Conformational and structural studies of cyclobutylsilane from temperature dependent infrared spectra of xenon solutions and ab initio calculations

The infrared spectra (3500–220 cm−1) in the gas phase of cyclobutylsilane, c-C4H7SiH3 has been recorded. Variable temperature (−65 to −100 °C) studies of the infrared spectra (3500–400 cm−1) dissolved in liquid xenon have been carried out. From these data, both the equatorial and axial conformers have been identified and their relative stability obtained. The enthalpy difference has been determined from 15 band pairs to be 67 ± 7 cm−1 (0.81 ± 0.08 kJ mol−1) with the equatorial conformer the more stable form. The percentage of the axial conformer is estimated to be 42 ± 1% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained for both conformers. The determined heavy atom distances (Å) for the equatorial [axial] form are Si–Cα = 1.879(3) [1.884(3)], Cα–Cβ, Cβ′ = 1.562(3) [1.561(3)], Cγ–Cβ, Cβ′ = 1.551(3) [1.553(3)] and angles in degrees (°) ∠SiCαCβ = 118.6(5) [113.2(5)], ∠CβCαCβ′ = 88.2(5) [88.4(5)], ∠CαCβCγ = 87.8(5) [88.9(5)], ∠CβCγCβ′ = 88.9(5) [89.0(5)] and a puckering angle of 29.0(5) [23.5(5)]. Support for the vibrational studies have been provided from MP2(full)/6-31G(d) ab initio calculations to predict harmonic force constants, fundamental wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are discussed and compared to the corresponding properties of some related molecules.

► The heavy atom structural parameters have been determined for both forms.
► Vibrational assignments have been obtained for both conformers.
► ΔE has been predicted for both forms and the planar transition state.
► Experimental enthalpy difference has been determined between the two conformers.