Icosahedral Carbaboranes with Peripheral Hydrogen-Chalcogenide Groups: Structures from Gas Electron Diffraction and Chemical Shielding in Solution.

Carbaboranes 1,2-(EH)2 -closo-1,2-C2 B10 H10 (E=S, Se) were prepared, in the case of E=Se for the first time. Their semi-experimental equilibrium molecular structures were established by the concerted use of quantum-chemical calculations and gas electron diffraction. A method was developed and implemented to quantify the contribution of experimental data to each refined structural parameter. The accuracy of the experimental structures and those calculated at the MP2 level of theory were gauged by comparison of experimental 11 B NMR chemical shifts with quantum-chemically computed values; the inclusion of electron correlation (GIAO-MP2) provided superior results. For the purpose of geometrical prediction, the remaining group 16 elements were considered, and the icosahedral structures for E=O and Te were also computed; for E=O the same theoretical approach was used as for E=S, and for E=Te a description similar to that for E=Se was employed.

Tris(perfluorotolyl)borane-A Boron Lewis Superacid.

Tris[tetrafluoro-4-(trifluoromethyl)phenyl]borane (BTolF) was prepared by treating boron tribromide with tetrameric F3 CC6 F4 -CuI . The F3 CC6 F4 -CuI was generated from F3 CC6 F4 MgBr and copper(I) bromide. Lewis acidities of BTolF evaluated by the Gutmann-Beckett method and calculated fluoride-ion affinities are 9 and 10 %, respectively, higher than that of tris(pentafluorophenyl)borane (BCF) and even higher than that of SbF5 . The molecular structures of BTolF and BCF were determined by gas-phase electron diffraction, that of BTolF also by single-crystal X-ray diffraction.

Structures of Trichloromethyl Thiocyanate, CCl3 SCN, in Gaseous and Crystalline State.

Trichloromethyl thiocyanate, CCl3 SCN, was structurally studied in both the gas and crystal phases by means of gas electron diffraction (GED) and single-crystal X-ray diffraction (XRD), respectively. Both experimental studies and quantum chemical calculations indicate a staggered orientation of the CCl3 group relative to the SCN group. This conclusion is supported by the similarity of the C-SCN bond length to that of the anti-structure of CH2 ClSCN (Berrueta Martínez et al. Phys. Chem. Chem. Phys. 2015, 17, 15805-15812). Bond lengths and angles are similar for gas and crystal CCl3 SCN structures; however, the crystal structure presents different intermolecular interactions. These include halogen and chalcogen type interactions, the geometry of which was studied. Characteristic C-Y⋅⋅⋅N angles (Y=Cl or S) close to 180° provide evidence for typical σ-hole interactions along the halogen/chalcogen-carbon bond in N⋅⋅⋅Cl and N⋅⋅⋅S, intermolecular units.

Gas-phase structure of 2,2,2-trichloroethyl chloroformate studied by electron diffraction and quantum-chemical calculations.

The molecular structure and conformational properties of 2,2,2-trichloroethyl chloroformate, ClC(O)OCH2CCl3 were determined experimentally using gas-phase electron diffraction (GED) and theoretically based on quantum-chemical calculations at the MP2 and DFT levels of theory. Further experimental measurements such as UV-visible, IR and Raman spectroscopy were complemented with the corresponding theoretical studies. All experimental results and calculations confirm the presence of two conformers namely anti-gauche (C1 symmetry) and anti-anti (Cs symmetry). The conformational preference was rationalised by NBO and AIM analyses. Molecular properties such as ionisation potential, electronegativity, chemical potential, chemical hardness and softness were deduced from HOMO-LUMO analyses. The TD-DFT approach was applied to assign the electronic transitions observed in the UV-visible spectrum. A detailed interpretation of the infrared and Raman spectra of the title compound are reported. Using calculated frequencies as a guide, IR and Raman spectra also provide evidence for the presence of both C1 and Cs conformers.

Structural Analysis of Perfluoropropanoyl Fluoride in the Gas, Liquid, and Solid Phases.

The coexistence of two conformers in perfluoropropanoyl fluoride, CF3CF2C(O)F, differing in the CC-CF dihedral angle (gauche 85(10)% and anti 15(10)%), has been determined by means of gas-phase electron diffraction (GED). Quantum-chemical calculations performed at the MP2 and B3LYP approximations and cc-pVTZ basis sets reproduce the experimental values with confidence. By contrast, FTIR spectra give no clear evidence for the anti-conformer in the gas phase. Information on this less abundant but stable rotamer is obtained from matrix-isolation/FTIR spectroscopy and liquid Raman spectroscopy. In situ crystallization and single-crystal X-ray diffraction (XRD) data reveal the presence of solely the gauche-conformation in the solid state. A set of intermolecular interactions including C═O···C═O, C-F···F-C, and F···C═O is detected. The nature of bonding and the relative stabilities of gauche- and anti-conformers are explored using natural bond orbitals.

Synthesis and characterization of 1,2,3,4,5-pentafluoroferrocene.

