Heinz Oberhammer

Gas‐phase structures of several organic and inorganic peroxides X‐O‐O‐X and X‐O‐O‐X′, which have been determined experimentally by gas electron diffraction and/or microwave spectroscopy, are discussed. The OO bond length in these peroxides varies from 1.481(8) Å in Me₃SiOOSiMe₃ to 1.214(2) Å in FOOF and the dihedral angle ϕ(XO‐OX) between 0° in HC(O)O‐OH and near 180° in Bu^tO‐OBu^t. Some of the peroxides cause problems for quantum chemistry, since several computational methods fail to reproduce the experimental structures. Extreme examples are MeO‐OMe and FO‐OF. In the case of MeO‐OMe only about half of the more than 100 computational methods reported in the literature reproduce the experimentally determined double‐minimum shape of the torsional potential around the OO bond correctly. For FO‐OF only a small number of close to 200 computational methods reproduce the OO and OF bond lengths better than ±0.02 Å.

Structural and conformational properties of the tetrahalides MX₄ with M = S, Se, Te and X = F, Cl, Br, I were studied with quantum chemical calculations (MP2/aug-cc-pVTZ and relativistic core potentials for heavy atoms). The pseudotrigonal bipyramidal geometries (C_2v symmetry) of the three tetrafluorides and of TeCl₄ , whose structures have been determined by gas electron diffraction, are very well reproduced with this computational method. No additional stable conformer with C_3v, C_S, C_4v or T_d symmetry is predicted for these four chalcogen tetrahalides. For all other MX₄ compounds, except SeI₄, the existence of two conformers with C_2v and T_d symmetry is predicted. The T_d structure is favoured in the case of SI₄ and SeBr₄ and SeI₄ is predicted to exist only as T_d conformer.

The sublimation processes of TeBr₄ at 471(5) K and TeI₄ at 373(5) K were studied with a combined gas‐phase electron diffraction and mass spectrometric technique (GED/MS). The mass spectra and the analysis of the GED intensities showed that a contribution of 40(3) mol‐% TeBr₂, 59(3) mol‐% Br₂, and 1 mol‐% TeBr₄ was formed in the vapor over TeBr₄(s). Solid tellurium tetraiodide decomposes to form I₂(g) and Te(s). A very small contribution of 3.3 ± 2.1 mol‐% of gaseous TeI₂ was also determined by both GED and MS. The “metallic” Te accumulated in the solid phase vaporizes at above ca. 670 K as the predominately Te₂ molcular species. Refinement of the GED intensities resulted in r_g(Te–Br) = 2.480(5) Å and ∠_gBr–Te–Br = 99.0(6)° for TeBr₂ and r_g(Te–I) = 2.693(9) Å and ∠_g(I–Te–I) = 103.1(22)° for TeI₂. The small contribution of TeBr₄ observed in the mass spectra of the vapor over TeBr₄ could not be observed in the GED data. Geometric parameters and vibrational frequencies for the tellurium dihalides TeX₂ with X = F, Cl, Br, and I were calculated with B3LYP, MP2, CCSD, and CCSD(T) methods by using aug‐cc‐pVTZ basis sets and various core potentials for the tellurium atom. Bonding properties in tellurium dihalides are discussed on the basis of natural bond orbital analyses. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The vapour over solid SeBr₄ at 10 °C was investigated with a combined gas-phase electron diffraction/mass spectrometric (GED/MS) method. The composition of the vapour derived from the mass spectra (43% SeBr₂, 56.7% Br₂ and 0.3% Se₂Br₂) was in agreement with the composition obtained from the analysis of the simultaneously recorded GED intensities (41(3)% SeBr₂, 59(3)% Br₂). The GED study results in the following geometric parameters (r_g, ∠_g values with total uncertainties): Se–Br = 2.306(5) Å and Br–Se–Br = 101.6(6)°. Most quantum chemical approximations (B3LYP, MP2, CCSD and CCSD(T) with relativistic effective core potentials and cc-pVTZ as well as aug-cc-pVTZ basis sets for the outer shells) overestimate the Se–Br bond length by 0.01 to 0.03 Å. All methods reproduce the bond angle correctly, except for the B3LYP method. Gas phase vibrational frequencies estimated from experimental vibrational amplitudes agree well with those measured by Raman spectroscopy in acetonitrile solutions. All computational methods overestimate vibrational frequencies, especially that for the symmetric stretch vibration, by about or 8 to 13%.

The molecular structure of axial and equatorial conformers of 1‐trifluoromethyl‐1‐silacyclohexane, (C₅H₁₀SiHCF₃), as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR) spectroscopy, and quantum chemical calculations (B3LYP, MP2, and CBS‐QB3). According to GED, the compound exists as a mixture of two C_s symmetry conformers possessing the chair conformation of the six‐membered ring and differing in the axial or equatorial position of the CF₃ group (axial=58(12) mol %/equatorial=42(12) mol %) at T=293 K. This result is in a good agreement with the theoretical prediction. This is, however, in sharp contrast to the conformational properties of the cyclohexane analogue. The main structural feature for both conformers is the unusually long exocyclic bond length SiC 1.934(10) Å. A low‐temperature ¹⁹F NMR experiment results in an axial/equatorial ratio of 17(2) mol %:83(2) mol % at 113 K and a ΔG ^≠ of 5.5(2) kcal mol⁻¹. CBS‐QB3 calculations in the gas‐phase and solvation effect calculations using the PCM(B3LYP/6‐311G*) and IPCM(B3LYP/6‐311G*) models were applied to estimate the axial/equatorial ratio in the 100–300 K temperature range, which showed excellent agreement with the experimental results. The minimum energy pathways for the chair‐to‐chair inversion of trifluoromethylsilacyclohexane and methylsilacyclohexane were also calculated using the STQN(Path) method.