Electron Diffraction and Theoretical Calculations

The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3–0.5 kJ mol^–1. The calculated lengths of the and bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either or bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe₃ groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct. 1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1:1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe₃ groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are , and . While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9° MP2(f.c.), 117.1° B3LYP, 116.9°].     

Molecular Structure and Internal Rotation Potential of Dimethylphenylphosphine, According to Gas-Phase Electron Diffraction Data and Quantum-Chemical Calculations

Geometric parameters of dimethylphenylphosphine molecule were determined by gas-phase electron diffraction using a dynamic model in which the rotation of the PMe₂ group is treated as large-amplitude motion. Refinement of the structural parameters and parameters of the potential function was performed taking into account the geometry relaxation on the basis of HF/6-311++G** calculations. The internal rotation potential has a single minimum at 0° ( is the angle between the bisector of the MePMe angle and the phenyl ring plane) and may be described by the function of the form V() = 0.5V ₂(1 - cos2), where V ₂ = 0.38±0.36 kcal mol^{- 1}. The data obtained are compared with those for related molecules. Steric effects affect the geometry of the phenylphosphine molecule more significantly than does p- interaction.     

Determination of Molecular Structure in Terms of Potential Energy Functions from Gas-Phase Electron Diffraction Supplemented by Other Experimental and Computational Data

Different approaches of equilibrium structure determinations by the gas-phase electron diffraction (ED) method or by its combination with other relevant techniques have been reviewed. Some problems and limitations of these approaches are discussed. Special attention is paid to various potential energy function models. Different types of equilibrium bond lengths obtained by the optimization of ED data or their combination with experimental and computational spectroscopic data are compared in tables. Relations between different types of vibrational corrections are discussed. Structure data determined by other methods or approaches are given for comparison.     

The Molecular Structure of Cyclopropene as Determined by Electron Diffraction

The molecular structure of cyclopropene has been determined by gas-phase electron difraction. A least-squares analysis was applied to the experimental molecular intensity to obtain the following geometric structural parameters: C_l-C₃=1.304±0.003A, C₃-C₃=1.519±0.002A, C₁-H=1.077±0.012A, C₃-H=1.112±0.015A.     

Electron Diffraction and Quantum Chemical Study of the Molecular Structure of para-Methylbenzenesulfonyl Fluoride and para-Methylbenzenesulfonyl Bromide

A combined electron diffraction and mass-spectrometric experiment has been performed at 350(2) and 340(2) K, respectively, to study the molecular structure of para-methylbenzenesulfonyl fluoride and para-methylbenzenesulfonyl bromide. The electron diffraction data are analyzed in terms of r structure. Several geometrical models with different orientations of the sulfonyl halide group relative to the plane of the benzene ring are considered. The angle between the plane of the benzene ring and the plane of the S–Hal bond equals 90°, indicating that the molecules have C _s symmetry. The structural parameters of the molecules and the internal rotation barriers of the sulfonyl halide and methyl groups have been calculated by the ab initio and semiempirical quantum chemical methods. The results of calculations are compared with electron diffraction data.     

Molecular Structure of Silatrane Determined by Gas Electron Diffraction and Quantum-Mechanical Calculations

The geometry of silatrane HSi(OCH₂CH₂)₃N has been determined by gas electron diffraction, ab initio calculations, and vibrational spectroscopy of crystal. Using the scaled force field from DFT calculations the amplitudes and perpendicular corrections were calculated. It was assumed that the silatrane molecule has C ₃ symmetry. The following values (r _g bond lengths in Å and _a bond angles in deg. with three standard deviations from the least-squared refinements using a diagonal weight matrix) are: SiN 2.406(27); NC 1.443(7); OC 1.399(11); SiO 1.648(3); CC 1.504(15); NSiO 78.8(21); SiOC 128.1(11); SiNC 105.4(14); CCO 117.0(26); CCN 108.2(30); CNC 113.2(17); OSiO 116.3(13). The 5-membered rings are flattened. The sum of its bond angles is equal to 537.5(42). It is shown that a very large difference is found for Si—N distance from ab initio and DFT calculating.     

Electron Diffraction Determination of Equilibrium Structure for Rigid Molecules. Equilibrium Configuration of 1,2-Thiaarsol According to Gas-Phase Electron Diffraction and Quantum-Chemical Data

For rigid polyatomic molecules, a procedure is described for determining their equilibrium geometrical structures by processing gas-phase electron diffraction, spectroscopy, and quantum-chemical data. The efficiency of the procedure is demonstrated by reference to the 1,2-thiaarsol molecule. For this molecule, quantum-chemical calculations of different degrees of complexity have been carried out, and harmonic and anharmonic force fields have been constructed. The force constants were employed for determining the equilibrium geometry from experimental data. Analysis of the results of this study suggests that the calculated vibrational corrections to the internuclear distances are almost independent of the level of the quantum-chemical calculations.Original Russian Text Copyright © 2004 by Yu. I. Tarasov, I. V. Kochikov, D. M. Kovtun, N. Vogt, B. K. Novosadov, and A. S. Saakyan__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 822–829. September–October, 2004.     

Looking Back and Ahead: Gas-Phase Electron Diffraction at 75

At 75, gas-phase electron diffraction is still the method of choice for selected problems in molecular structure determination. It works best when being applied with other techniques in a concerted way.     

Molecular Structure and Conformational Preferences of Trimethyl Phosphorotrithioite,P(SMe)<Subscript>3</Subscript>, Evaluated by Gas-Phase Electron Diffraction and Quantum Chemical Calculations

Free P(SMe)₃ molecule was studied by gas electron diffraction (GED) and by B3PW91/6-311+G* (DFT) and MP2/6-31+G* calculations. Each conformer is characterized by three dihedral angles τ(CSPlp), where lp denotes the direction of the lone electron lone pair on the P atom. DFT calculations indicate that the most stable conformer is an anti,gauche+,gauche- (ag+g-) conformer of C _s symmetry; the next are the ag+g+ (ΔE = 2.5 kJ mol⁻¹), g+g+g+ (ΔE = 5.2 kJ mol⁻¹), and aa+g+ (Δ E = 12.5 kJ mol⁻¹) conformers. The MP2 calculations give the similar order, with the relative energies of 0.3, 4.3, and 10.6 kJ mol⁻¹, respectively. The experimental GED data agree well with the presence of only two conformers: χ(ag+g+) = 80(20)% and χ(ag+g-) = 20(10)%.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005, pp. 742–750.Original Russian Text Copyright © 2005 by Belyakov, Khramov, Baskakova, Naumov.     

Molecular Structure of BiBr3: An Electron Diffraction Study

The molecular structure of BiBr₃ was determined by gas-phase electron diffraction. The principal geometrical parameters are r (Bi—Br) = 2.567 ± 0.005 Å and _221D;Br—Bi—Br = 98.6 ± 0.2°. The force field of the molecule was obtained by a normal coordinate analysis utilizing both experimental vibrational frequencies and electron diffraction mean amplitudes of vibration. The variation of bond lengths and bond angles within the Group 15 trihalides is consistent with the expected trend, except that all bismuth trihalide bond angles appear to be somewhat large.