Molecular structure of 3-methylthiophene studied by gas electron diffraction combined with microwave spectroscopic data

The molecular structure of 3-methylthiophene

has been determined by gas electron diffraction (GED) combined with microwave (MW) spectroscopic data. Ab initio calculations at the HF/3–21G* level were carried out and used as structural constraints in the data analysis. The torsional vibration of the methyl group was treated as a large-amplitude motion. The structural parameters were determined to be: r_g(S---C₂) = 1.719(2) Å, r_g(C₂=C₃) = 1.370(3) Å, r_g(C₃---C₆) = 1.497(6) Å, r_g(C₂---H) = 1.101(5) Å, CSC = 91.6(2)°, SC₂C₃ = 113.3(5)°, SC₅C₄ = 111.3(3)°, C₂C₃C₆ = 123.2(11)° and C₃C₆H = 112(2)°. The values of r(S---C₂) − r(S---C₅) and r(C₂=C₃) − r(C₄=C₅) were fixed at the 3–21G* value of 0.002Å. Parenthesized values are the estimated limits of error (3σ) referring to the last significant digit.     

Molecular structure and conformation of diethylmethylamine determined by gas electron diffraction and vibrational spectroscopy combined with theoretical calculations

We have investigated the molecular structure and conformation of diethylmethylamine, C(4)H₃C(2)H₂N(1)[CH₃]C(3)H₂C(5)H₃, by gas electron diffraction and vibrational spectroscopy with the aid of theoretical calculations. Diffraction data are consistent with a conformational mixture of 35(14)% tt + 27(14)% g⁺t + 20(17)% g⁻t + 18(23)% g⁺g⁺ where the numbers in parentheses denote three times the standard errors (3σ). Normal-coordinate analysis based on B3LYP/6-311+G^** calculations supports the existence of the four conformers. The dihedral angle ₁(C4C2N1C3) (= −₂(C5C3N1C2)) of the tt conformer was 170(4)° whereas the ₁ and ₂ values of the other conformers were fixed at the B3LYP/6-311++G(2df,p) values: 72.4° and −163.3° for the g⁺t, −66.0° and −158.2° for the g⁻t, and 60.3° and 63.5° for the g⁺g⁺. Average values of the structural parameters (r_g/Å and _α/°) with 3σ are: r(N–C) = 1.462(2), r(C–C) = 1.523(3), r(C–H) = 1.113(2), CNC = 111.6(5), NCC = 114.5(5), NCH/CCH_Me = 110.6(5).     

Molecular structure of carbonyl fluoride as studied by gas electron diffraction and microwave data

The molecular structure of carbonyl fluoride has been determined by electron diffraction. The results have been used in conjunction with the rotational constants reported by Carpenter in a combined structure analysis. The values so obtained are r_z (C=O) = 1.1717 ± 0.0013 Å, r_z (C-F) = 1.3157 ± 0.0005 Å, and ∠_zF-C-F = 107.71 ± 0.08°. These agree with the corresponding parameters estimated by Carpenter from the rotational constants alone. The effective constants, α₃, representing the cubic anharmonicity of bond stretching vibrations have been estimated.     

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 structure of acetyl cyanide as studied by gas electron diffraction

The structure of acetyl cyanide has been determined by making joint use of the electron diffraction intensities measured in the present study and the rotational constants reported by Krisher and Wilson. The thermal average bond distances are: r_g(C-H) = 1.116±0.011 Å, r_g(CN) = 1.167±0.010 Å, r_g(C=O) = 1.208±0.009 Å, r_g(=C-C) = 1.477±0.008 Å and r_g(C-C_methyl) = 1.518±0.009 Å. The bond angles in the zero-point average structure (r_av) are: (C_methyl-C=O) = 124.6±0.7°, (C-C-C) = 114.2±0.9°, (C-CN) = 179.2±2.2° and (H-C-H) = 109.2±0.7°. The uncertainties represent the estimated limits of experimental error. The C-C single bond placed between the double and triple bonds is longer than those in vinylacetylene, acrylonitrile and propynal. Other structural parameters are also compared with those in related molecules. The infrared and Raman spectra of this molecule have been measured, and Urey-Bradley force constants have been determined.     

Molecular structure of bismuth trichloride from combined electron diffraction and vibrational spectroscopic study

The results of an electron diffraction reanalysis, augmented with a combined electron diffraction and vibrational spectroscopic elucidation, of the molecular structure of BiCl₃ are reported. The principal parameters arer _g (Bi-Cl)=2.424±0.005 å (r =2.417±0.005 å) and <Cl-Bi-Cl=97.5±0.2. They are in excellent agreement with previous electron diffraction analysis [1], utilizing a more limited data range from the same experiment. They are also fully consistent with the expected trends of geometrical variation in the Group V trihalide series. The force fields of BiCl₃, determined by normal coordinate analysis and by combined analysis, agree within experimental error.     

