ASYMMETRIC POTENTIAL FUNCTIONS FOR RING INVERSION IN CYCLIC IMINES FROM FAR INFRARED SPECTRA

$^{1}$ S.I. Chan, T.R. Borgers, J.W. Russell, H.L. Strauss, and W.D. Gwinn, J. Chem. Phys. 44, 1103 (1966). $^{2}$ J.R. Durig and R.C. Lord, J. Chem Phys. 45, 61 (1966). $^{3}$ Jaan Laane and R.C. Lord, J. Chem. Phys. 47, 4941 (1967); 48, 1508 (1968). $^{4}$ T. Ueda and T. Shimanouchi, J. Chem. Phys. 47, 4042 (1967); 47, 5018 (1967). $^{5}$ T.M. Hard and R.C. Lord, Appl. Optics 7, 589 (1968).Author Institution: Spectroscopy Laboratory, Massachusetts Institute of TechnologySymmetrical potential functions for the ring-inversion vibrations of a number of four- and five-membered rings have been evaluated from vibrational transitions observed in the far $infrared.^{1-4}$ We have now been able to assign the transitions for the ring puckering vibrations in the four-membered ring trimethylenimine, $(CH_{2})_{3}NH$, and the five-membered ring $2,5-dihydropyrrole, (CH_{2}CH = CHCH_{2})NH$. The NH group in these molecules introduces an asymmetry into the potential function for the inversion of these rings and leads to a considerable complexity of the eigenvalues. A high-resolution double-beam $spectrometer^{5}$ and multipass absorption cell enabled the observation of sufficient transitions of the above molecules and their ND-derivatives to provide data for complete determination of their potential functions up to moderate energies ($1000 cm^{-1}$ or higher). Above these energies the curves are essentially quartic in shape. The potential function was assumed to have the form $V(x) = ax^{2}+bx^{2}+cx^{4}$, where x is the ring-puckering normal coordinate. The coefficients a, b and c were evaluated by means of the computer program of Ueda and $Shimanouchi.^{4}$. Once the far infrared spectrum has been understood, it becomes possible to interpret the satellite fine structure due to difference bands associated with certain of the mid infrared fundamentals