Conformational Properties of 1-Fluoro-1-silacyclohexane, C₅H₁₀SiHF: Gas Electron Diffraction, Low-Temperature NMR, Temperature-Dependent Raman Spectroscopy, and Quantum Chemical Calculations

Abstract

The molecular structures of axial and equatorial conformers of 1-fluorosilacyclohexane, C 5 H 10 SiHF, as well as the thermodynamic equilibrium between these species were investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance, temperature-dependent Raman spectroscopy, and quantum chemical calculations (MP2, DFT, and composite methods). According to GED, the compound exists in the gas phase as a mixture of two conformers possessing the chair conformation of the six-membered ring and C s symmetry and differing in the axial or equatorial position of the Si-F bond (axial ) 63( 8) mol %/equatorial ) 37(8) mol %) at T ) 293 K, corresponding to an A value of -0.31(20) kcal mol -1 . Density functional theory (DFT) calculations were employed to obtain the minimal energy path of the conformational inversion. The MP2, G3B3, and CBS-QB3 methods were also employed to calculate the equilibrium geometries and energies of the local minima in the gas phase and in solution. The gas-phase results are in good agreement with the experiment, whereas a combined PCM/IPCM(B3LYP/6-311G(d)) approach overestimates the stabilization of the axial conformer by 0.3-0.4 kcal mol -1 in solution at 112 K. Temperature-dependent Raman spectroscopy in the temperature ranges of 210-300 K (neat liquid), 120-300 K (pentane solution), and 200-293 K (dichloromethane solution) also indicates that the axial conformer is favored over the equatorial one by 0.25(5), 0.22(5), and 0.28(5) kcal mol -1 (∆H values), respectively.

Conformational energies of silacyclohexanes C5H10SiHMe, C5H10SiH(CF3) and C5H10SiCl(SiCl3) from variable temperature Raman spectra

“…The Raman spectra thus confirm results from a combined ab initio , electron diffraction and NMR investigation concluding that the axial conformer is more stable than the equatorial. Moreover, solvent polarity has no noticeable effect on the conformational energies, which is also found for 1‐methyl‐1‐silacyclohexane (described in this paper), 1‐fluoro‐1‐silacyclohexane and 1‐silyl‐1‐silacyclohexane …”

“…Moreover, a preference for axial is also predicted for 1‐chloro and 1‐fluorosilacyclohexane, but not for 1‐trichloromethylsilacyclohexane . For these molecules, the predicted energy differences have been verified experimentally just for 1‐fluorosilacyclohexane, employing microwave spectroscopy, low temperature nuclear magnetic resonance (NMR) and low temperature Raman spectroscopy and gas electron diffraction …”

Molecular structure and conformational analysis of 3-methyl-3-silathiane by gas phase electron diffraction, FTIR spectroscopy and quantum chemical calculations

“…The conformational equilibrium between the chair-axial and chair-equatorial conformers of 1-X-1-silacyclohexanes C 5 H 10 SiHX is shifted toward the axial conformer for electronegative substituents (X ¼ Hal [1,2], CF 3 [3]), the axial predominance being increased by lowering the temperature. An exception is 1-silyl-1-silacyclohexane [4] for which the SiH 3 -axial conformer predominates in gas phase (GED), whereas in solution at low temperature (NMR) the ratio of the conformers is reversed.…”

Total Synthesis of Potent Antitumor Agent (−)‐Lasonolide A: A Cycloaddition‐Based Strategy

“…The lower barrier is observed for compound 2 bearing strong electronegative group CF 3 and having much larger dipole moment. Lowering of the inversion barriers of the Si‐cycles relative to their carbon analogs is well documented in a number of works31–35 and is due to higher flexibility of the former because of longer SiC compared to the CC bond as well as flattening of the ‘Si‐part’ of the ring. Thus, the angle formed by the C3–C4–C5 plane and the C2–C3–C5–C6 plane of the ring in the optimized structures of the axial and equatorial conformers of thiane‐1‐imide 4 (Scheme ) is 53°, whereas the corresponding angle between the CSiC plane and the same C2–C3–C5–C6 plane of the ring in both conformers of silathiane‐1‐imide 1 (Scheme ) is only 41°, that should also facilitate the ring inversion.…”