Spectroscopic Determination of the Barrier to Planarity in Cyclopentane

Abstract

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 1824 ± 50 cm−1 (5.21 kcal) and the equilibrium out-of-plane amplitude qe=0.47 ± 0.025 Å.

NMR Conformational Analysis and Theoretical Calculations for 2‐Aryl‐ 1,3‐dihydroxy‐4,4,5,5‐tetramethylimidazolidines

“…In fact, several pentacyclic systems have an axial ± equatorial interconversion energy barrier of ca. 20 kJ/mol [19], this being dependent on the substitutent groups [20]. For this reason, all the structures discussed below require mirror-image structures to be equally present.…”

Parametrization of the Torsion Potential in All-Atom Models of Hydrocarbon Molecules Using a Simplified Expression for the Deformation Energy of Valence Bonds and Angles

“…In the equilibrium state, twisted and bent forms of the cyclopentane molecule alternate almost freely [44][45][46] so that the potential barriers during pseudorotation do not exceed 0.06-0.03 kcal/mol [44,45]. According to the experimental data, the average of amplitude q e characterizing the puckering of the cyclopentane molecule in the equilibrium state is 0.47±0.025 Å [47] or 0.458 Å [48]. Note that the transition from the equilibrium state the planar five-membered ring (q f = 0 Å) requires 5.21±0.14 kcal/mol [47] or 5.16 kcal/mol [48].…”

Fourier transform carbon-13 spin–lattice relaxation study of motion of cyclic hydrocarbons

“…The x test for methoxycyclopentane indicates that Rl and R, are in the inertial limit (X = 0.7 and 1.3 respectively) while R, is not (X = 4.6). In all of these cases contributions to T,"Yrom ring flipping are possible, there being other evidence for large amplitude motions of this type in small rings (25)(26)(27)(28). The TId" values in cyclopentanol, compared to the other five membered ring derivatives, indicate considerable molecular association.…”