Spectroscopic Determination of the Barrier to Planarity in Cyclopentane

A measurement of the Raman spectrum of cyclopentane in the gas phase under moderately high resolution reveals that the ground state radial transition at 272.5 cm⁻¹ has two associated Q branches at 265.3 and 257.8 cm⁻¹, corresponding to the first two excited transitions of the radial mode. With this information and the pseudorotational combination and difference bands with the CH₂ deformation observed by During and Wertz, we have used both the perturbation treatment developed by Ikeda et al. and the direct matrix diagonalization program developed earlier to obtain the potential energy surface for cyclopentane in the ground state and in the first excited state of the CH₂ deformation. In the ground state the barrier to planarity is found to be $\text{1824 ± 50\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ (5.21 kcal) and the equilibrium out‐of‐plane amplitude $q_e\text{=0.47 ± 0.025\hspace{0.17em}Å.}$