Electropolymerization of 3-methylthiophene at liquid 3-methylthiophene | aqueous solution | graphite three-phase junction

Droplets of 3-methylthiophene are mechanically attached to the surface of paraffin-impregnated graphite electrode (PIGE) and immersed into aqueous solution of LiClO₄. It is demonstrated that the oxidative electropolymerization (observed in non-aqueous solutions) can be accomplished by potential cycling between −0.3 and 1.4 V vs. saturated calomel electrode (SCE). Since the droplets do not contain a dissolved electrolyte, the electrochemical reaction starts at the very edge of the three-phase junction organic droplet | graphite | aqueous electrolyte.     

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 force field of boron tribromide as determined from combined analysis of gas electron diffraction and spectroscopic data and supported by quantum-chemical density-functional calculations

The thermal-average parameters of BBr₃ at 21(1) °C were obtained from a conventional analysis of gas electron diffraction (GED) data (r_g(B---Br) = 190.0(4) pm). The equilibrium structure and the force constants were refined from a joint analysis of the GED intensities and vibrational frequencies using different approximations. The simplest approximation (quadratic potential function in rectilinear coordinates) is suitable for the refinements of the equilibrium bond length (r^h_e(B---Br) = 189.6(4) pm) and the force constants of BBr₃. The molecule is planar within the error limits. Quantum-chemical density-functional calculations supported planarity of the molecule.     

Molecular structure and conformation of diisopropylamine studied by gas electron diffraction combined with vibrational spectroscopy and molecular mechanics

The molecular structure and conformation of diisopropylamine have been determined by gas electron diffraction with the aid of vibrational spectroscopy and molecular mechanics calculations.Only one conformer with the skeletal geometry of C₂ symmetry has been detected. The dihedral angle, CNCH, has been determined to be 52(4)°. The difference between the NCC angles at the gauche and trans positions with respect to the opposite NC bond is 2.4°. The CNC bond angle, 120.1(10)°, and the CN bond length, 1.470(4) Å, are 8.3° and 0.014 Å larger than the corresponding values of dimethylamine respectively     

Molecular structure of 1,1-dichloroethylene as studied by gas electron diffraction and microwave data. Estimation of equilibrium structure by an extended anharmonic model analysis

The r_z structure of 1,1-dichloroethylene has been determined by a joint analysis of the electron diffraction intensity and the rotational constants as follows: $\text{r}{}_{\mathrm{<mtext>z</mtext>}}\text{(}\text{CH}\text{) = 1.088 ± 0.011, r}{}_{\mathrm{<mtext>z</mtext>}}\text{(}\text{CC}\text{) = 1.329 ± 0.003, r}{}_{\mathrm{<mtext>z</mtext>}}\text{(}\text{CCl}\text{) = 1.725 ± 0.002}\text{A}\text{?}\text{, ∠}{}_{\mathrm{<mtext>z</mtext>}}\text{HCH}\text{= 121.4 ± 0.7}\text{and}\text{∠}{}_{\mathrm{<mtext>z</mtext>}}\text{ClCCl}\text{= 114.1 ± 0.2°}$. The uncertainties represent estimated limits of error. The observed structural parameters are compared with those for related compounds and the systematic trends in the bond lengths and bond angles are discussed. The effective constants representing anharmonicity have been obtained from an analysis of the isotopic differences in the r_z structure. By using the r_z parameters and the effective constants, the equilibrium structure has been estimated as follows: $\text{r}{}_{\mathrm{<mtext>e</mtext>}}\text{(}\text{CH}\text{) = 1.079 ± 0.012, r}{}_{\mathrm{<mtext>e</mtext>}}\text{(}\text{CC}\text{) = 1.324 ± 0.005, r}{}_{\mathrm{<mtext>e</mtext>}}\text{(}\text{CCl}\text{) = 1.721 ± 0.003}\text{A}\text{?}\text{, ∠}{}_{\mathrm{<mtext>e</mtext>}}\text{HCH}\text{= 120.5 ± 0.8}\text{and}\text{∠}{}_{\mathrm{<mtext>e</mtext>}}\text{ClCCl}\text{= 114.0 ± 0.3°}$.     

