Structure and conformation studies from temperature dependent infrared spectra of xenon solutions and ab initio calculations of cyclobutylgermane

The infrared spectra (3500–220 cm−1) of cyclobutylgermane, c-C4H7GeH3 have been recorded of the gas. Also variable temperature (−65 to −100 °C) studies of the infrared spectra (3500–400 cm−1) of the sample dissolved in liquid xenon were recorded and both the equatorial and axial conformers were identified. The enthalpy difference has been determined from 10 band pairs 8 temperatures to give 112 ± 11 cm−1 (1.34 ± 0.13 kJ mol−1) with the equatorial conformer the more stable form. The percentage of the axial conformer present at ambient temperature is estimated to be 37 ± 1%. From ab initio calculations conformational stabilities have been predicted from both MP2(full) and density functional theory calculations from a variety of basic sets. The r0 structure parameters have been obtained for both conformers from the previously reported rotational constants from the three isotopologues. The determined heavy atom distances for the equatorial [axial] form are (Å) Ge–Cα = 1.952(3) [1.950(3)], Cα–Cβ,Cβ′=1.557(3)[1.565(3)]Cα–Cβ,Cβ′=1.557(3)[1.565(3)], Cγ–Cβ,Cβ′=1.551(3)Cγ–Cβ,Cβ′=1.551(3) [1.551(3)] and angles in degrees (°) ∠GeCαCβ = 118.6(5) [113.4(5)], ∠CβCαCβ′=88.3(5)[88.0(5)]∠CβCαCβ′=88.3(5)[88.0(5)], ∠CαCβCγ = 87.8(5) [88.8(5)], ∠CβCγCβ′=88.7(5)[89.0(5)]∠CβCγCβ′=88.7(5)[89.0(5)] and a puckering angle of 29.1(5) [25.1(5)]. Data from ab initio calculations were used to predict vibrational harmonic force constants, fundamental wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are compared to the corresponding properties of some related molecules.

Axial (598 cm−1) and equatorial (590 cm−1) bands at various temperatures.Figure optionsDownload as PowerPoint slideHighlights
► The r0 adjusted structural parameters have been determined for both conformers.
► Experimental enthalpy difference has been determined between the two conformers.
► Vibrational assignments have been obtained for both conformers.
► ΔE has been predicted for both forms and the planar transition state.