Spectra and Structure of Small Ring Compounds. XIX. Vibrational Analysis and the Barrier to Pseudorotation of Germylcyclopentane

The infrared spectrum of germylcyclopentane in the gaseous state has been recorded from 4000 to 33 cm⁻¹ and that of 1, 1‐dideutero‐1‐germylcyclopentane has been recorded from 4000 to 250 cm⁻¹. The Raman spectra of the two liquids have been recorded and depolarization values determined. The data rule out the planar configuration for the germylcyclopentane molecule and are shown to be consistent with the twisted $C_2$ molecular structure. The 39 normal modes have been assigned on the basis of their depolarization values, infrared band contours, isotopic shift factors, band intensities, and “group frequencies.” The ring‐bending mode was found at 113 cm⁻¹ in the vapor phase of the “light” compound. The ring‐twisting mode for the $d_0$ and $d_2$ compounds was found at 273 and 269 cm⁻¹, respectively, in the Raman spectrum. The 113‐cm⁻¹ band in the light compound was found to be the first of a series of $Q$ branches running to lower frequency. This series has been interpreted in terms of a relatively high barrier to pseudorotation. From midinfrared sum bands a similar series of transitions were obtained for the pseudorotation of the $d_2$ compound. A potential function of the form $\text{V\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}(2043\hspace{0.17em}/\hspace{0.17em}2) (1\hspace{0.17em}−\hspace{0.17em}}\cos \text{2θ)\hspace{0.17em}−\hspace{0.17em}(21\hspace{0.17em}/\hspace{0.17em}2) (1\hspace{0.17em}−\hspace{0.17em}}\cos \text{6θ)}$ was found to fit nine observed far‐infrared transitions of the ring‐puckering mode. The calculated barrier to pseudorotation was 5.9 ± 0.1 kcal/mole for the “light” compound, and the spectroscopically derived parameter ${\mathrm{mq}}_0^2$ was found to be 18.7 × 10⁻⁴⁰ g·cm². The barrier in the corresponding $d_2$ compound is estimated to be 6.0 ± 0.3 kcal/mole. The barriers are considerably higher than one might expect from existing theories and data on torsional barriers.