The microwave spectrum of 1-fluorobicyclo[2.2.2] octane: An interal motion with large amplitude

Abstract

Eleven rotational transitions of the 1-fluorobicyclo[2.2.2]octane were observed. The spectrum consists of very rich satellites, which are assigned to the excited states of the skeletal torsion. There are two strong lines of nearly equal intensity, and the progression, which is regular in the high-frequency side of the main lines, becomes suddenly irregular near the ground-state line. These features of the spectrum are indicative of double-minimum potential for the torsion, and in fact the vibration-rotation constants α v are well accounted for by assuming the potential function of the form, V(φ) = −V 2φ 2 + V 4φ 4 + V 6φ 6 , where φ denotes the torsional coordinate. It is, however, found that additional information is necessary to specify the torsional coordinate uniquely, and the final results are obtained only after the torsional energy levels (or level separations) are measured quantitatively, by the measurement of relative intensities in the present case. The three potential constants obtained, V 2 = 1477 cm −1 (4.2 2 kcal/mole), V 4 = 6243 cm −1 (17. 8 kcal/mole), and V 6 = 22 337 cm −1 (64 kcal/mole), are therefore not very accurate, but the potential barrier V 0 between the two minima and the equilibrium angle φ 0 are determined to be 191 ± 52 cal/mole and 16.4 ± 2.6°, respectively. The spectra of the 13C species suggest that the bicyclo[2.2.2]octane ring is deformed on substitution of a fluorine atom at the C 1 position: the C 1C 2 bond length becomes shorter and the FC 1C 2 angle narrower. The spectrum of the 1-chlorobicyclo[2.2.2]octane was reinvestigated, and was analyzed also in terms of the double-minimum potential for the skeletal torsion. The values of V 0 and φ 0 obtained are 134 ± 67 cal/mole and 16.2 ± 4°, respectively. A model, in which the double-minimum potential results from balance of intramolecular forces due to internal rotation and to the strain of the valence angles, is critically examined, and it is found that the potential constants obtained by the present analysis are compatible with this model.