Starting from ferrocene, pentafluoroferrocene [Fe(C5F5)(C5H5)] can be prepared in five steps via a one-pot lithiation-electrophilic fluorination strategy. Pentafluoroferrocene was characterized by multinuclear NMR and IR spectroscopy, by cyclovoltammetry as well as X-ray (solid) and electron diffraction (gas) and the experimental results compared with DFT calculations.

Tridentate Lewis Acids Based on 1,3,5-Trisilacyclohexane Backbones and an Example of Their Host-Guest Chemistry.

Directed tridentate Lewis acids based on the 1,3,5-trisilacyclohexane skeleton with three ethynyl groups [CH2Si(Me)(C2H)]3 were synthesised and functionalised by hydroboration with HB(C6F5)2, yielding the ethenylborane {CH2Si(Me)[C2H2B(C6F5)2]}3, and by metalation with gallium and indium organyls affording {CH2Si(Me)[C2M(R)2]}3 (M = Ga, In, R = Me, Et). In the synthesis of the backbone the influence of substituents (MeO, EtO and iPrO groups at Si) on the orientation of the methyl group was studied with the aim to increase the abundance of the all-cis isomer. New compounds were identified by elemental analyses, multi-nuclear NMR spectroscopy and in some cases by IR spectroscopy. Crystal structures were obtained for cis-trans-[CH2Si(Me)(Cl)]3, all-cis-[CH2Si(Me)(H)]3, all-cis-[CH2Si(Me)(C2H)]3, cis-trans-[CH2Si(Me)(C2H)]3 and all-cis-[CH2Si(Me)(C2SiMe3)]3. A gas-phase electron diffraction experiment for all-cis-[CH2Si(Me)(C2H)]3 provides information on the relative stabilities of the all-equatorial and all-axial form; the first is preferred in both solid and gas phase. The gallium-based Lewis acid {CH2Si(Me)[C2Ga(Et)2]}3 was reacted with a tridentate Lewis base (1,3,5-trimethyl-1,3,5-triazacyclohexane) in an NMR titration experiment. The generated host-guest complexes involved in the equilibria during this reaction were identified by DOSY NMR spectroscopy by comparing measured diffusion coefficients with those of the suitable reference compounds of same size and shape.

The Structure and Conformation of (CH3 )3 CSNO.

The gas-phase molecular structure of (CH3 )3 CSNO was investigated by using electron diffraction, allowing the first experimental geometrical parameters for an S-nitrosothiol species to be elucidated. Depending on the orientation of the -SNO group, two conformers (anti and syn) are identified in the vapor of (CH3 )3 CSNO at room temperature, the syn conformer being less abundant. The conformational landscape is further scrutinized by using vibrational spectroscopy techniques, including gas-phase and matrix-isolation IR spectroscopy, resulting in a contribution of ca. 80:20 for the anti:syn abundance ratio, in good agreement with the computed value at the MP2(full)/cc-pVTZ level of approximation. The UV/Vis and resonance Raman spectra also show the occurrence of the conformational equilibrium in the liquid phase, with a moderate post-resonance Raman signature associated with the 350 nm electronic absorption.

Influence of Antipodally Coupled Iodine and Carbon Atoms on the Cage Structure of 9,12-I2-closo-1,2-C2B10H10: An Electron Diffraction and Computational Study.

Because of the comparable electron scattering abilities of carbon and boron, the electron diffraction structure of the C2v-symmetric molecule closo-1,2-C2B10H12 (1), one of the building blocks of boron cluster chemistry, is not as accurate as it could be. On that basis, we have prepared the known diiodo derivative of 1, 9,12-I2-closo-1,2-C2B10H10 (2), which has the same point-group symmetry as 1 but in which the presence of iodine atoms, with their much stronger ability to scatter electrons, ensures much better structural characterization of the C2B10 icosahedral core. Furthermore, the influence on the C2B10 geometry in 2 of the antipodally positioned iodine substituents with respect to both carbon atoms has been examined using the concerted application of gas electron diffraction and quantum chemical calculations at the MP2 and density functional theory (DFT) levels. The experimental and computed molecular geometries are in good overall agreement. Molecular dynamics simulations used to obtain vibrational parameters, which are needed for analyzing the electron diffraction data, have been performed for the first time for this class of compound. According to DFT calculations at the ZORA-SO/BP86 level, the (11)B chemical shifts of the boron atoms to which the iodine substituents are bonded are dominated by spin-orbit coupling. Magnetically induced currents within 2 have been calculated and compared to those for [B12H12](2-), the latter adopting a regular icosahedral structure with Ih point-group symmetry. Similar total current strengths are found but with a certain anisotropy, suggesting that spherical aromaticity is present; electron delocalization in the plane of the hetero atoms in 2 is slightly hindered compared to that for [B12H12](2-), presumably because of the departure from ideal icosahedral symmetry.

Spectroscopic characterization and constitutional and rotational isomerism of ClC(O)SCN and ClC(O)NCS.

