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Unexpected Conformational Properties of 1‐Trifluoromethyl‐1‐Silacyclohexane, C 5 H 10 SiHCF 3 : Gas Electron Diffraction, Low‐Temperature NMR Spectropic Studies, and Quantum Chemical Calculations
Author(s) -
Girichev Georgiy V.,
Giricheva Nina I.,
Bodi Andras,
Gudnason Palmar I.,
Jonsdottir Sigridur,
Kvaran Agust,
Arnason Ingvar,
Oberhammer Heinz
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600683
Subject(s) - conformational isomerism , trifluoromethyl , chemistry , cyclohexane , crystallography , ring flip , electron diffraction , molecular geometry , solvation , gas electron diffraction , diffraction , molecule , ring (chemistry) , alkyl , physics , reflection high energy electron diffraction , organic chemistry , optics
The molecular structure of axial and equatorial conformers of 1‐trifluoromethyl‐1‐silacyclohexane, (C 5 H 10 SiHCF 3 ), 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 CF 3 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 SiC 1.934(10) Å. A low‐temperature 19 F NMR experiment results in an axial/equatorial ratio of 17(2) mol %:83(2) mol % at 113 K and a Δ G ≠ 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.