The Perfluoro-o-phenylene-mercury Trimer [Hg(o-C6F4)]3 - a Textbook Example of Phase-Dependent Structural Differences.

The geometric and electronic structure of [Hg(o-C6F4)]3 (1) in the gas phase, i. e. free of intermolecular interactions, was determined by a synchronous gas-phase electron diffraction/mass spectrometry experiment (GED/MS), complemented by quantum chemical calculations. 1 is stable up to 498 K and the gas phase contains a single molecular form: the trimer [Hg(o-C6F4)]3. It has a planar structure of D3h symmetry with a Hg-C distance of 2.075(5) Šand a Hg-Hg distance of 3.614(7) Š(both rh1). Structural differences between the crystalline and gaseous state have been analyzed. Different DFT functional-basis combinations were tested, demonstrating the importance to consider the relativistic effects of the mercury atoms. The combination PBE0/MWB(Hg),cc-pVTZ(C,F) turned out to be the most appropriate for the geometry optimization of such organomercurials. The electronic structure of 1, the nature of the chemical bonding in C-Hg-C fragments and the nature of the Hg⋅⋅⋅Hg interactions have been analyzed in terms of the Natural Bond Orbital (NBO) and Quantum Theory of Atoms in Molecules (QTAIM) approaches. The influence of the nature of halogen substitution on the structure of the molecules in the series [Hg(o-C6H4)]3, [Hg(o-C6F4)]3, [Hg(o-C6Cl4)]3, [Hg(o-C6Br4)]3 was also analyzed.

The Nature of Chalcogen-Bonding-Type Tellurium-Nitrogen Interactions: A First Experimental Structure from the Gas Phase.

(C6 F5 )Te(CH2 )3 NMe2 (1), a perfluorophenyltellurium derivative capable of forming intramolecular N⋅⋅⋅Te interactions, was prepared and characterized. The donor-free reference substance (C6 F5 )TeMe (2) and the unsupported adduct (C6 F5 )(Me)Te⋅NMe2 Et (2 b) were studied in parallel. Molecular structures of 1, 2 and 2 b were determined by single-crystal X-ray diffraction and for 1 and 2 by gas-phase electron diffraction. The structure of 1 shows N⋅⋅⋅Te distances of 2.639(1) Š(solid) and 2.92(3) Š(gas). Ab initio plus NBO and QTAIM calculations show significant charge transfer effects within the N⋅⋅⋅Te interactions and indicate σ-hole interactions.

Molecular structure and electron distribution of 4-nitropyridine N-oxide: Experimental and theoretical study of substituent effects.

The molecular structure of 4-nitropyridine N-oxide, 4-NO2-PyO, has been determined by gas-phase electron diffraction monitored by mass spectrometry (GED/MS) and by quantum chemical calculations (DFT and MP2). Comparison of these results with those for non-substituted pyridine N-oxide and 4-methylpyridine N-oxide CH3-PyO, demonstrate strong substitution effects on structural parameters and electron density distribution. The presence of the electron-withdrawing -NO2 group in para-position of 4-NO2-PyO results in an increase of the ipso-angle and a decrease of the semipolar bond length r(N→O) in comparison to the non-substituted PyO. The presence of the electron-donating -CH3 group in 4-CH3-PyO leads to opposite structural changes. Electron density distribution in pyridine-N-oxide and its two substituted compounds are discussed in terms of natural bond orbitals (NBO) and quantum theory atoms in molecule (QTAIM).

Disulfuryl Dichloride ClSO2 OSO2 Cl: A Conformation and Polymorphism Chameleon.

Disulfuryl dichloride ClSO2 OSO2 Cl was characterized by vibrational spectroscopy in the gaseous and liquid phase as well as in the Ar-matrix. By varying the temperature, certain bands could be assigned to several conformers. Gas-phase electron diffraction revealed a dominance of the anti-conformer at ambient temperature. The same conformation was found in the solid state. Via the in situ technique for crystallization, not less than four different modifications were identified. Among these different modifications, the structural parameters of the molecules remain relatively constant, but the aggregation pattern changes. Although the molecules aggregate by chlorine⋅⋅⋅oxygen contacts in each modification, the geometrical parameters of these interaction show significant differences and were evaluated and are in part inconsistent with the halogen bonding concept.

The molecular structure of 4-methylpyridine-N-oxide: Gas-phase electron diffraction and quantum chemical calculations.

The molecular structure of 4-methylpiridine-N-oxide, 4-MePyO, has been studied by gas-phase electron diffraction monitored by mass spectrometry (GED/MS) and quantum chemical (DFT) calculations. Both, quantum chemistry and GED analyses resulted in C S molecular symmetry with the planar pyridine ring. Obtained molecular parameters confirm the hyperconjugation in the pyridine ring and the sp2 hybridization concept of the nitrogen and carbon atoms in the ring. The experimental geometric parameters are in a good agreement with the parameters for non-substituted N-oxide and reproduced very closely by DFT calculations. The presence of the electron-donating CH3 substituent in 4-MePyO leads to a decrease of the ipso-angle and to an increase of r(N→O) in comparison with the non-substituted PyO. Electron density distribution analysis has been performed in terms of natural bond orbitals (NBO) scheme. The nature of the semipolar N→O bond is discussed.

Structure Determination Techniques Flex Their Muscles.

A historical challenge: Gas-phase electron diffraction and single-crystal X-ray diffraction are both established techniques, but they were both pushed to their limits by the challenge posed by the highly flexible tetranitromethane molecule. New approaches had to be developed for the structure of the molecule to be elucidated.

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.

Tetranitromethane: A Nightmare of Molecular Flexibility in the Gaseous and Solid States.

After numerous attempts over the last seven decades to obtain a structure for the simple, highly symmetric molecule tetranitromethane (C(NO2 )4 , TNM) that is consistent with results from diffraction experiments and spectroscopic analysis, the structure has now been determined in the gas phase and the solid state. For the gas phase, a new approach based on a four-dimensional dynamic model for describing the correlated torsional dynamics of the four C-NO2 units was necessary to describe the experimental gas-phase electron diffraction intensities. A model describing a highly disordered high-temperature crystalline phase was also established, and the structure of an ordered low-temperature phase was determined by X-ray diffraction. TNM is a prime example of molecular flexibility, bringing structural methods to the limits of their applicability.

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.