Chlorocarbonylthio- and isothiocyanate (ClC(O)SCN and ClC(O)NCS) have been isolated and characterized by IR (Ar matrix, gas), Raman (liquid), (13)C NMR and UV-visible spectroscopies. Vibrational and quantum chemical studies suggest the presence of the syn and anti conformers (SCN group with respect to the C═O bond) in the gas phase for both constitutional isomers. syn-ClC(O)SCN is preferred by ΔH° (anti/syn) = 1.3(0.3) kcal mol(-1). The solid-state structure of ClC(O)SCN has been determined by single crystal X-ray diffraction analysis at low temperature. The crystalline solid consists exclusively of molecules in the syn conformation. On the other hand, the anti form is more stable for the ClC(O)NCS isomer. The structure of ClC(O)NCS and its conformational composition were determined by gas electron diffraction. An unusual low syn → anti interconversion energy barrier of 0.98 (0.15) kcal mol(-1) was detected for ClC(O)NCS at cryogenic temperatures. The photochemistry of both constitutional isomers isolated in solid argon at 15 K was studied. Rearrangement of ClC(O)SCN to ClC(O)NCS was observed in the neat liquid and under UV-vis irradiation of ClC(O)SCN isolated in solid argon. Properties have been discussed in terms of the valence electronic structure, including the analysis of the He(I) photoelectron spectrum of ClC(O)SCN.

Low pressure gas electron diffraction: An experimental setup and case studies.

Principles of low pressure gas electron diffraction are introduced. An experimental setup has been constructed for measuring the electron diffraction patterns of gaseous samples at pressures below 10-3 mbar. Test measurements have been performed for benzoic acid at T = 287 K corresponding to a vapor pressure of the substance P = 2 × 10-4 mbar, for iodoform CHI3 at T = 288 K (P = 4 × 10-4 mbar), and for carbon tetraiodide CI4 at T = 290 K (P = 1 × 10-4 mbar). Due to the low experimental temperature, thermal decomposition of CI4 has been prevented, which was unavoidable in previous classical measurements at higher temperatures. From the obtained data, the molecular structures have been successfully refined. The most important semi-empirical equilibrium molecular parameters are re(Car-Car)av = 1.387(5) Å in benzoic acid, re(C-I) = 2.123(3) Å in iodoform, and re(C-I) = 2.133(7) Å in carbon tetraiodide. The determined parameters showed consistency with the theoretically predicted values. A critical comparison with the results of the earlier investigations has also been done.

Combined gas electron diffraction and mass spectrometric experimental setup at Bielefeld University.

We have designed and constructed a combined experimental setup for synchronous measurements of electron diffraction patterns and mass-spectra of gas samples. Test measurements have been performed for acetic acid at two temperatures, 296 K and 457 K. Electron diffraction data have been analyzed taking into account mass spectra measured in the same experiments. From the diffraction intensities, molecular structures and mole fractions of the acetic acid monomer and dimer have been refined. The obtained results demonstrate the importance of measuring mass spectra in gas electron diffraction experiments. In particular, it is possible to detect the sample decomposition, which can be used for the optimization of experimental conditions and for the data interpretation. The length of the hydrogen bond in the acetic acid dimer determined in this work, re(O⋯H) = 1.657(9) Å, is in good agreement with modern theoretical predictions. We recommend measuring the diffraction patterns of acetic acid for the calibration of the sample pressure in the diffraction volume.

Pentafluoroethyl-substituted α-silanes: model compounds for new insights.

To further investigate the α-effect in silanes bearing a geminal donor atom, the model compounds (C2F5)3SiCH2NMe2, (C2F5)3SiCH2OMe and (C2F5)3SiONMe2 were prepared by introduction of pentafluoroethyl groups via nucleophilic substitution of the corresponding chloro-derivatives with pentafluoroethyl lithium. The substances were characterised by NMR spectroscopy and X-ray diffraction via in situ crystallization techniques. The solid state structures of these highly electronegatively substituted α-silanes contain monomeric molecules. The Si-C-N angle in (C2F5)3SiCH2NMe2 shows a value of 115.3(2)° and the Si-C-O angle in (C2F5)3SiCH2OMe a value of 105.4(1)°. Both values are smaller than the Si-C-C angle of the reference compound (C2F5)3SiCH2CH3 with a value of 118.6(2)° indicating attractive interaction between the silicon atom and the respective donor atoms. The Si-O-N angle in (C2F5)3SiONMe2 is extremely narrow at 82.0(1)°. This behaviour was further investigated by gas electron diffraction and by quantum-chemical calculations. The NBO method finds no significant orbital interactions between Si and N/O atoms in the Si-C-N, Si-C-O and Si-O-N units. The IQA model describes the compounds as strongly stabilised by electrostatic interactions between formally non-bonded silicon and donor atoms.

Silanetriols in the gas phase: single molecules vs. hydrogen-bonded dimers.

The first gas phase structure of a silanetriol, tert-butylsilane-triol [(t)BuSi(OH)(3)], determined by gas electron diffraction (GED), is reported. Quantum chemical calculations have been performed to elucidate potential intermolecular interactions between silanetriol molecules in the gas phase. The results are set into contrast to solid state structures of (t)BuSi(OH)(3) and related compounds.