Conformational Properties of 1-Silyl-1-Silacyclohexane, C₅H₁₀SiHSiH₃: Gas Electron Diffraction, Low-Temperature NMR, Temperature-Dependent Raman Spectroscopy, and Quantum Chemical Calculations

The molecular structure of axial and equatorial conformers of 1-silyl-silacyclohexane, C₅H₁₀SiHSiH₃, and the thermodynamic equilibrium between these species were investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR), temperature-dependent Raman spectroscopy, and quantum chemical calculations (CCSD(T), MP2 and DFT methods). According to GED, the compound exists 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 SiH₃ group (axial = 57(7) mol %/equatorial = 43(7) mol %) at T = 321 K. This corresponds to an A value (free energy difference = G_axial − G_equatorial) of −0.17(15) kcal mol⁻¹. A low-temperature ¹³C NMR experiment using SiD₄ as a solvent resulted in an axial/equatorial ratio of 45(3)/55(3) mol % at 110 K corresponding to an A value of 0.05(3) kcal mol⁻¹, and a ΔG^# value of 5.7(2) kcal mol⁻¹ was found at 124 K. Temperature-dependent Raman spectroscopy in the temperature range of 210−300 K of the neat liquid, a THF solution, and a heptane solution indicates that the axial conformer is favored over the equatorial one by 0.26(10), 0.23(10), and 0.22(10) kcal mol⁻¹ (ΔH values), respectively. CCSD(T)/CBS and MP2/CBS calculations in general predict both conformations to have very similar stability and are, thus, in excellent agreement with the DNMR result but in a slight disagreement with the GED and Raman results. Two DFT functionals, that account for dispersion interactions, M06-2X/pc-3 and B2PLYP-D/QZVPP, deviate from the high-level coupled cluster and MP2 calculations by only 0.1 kcal mol⁻¹ on average, whereas B3LYP/pc-3 calculations greatly overestimate the stability of the equatorial conformer.