Molecular structure and conformational behavior of 1-methyl-1-phenylsilacyclohexane studied by gas electron diffraction, IR spectroscopy and quantum chemical calculations

- 1-Methyl-1-phenylsilacyclohexane was studied by GED and quantum chemistry.
- The value ΔG for Phax-twist ↔ Pheq-orth is 0.18-0.82 kcal/mol from DFT, MP2.
- The ratio PhaxMeeq:PheqMeax is 58(15):42(15)% from GED data.
- The rotation of the methyl group influences on orientation of the phenyl ring.
- IR spectra proves the existence of both conformers in the liquid phase.

Conformational properties and molecular structure of 1-methyl-1-phenylsilacyclohexane 1 were studied by quantum chemical (QC) calculations, combined gas electron diffraction/mass spectrometry (GED/MS) and IR spectroscopy. Molecule 1 may exist in four forms that differ by axial or equatorial positions of the substituents and by relative orientation of the two rings. Two of these conformers were found to dominate in the gas phase: 1-Pheq-orth conformer with the phenyl ring plane bisecting the CSiC endocyclic angle, and 1-Phax-twist with the phenyl ring and the CSiC exocyclic angle oriented nearly perpendicularly to each other. From the GED data, the summarized molar fractions of the conformers were found to be Pheq:Phax = 42(15):58(15)% which corresponds to ΔG = Geq-Gax = 0.19(37) kcal/mol. Experimental and calculated IR spectra provides an evidence of existence of both conformers in the liquid phase. The QC calculations yielded the ΔG = G(Pheq-orth)-G(Phax-twist) values of 0.18-0.36 DFT and 0.82 MP2 kcal/mol.

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