Molecular structure of N-chlorosuccinimide studied by gas-phase electron diffraction and quantum-chemical methods

The molecular structure of N-chlorosuccinimide has been studied by GED method at the nozzle temperature of 116 °C. Vibrational corrections to the r _a parameters, Δ(r _a − r _e), have been calculated using the scaled quadratic and cubic force constants from B3LYP/6-31G(df,p) calculations. The force field scaling has been carried out using the IR and Raman spectra of the solid N-chlorosuccinimide. The molecular skeleton and the bond conformation around nitrogen were found to be planar within large experimental errors. The equilibrium geometrical parameters derived from the experimental data assuming C _2v molecular symmetry and those from MP2(fc)/6-311G(3df,2pd) calculations are in a good agreement.     

Molecular structure of carphedon as studied by gas electron diffraction and quantum chemical calculations

The gas-phase structure and conformational properties of carphedon (C₁₂H₁₄N₂O₂, phenylpiracetam, 2-oxo-4-phenyl-1-pyrrolidineacetamide) have been determined by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 with 6-31G and cc-pVDZ basis sets). Since quantum chemical calculations demonstrate that the orientation of the acetamide group is fixed by a strong intramolecular N–H(amide)···O(pyrrolidone) hydrogen bond, the number of possible conformers is reduced considerably. Depending on the conformation of the pyrrolidine ring, envelope with out-of-plane C4 atom and acetamide group on the same side of the plane (“+”) or envelope with C4 and acetamide group on opposite sides (“?”), and on the orientation of the phenyl ring, axial (Ax), or equatorial (Eq), four relevant conformations, Ax?, Ax+, Eq?, and Eq+, exist. According to both quantum chemical methods (B3LYP and MP2 with cc-pVDZ basis sets) these four conformers differ by less than 2 kcal/mol in free energies. However, the two methods predict different relative free energies. The GED data were analyzed with different models. With a single-conformer model the best fit of the experimental GED intensities (agreement factor R _f = 4 %) is obtained with the Ax+ conformer. Using a two-conformer model the fit improves considerable for a 50(11):50(11) mixture of Ax? and Eq+ conformers (R _f = 2.7 %). No further improvement is obtained with a three-conformer model and large uncertainties for relative contributions occur. The geometric parameters of gaseous carphedon are compared with those in the crystal phase, where two molecules are connected by two intermolecular N–H···O hydrogen bonds, and with gas-phase values of piracetam.     

Molecular potential functions from joint analysis of gas electron diffraction and spectroscopic data

A joint analysis of electron diffraction and spectroscopic data is carried out for BF₃, PBr₃, AsBr₃, SbCl₃, SeO₂ and ClO₂ in terms of the harmonic force fields. The scheme of analysis was extended to include the following spectroscopic observables: vibrational frequencies, rotational, Coriolis coupling and centrifugal distortion constants and, whenever available, those for the isotopic species. For ClO₂ a simplified anharmonic analysis was also performed, the anharmonic spectroscopic observables being involved in this case.

Compelling evidence has been presented that the conventional harmonic approximation for the force field in terms of rectilinear internal coordinates yields the simplest satisfactory representation of diffraction and spectroscopic observations. However, a consistently better fit to experimental data was found when natural curvilinear internal coordinates were used. It is shown here that for the systems considered the joint analysis of data from various sources ensures that reliable and accurate values for equilibrium distances and force field parameters are obtained. The optimized values of spectroscopic constants, structural and force field parameters obtained are presented and compared with those available from the literature.     

The structure of 1,3-dichloropropyne as determined by gas electron diffraction and comparison with microwave data

The structure of 1,3-dichloropropyne has been studied by gas electron diffraction. The resulting parameters r_a have been converted into r_α^o distances. A geometrical structure has been fitted to these internuclear distances. Thus the following parameters (r_α^o) have been determined: r(C₁-Cl₁) = 1.629 (10) A, r(C₁C₂) = 1.201 (13) Å, r(C₃-Cl₂) = 1.791 (6) A, ∠(C₂-C₃-Cl₂) = 111.1° (1.0°), ∠(H-C₃-H) = 98.8° (3.1°), ∠(C₂-C₃-H) = 108.7° (3.2°). ∠(Cl₁-C₁C₂) = 176.6° (1.1°), ∠(C₁C₂-C₃) = 182.7° (1.4°). Inconsistencies have been detected between our results and the rotational constants reported by Günther and Zeil. Discussion of the problem including rotational constants of the first excited vibrational state leads to the conclusion that the observed discrepancies are due to temperature effects.     

Molecular structure of 5-fluorouracil from gas-phase electron diffraction data and quantum-chemical calculations

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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.